Merge pull request #29403 from yuwata/journal-decouple-journald-and-journal-remote

[thirdparty/systemd.git] / src / core / namespace.c

/* SPDX-License-Identifier: LGPL-2.1-or-later */

#include <errno.h>
#include <linux/loop.h>
#include <sched.h>
#include <stdio.h>
#include <sys/file.h>
#include <sys/mount.h>
#include <unistd.h>
#if WANT_LINUX_FS_H
#include <linux/fs.h>
#endif

#include "alloc-util.h"
#include "base-filesystem.h"
#include "chase.h"
#include "dev-setup.h"
#include "devnum-util.h"
#include "env-util.h"
#include "escape.h"
#include "extension-util.h"
#include "fd-util.h"
#include "format-util.h"
#include "glyph-util.h"
#include "label-util.h"
#include "list.h"
#include "lock-util.h"
#include "loop-util.h"
#include "loopback-setup.h"
#include "missing_syscall.h"
#include "mkdir-label.h"
#include "mount-util.h"
#include "mountpoint-util.h"
#include "namespace-util.h"
#include "namespace.h"
#include "nsflags.h"
#include "nulstr-util.h"
#include "os-util.h"
#include "path-util.h"
#include "selinux-util.h"
#include "socket-util.h"
#include "sort-util.h"
#include "stat-util.h"
#include "string-table.h"
#include "string-util.h"
#include "strv.h"
#include "tmpfile-util.h"
#include "umask-util.h"
#include "user-util.h"

#define DEV_MOUNT_OPTIONS (MS_NOSUID|MS_STRICTATIME|MS_NOEXEC)

typedef enum MountMode {
        /* This is ordered by priority! */
        INACCESSIBLE,
        OVERLAY_MOUNT,
        MOUNT_IMAGES,
        BIND_MOUNT,
        BIND_MOUNT_RECURSIVE,
        PRIVATE_TMP,
        PRIVATE_TMP_READONLY,
        PRIVATE_DEV,
        BIND_DEV,
        EMPTY_DIR,
        PRIVATE_SYSFS,
        BIND_SYSFS,
        PROCFS,
        READONLY,
        READWRITE,
        NOEXEC,
        EXEC,
        TMPFS,
        RUN,
        EXTENSION_DIRECTORIES, /* Bind-mounted outside the root directory, and used by subsequent mounts */
        EXTENSION_IMAGES, /* Mounted outside the root directory, and used by subsequent mounts */
        MQUEUEFS,
        READWRITE_IMPLICIT, /* Should have the lowest priority. */
        _MOUNT_MODE_MAX,
} MountMode;

typedef struct MountEntry {
        const char *path_const;   /* Memory allocated on stack or static */
        MountMode mode:5;
        bool ignore:1;            /* Ignore if path does not exist? */
        bool has_prefix:1;        /* Already is prefixed by the root dir? */
        bool read_only:1;         /* Shall this mount point be read-only? */
        bool nosuid:1;            /* Shall set MS_NOSUID on the mount itself */
        bool noexec:1;            /* Shall set MS_NOEXEC on the mount itself */
        bool exec:1;              /* Shall clear MS_NOEXEC on the mount itself */
        bool applied:1;           /* Already applied */
        char *path_malloc;        /* Use this instead of 'path_const' if we had to allocate memory */
        const char *unprefixed_path_const; /* If the path was amended with a prefix, these will save the original */
        char *unprefixed_path_malloc;
        const char *source_const; /* The source path, for bind mounts or images */
        char *source_malloc;
        const char *options_const;/* Mount options for tmpfs */
        char *options_malloc;
        unsigned long flags;      /* Mount flags used by EMPTY_DIR and TMPFS. Do not include MS_RDONLY here, but please use read_only. */
        unsigned n_followed;
        LIST_HEAD(MountOptions, image_options);
} MountEntry;

/* If MountAPIVFS= is used, let's mount /sys, /proc, /dev and /run into the it, but only as a fallback if the user hasn't mounted
 * something there already. These mounts are hence overridden by any other explicitly configured mounts. */
static const MountEntry apivfs_table[] = {
        { "/proc",               PROCFS,       false },
        { "/dev",                BIND_DEV,     false },
        { "/sys",                BIND_SYSFS,   false },
        { "/run",                RUN,          false, .options_const = "mode=0755" TMPFS_LIMITS_RUN, .flags = MS_NOSUID|MS_NODEV|MS_STRICTATIME },
};

/* ProtectKernelTunables= option and the related filesystem APIs */
static const MountEntry protect_kernel_tunables_proc_table[] = {
        { "/proc/acpi",          READONLY,           true  },
        { "/proc/apm",           READONLY,           true  }, /* Obsolete API, there's no point in permitting access to this, ever */
        { "/proc/asound",        READONLY,           true  },
        { "/proc/bus",           READONLY,           true  },
        { "/proc/fs",            READONLY,           true  },
        { "/proc/irq",           READONLY,           true  },
        { "/proc/kallsyms",      INACCESSIBLE,       true  },
        { "/proc/kcore",         INACCESSIBLE,       true  },
        { "/proc/latency_stats", READONLY,           true  },
        { "/proc/mtrr",          READONLY,           true  },
        { "/proc/scsi",          READONLY,           true  },
        { "/proc/sys",           READONLY,           true  },
        { "/proc/sysrq-trigger", READONLY,           true  },
        { "/proc/timer_stats",   READONLY,           true  },
};

static const MountEntry protect_kernel_tunables_sys_table[] = {
        { "/sys",                READONLY,           false },
        { "/sys/fs/bpf",         READONLY,           true  },
        { "/sys/fs/cgroup",      READWRITE_IMPLICIT, false }, /* READONLY is set by ProtectControlGroups= option */
        { "/sys/fs/selinux",     READWRITE_IMPLICIT, true  },
        { "/sys/kernel/debug",   READONLY,           true  },
        { "/sys/kernel/tracing", READONLY,           true  },
};

/* ProtectKernelModules= option */
static const MountEntry protect_kernel_modules_table[] = {
        { "/usr/lib/modules",    INACCESSIBLE, true  },
};

/* ProtectKernelLogs= option */
static const MountEntry protect_kernel_logs_proc_table[] = {
        { "/proc/kmsg",          INACCESSIBLE, true },
};

static const MountEntry protect_kernel_logs_dev_table[] = {
        { "/dev/kmsg",           INACCESSIBLE, true },
};

/*
 * ProtectHome=read-only table, protect $HOME and $XDG_RUNTIME_DIR and rest of
 * system should be protected by ProtectSystem=
 */
static const MountEntry protect_home_read_only_table[] = {
        { "/home",               READONLY,     true  },
        { "/run/user",           READONLY,     true  },
        { "/root",               READONLY,     true  },
};

/* ProtectHome=tmpfs table */
static const MountEntry protect_home_tmpfs_table[] = {
        { "/home",               TMPFS,        true, .read_only = true, .options_const = "mode=0755" TMPFS_LIMITS_EMPTY_OR_ALMOST, .flags = MS_NODEV|MS_STRICTATIME },
        { "/run/user",           TMPFS,        true, .read_only = true, .options_const = "mode=0755" TMPFS_LIMITS_EMPTY_OR_ALMOST, .flags = MS_NODEV|MS_STRICTATIME },
        { "/root",               TMPFS,        true, .read_only = true, .options_const = "mode=0700" TMPFS_LIMITS_EMPTY_OR_ALMOST, .flags = MS_NODEV|MS_STRICTATIME },
};

/* ProtectHome=yes table */
static const MountEntry protect_home_yes_table[] = {
        { "/home",               INACCESSIBLE, true  },
        { "/run/user",           INACCESSIBLE, true  },
        { "/root",               INACCESSIBLE, true  },
};

/* ProtectSystem=yes table */
static const MountEntry protect_system_yes_table[] = {
        { "/usr",                READONLY,     false },
        { "/boot",               READONLY,     true  },
        { "/efi",                READONLY,     true  },
};

/* ProtectSystem=full includes ProtectSystem=yes */
static const MountEntry protect_system_full_table[] = {
        { "/usr",                READONLY,     false },
        { "/boot",               READONLY,     true  },
        { "/efi",                READONLY,     true  },
        { "/etc",                READONLY,     false },
};

/*
 * ProtectSystem=strict table. In this strict mode, we mount everything
 * read-only, except for /proc, /dev, /sys which are the kernel API VFS,
 * which are left writable, but PrivateDevices= + ProtectKernelTunables=
 * protect those, and these options should be fully orthogonal.
 * (And of course /home and friends are also left writable, as ProtectHome=
 * shall manage those, orthogonally).
 */
static const MountEntry protect_system_strict_table[] = {
        { "/",                   READONLY,           false },
        { "/proc",               READWRITE_IMPLICIT, false },      /* ProtectKernelTunables= */
        { "/sys",                READWRITE_IMPLICIT, false },      /* ProtectKernelTunables= */
        { "/dev",                READWRITE_IMPLICIT, false },      /* PrivateDevices= */
        { "/home",               READWRITE_IMPLICIT, true  },      /* ProtectHome= */
        { "/run/user",           READWRITE_IMPLICIT, true  },      /* ProtectHome= */
        { "/root",               READWRITE_IMPLICIT, true  },      /* ProtectHome= */
};

static const char * const mount_mode_table[_MOUNT_MODE_MAX] = {
        [INACCESSIBLE]          = "inaccessible",
        [OVERLAY_MOUNT]         = "overlay",
        [MOUNT_IMAGES]          = "mount-images",
        [BIND_MOUNT]            = "bind",
        [BIND_MOUNT_RECURSIVE]  = "rbind",
        [PRIVATE_TMP]           = "private-tmp",
        [PRIVATE_TMP_READONLY]  = "private-tmp-read-only",
        [PRIVATE_DEV]           = "private-dev",
        [BIND_DEV]              = "bind-dev",
        [EMPTY_DIR]             = "empty",
        [PRIVATE_SYSFS]         = "private-sysfs",
        [BIND_SYSFS]            = "bind-sysfs",
        [PROCFS]                = "procfs",
        [READONLY]              = "read-only",
        [READWRITE]             = "read-write",
        [NOEXEC]                = "noexec",
        [EXEC]                  = "exec",
        [TMPFS]                 = "tmpfs",
        [RUN]                   = "run",
        [EXTENSION_DIRECTORIES] = "extension-directories",
        [EXTENSION_IMAGES]      = "extension-images",
        [MQUEUEFS]              = "mqueuefs",
        [READWRITE_IMPLICIT]    = "read-write-implicit",
};

/* Helper struct for naming simplicity and reusability */
static const struct {
        const char *level_env;
        const char *level_env_print;
} image_class_info[_IMAGE_CLASS_MAX] = {
        [IMAGE_SYSEXT] = {
                .level_env = "SYSEXT_LEVEL",
                .level_env_print = " SYSEXT_LEVEL=",
        },
        [IMAGE_CONFEXT] = {
                .level_env = "CONFEXT_LEVEL",
                .level_env_print = " CONFEXT_LEVEL=",
        }
};

DEFINE_PRIVATE_STRING_TABLE_LOOKUP_TO_STRING(mount_mode, MountMode);

static const char *mount_entry_path(const MountEntry *p) {
        assert(p);

        /* Returns the path of this bind mount. If the malloc()-allocated ->path_buffer field is set we return that,
         * otherwise the stack/static ->path field is returned. */

        return p->path_malloc ?: p->path_const;
}

static const char *mount_entry_unprefixed_path(const MountEntry *p) {
        assert(p);

        /* Returns the unprefixed path (ie: before prefix_where_needed() ran), if any */

        return p->unprefixed_path_malloc ?: p->unprefixed_path_const ?: mount_entry_path(p);
}

static void mount_entry_consume_prefix(MountEntry *p, char *new_path) {
        assert(p);
        assert(p->path_malloc || p->path_const);
        assert(new_path);

        /* Saves current path in unprefixed_ variable, and takes over new_path */

        free_and_replace(p->unprefixed_path_malloc, p->path_malloc);
        /* If we didn't have a path on the heap, then it's a static one */
        if (!p->unprefixed_path_malloc)
                p->unprefixed_path_const = p->path_const;
        p->path_malloc = new_path;
        p->has_prefix = true;
}

static bool mount_entry_read_only(const MountEntry *p) {
        assert(p);

        return p->read_only || IN_SET(p->mode, READONLY, INACCESSIBLE, PRIVATE_TMP_READONLY);
}

static bool mount_entry_noexec(const MountEntry *p) {
        assert(p);

        return p->noexec || IN_SET(p->mode, NOEXEC, INACCESSIBLE, PRIVATE_SYSFS, BIND_SYSFS, PROCFS);
}

static bool mount_entry_exec(const MountEntry *p) {
        assert(p);

        return p->exec || p->mode == EXEC;
}

static const char *mount_entry_source(const MountEntry *p) {
        assert(p);

        return p->source_malloc ?: p->source_const;
}

static const char *mount_entry_options(const MountEntry *p) {
        assert(p);

        return p->options_malloc ?: p->options_const;
}

static void mount_entry_done(MountEntry *p) {
        assert(p);

        p->path_malloc = mfree(p->path_malloc);
        p->unprefixed_path_malloc = mfree(p->unprefixed_path_malloc);
        p->source_malloc = mfree(p->source_malloc);
        p->options_malloc = mfree(p->options_malloc);
        p->image_options = mount_options_free_all(p->image_options);
}

static int append_access_mounts(MountEntry **p, char **strv, MountMode mode, bool forcibly_require_prefix) {
        assert(p);

        /* Adds a list of user-supplied READWRITE/READWRITE_IMPLICIT/READONLY/INACCESSIBLE entries */

        STRV_FOREACH(i, strv) {
                bool ignore = false, needs_prefix = false;
                const char *e = *i;

                /* Look for any prefixes */
                if (startswith(e, "-")) {
                        e++;
                        ignore = true;
                }
                if (startswith(e, "+")) {
                        e++;
                        needs_prefix = true;
                }

                if (!path_is_absolute(e))
                        return log_debug_errno(SYNTHETIC_ERRNO(EINVAL),
                                               "Path is not absolute: %s", e);

                *((*p)++) = (MountEntry) {
                        .path_const = e,
                        .mode = mode,
                        .ignore = ignore,
                        .has_prefix = !needs_prefix && !forcibly_require_prefix,
                };
        }

        return 0;
}

static int append_empty_dir_mounts(MountEntry **p, char **strv) {
        assert(p);

        /* Adds tmpfs mounts to provide readable but empty directories. This is primarily used to implement the
         * "/private/" boundary directories for DynamicUser=1. */

        STRV_FOREACH(i, strv) {

                *((*p)++) = (MountEntry) {
                        .path_const = *i,
                        .mode = EMPTY_DIR,
                        .ignore = false,
                        .read_only = true,
                        .options_const = "mode=0755" TMPFS_LIMITS_EMPTY_OR_ALMOST,
                        .flags = MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_STRICTATIME,
                };
        }

        return 0;
}

static int append_bind_mounts(MountEntry **p, const BindMount *binds, size_t n) {
        assert(p);

        for (size_t i = 0; i < n; i++) {
                const BindMount *b = binds + i;

                *((*p)++) = (MountEntry) {
                        .path_const = b->destination,
                        .mode = b->recursive ? BIND_MOUNT_RECURSIVE : BIND_MOUNT,
                        .read_only = b->read_only,
                        .nosuid = b->nosuid,
                        .source_const = b->source,
                        .ignore = b->ignore_enoent,
                };
        }

        return 0;
}

static int append_mount_images(MountEntry **p, const MountImage *mount_images, size_t n) {
        assert(p);

        for (size_t i = 0; i < n; i++) {
                const MountImage *m = mount_images + i;

                *((*p)++) = (MountEntry) {
                        .path_const = m->destination,
                        .mode = MOUNT_IMAGES,
                        .source_const = m->source,
                        .image_options = m->mount_options,
                        .ignore = m->ignore_enoent,
                };
        }

        return 0;
}

static int append_extensions(
                MountEntry **p,
                const char *root,
                const char *extension_dir,
                char **hierarchies,
                const MountImage *mount_images,
                size_t n,
                char **extension_directories) {

        _cleanup_strv_free_ char **overlays = NULL;
        int r;

        if (n == 0 && strv_isempty(extension_directories))
                return 0;

        assert(p);
        assert(extension_dir);

        /* Prepare a list of overlays, that will have as each element a string suitable for being
         * passed as a lowerdir= parameter, so start with the hierarchy on the root.
         * The overlays vector will have the same number of elements and will correspond to the
         * hierarchies vector, so they can be iterated upon together. */
        STRV_FOREACH(hierarchy, hierarchies) {
                _cleanup_free_ char *prefixed_hierarchy = NULL;

                prefixed_hierarchy = path_join(root, *hierarchy);
                if (!prefixed_hierarchy)
                        return -ENOMEM;

                r = strv_consume(&overlays, TAKE_PTR(prefixed_hierarchy));
                if (r < 0)
                        return r;
        }

        /* First, prepare a mount for each image, but these won't be visible to the unit, instead
         * they will be mounted in our propagate directory, and used as a source for the overlay. */
        for (size_t i = 0; i < n; i++) {
                _cleanup_free_ char *mount_point = NULL;
                const MountImage *m = mount_images + i;

                r = asprintf(&mount_point, "%s/%zu", extension_dir, i);
                if (r < 0)
                        return -ENOMEM;

                for (size_t j = 0; hierarchies && hierarchies[j]; ++j) {
                        _cleanup_free_ char *prefixed_hierarchy = NULL, *escaped = NULL, *lowerdir = NULL;

                        prefixed_hierarchy = path_join(mount_point, hierarchies[j]);
                        if (!prefixed_hierarchy)
                                return -ENOMEM;

                        escaped = shell_escape(prefixed_hierarchy, ",:");
                        if (!escaped)
                                return -ENOMEM;

                        /* Note that lowerdir= parameters are in 'reverse' order, so the
                         * top-most directory in the overlay comes first in the list. */
                        lowerdir = strjoin(escaped, ":", overlays[j]);
                        if (!lowerdir)
                                return -ENOMEM;

                        free_and_replace(overlays[j], lowerdir);
                }

                *((*p)++) = (MountEntry) {
                        .path_malloc = TAKE_PTR(mount_point),
                        .image_options = m->mount_options,
                        .ignore = m->ignore_enoent,
                        .source_const = m->source,
                        .mode = EXTENSION_IMAGES,
                        .has_prefix = true,
                };
        }

        /* Secondly, extend the lowerdir= parameters with each ExtensionDirectory.
         * Bind mount them in the same location as the ExtensionImages, so that we
         * can check that they are valid trees (extension-release.d). */
        STRV_FOREACH(extension_directory, extension_directories) {
                _cleanup_free_ char *mount_point = NULL, *source = NULL;
                const char *e = *extension_directory;
                bool ignore_enoent = false;

                /* Pick up the counter where the ExtensionImages left it. */
                r = asprintf(&mount_point, "%s/%zu", extension_dir, n++);
                if (r < 0)
                        return -ENOMEM;

                /* Look for any prefixes */
                if (startswith(e, "-")) {
                        e++;
                        ignore_enoent = true;
                }
                /* Ignore this for now */
                if (startswith(e, "+"))
                        e++;

                source = strdup(e);
                if (!source)
                        return -ENOMEM;

                for (size_t j = 0; hierarchies && hierarchies[j]; ++j) {
                        _cleanup_free_ char *prefixed_hierarchy = NULL, *escaped = NULL, *lowerdir = NULL;

                        prefixed_hierarchy = path_join(mount_point, hierarchies[j]);
                        if (!prefixed_hierarchy)
                                return -ENOMEM;

                        escaped = shell_escape(prefixed_hierarchy, ",:");
                        if (!escaped)
                                return -ENOMEM;

                        /* Note that lowerdir= parameters are in 'reverse' order, so the
                         * top-most directory in the overlay comes first in the list. */
                        lowerdir = strjoin(escaped, ":", overlays[j]);
                        if (!lowerdir)
                                return -ENOMEM;

                        free_and_replace(overlays[j], lowerdir);
                }

                *((*p)++) = (MountEntry) {
                        .path_malloc = TAKE_PTR(mount_point),
                        .source_malloc = TAKE_PTR(source),
                        .mode = EXTENSION_DIRECTORIES,
                        .ignore = ignore_enoent,
                        .has_prefix = true,
                        .read_only = true,
                };
        }

        /* Then, for each hierarchy, prepare an overlay with the list of lowerdir= strings
         * set up earlier. */
        for (size_t i = 0; hierarchies && hierarchies[i]; ++i) {
                _cleanup_free_ char *prefixed_hierarchy = NULL;

                prefixed_hierarchy = path_join(root, hierarchies[i]);
                if (!prefixed_hierarchy)
                        return -ENOMEM;

                *((*p)++) = (MountEntry) {
                        .path_malloc = TAKE_PTR(prefixed_hierarchy),
                        .options_malloc = TAKE_PTR(overlays[i]),
                        .mode = OVERLAY_MOUNT,
                        .has_prefix = true,
                        .ignore = true, /* If the source image doesn't set the ignore bit it will fail earlier. */
                };
        }

        return 0;
}

static int append_tmpfs_mounts(MountEntry **p, const TemporaryFileSystem *tmpfs, size_t n) {
        assert(p);

        for (size_t i = 0; i < n; i++) {
                const TemporaryFileSystem *t = tmpfs + i;
                _cleanup_free_ char *o = NULL, *str = NULL;
                unsigned long flags;
                bool ro = false;
                int r;

                if (!path_is_absolute(t->path))
                        return log_debug_errno(SYNTHETIC_ERRNO(EINVAL),
                                               "Path is not absolute: %s",
                                               t->path);

                str = strjoin("mode=0755" NESTED_TMPFS_LIMITS ",", t->options);
                if (!str)
                        return -ENOMEM;

                r = mount_option_mangle(str, MS_NODEV|MS_STRICTATIME, &flags, &o);
                if (r < 0)
                        return log_debug_errno(r, "Failed to parse mount option '%s': %m", str);

                ro = flags & MS_RDONLY;
                if (ro)
                        flags ^= MS_RDONLY;

                *((*p)++) = (MountEntry) {
                        .path_const = t->path,
                        .mode = TMPFS,
                        .read_only = ro,
                        .options_malloc = TAKE_PTR(o),
                        .flags = flags,
                };
        }

        return 0;
}

static int append_static_mounts(MountEntry **p, const MountEntry *mounts, size_t n, bool ignore_protect) {
        assert(p);
        assert(mounts);

        /* Adds a list of static pre-defined entries */

        for (size_t i = 0; i < n; i++)
                *((*p)++) = (MountEntry) {
                        .path_const = mount_entry_path(mounts+i),
                        .mode = mounts[i].mode,
                        .ignore = mounts[i].ignore || ignore_protect,
                };

        return 0;
}

static int append_protect_home(MountEntry **p, ProtectHome protect_home, bool ignore_protect) {
        assert(p);

        switch (protect_home) {

        case PROTECT_HOME_NO:
                return 0;

        case PROTECT_HOME_READ_ONLY:
                return append_static_mounts(p, protect_home_read_only_table, ELEMENTSOF(protect_home_read_only_table), ignore_protect);

        case PROTECT_HOME_TMPFS:
                return append_static_mounts(p, protect_home_tmpfs_table, ELEMENTSOF(protect_home_tmpfs_table), ignore_protect);

        case PROTECT_HOME_YES:
                return append_static_mounts(p, protect_home_yes_table, ELEMENTSOF(protect_home_yes_table), ignore_protect);

        default:
                assert_not_reached();
        }
}

static int append_protect_system(MountEntry **p, ProtectSystem protect_system, bool ignore_protect) {
        assert(p);

        switch (protect_system) {

        case PROTECT_SYSTEM_NO:
                return 0;

        case PROTECT_SYSTEM_STRICT:
                return append_static_mounts(p, protect_system_strict_table, ELEMENTSOF(protect_system_strict_table), ignore_protect);

        case PROTECT_SYSTEM_YES:
                return append_static_mounts(p, protect_system_yes_table, ELEMENTSOF(protect_system_yes_table), ignore_protect);

        case PROTECT_SYSTEM_FULL:
                return append_static_mounts(p, protect_system_full_table, ELEMENTSOF(protect_system_full_table), ignore_protect);

        default:
                assert_not_reached();
        }
}

static int mount_path_compare(const MountEntry *a, const MountEntry *b) {
        int d;

        /* ExtensionImages/Directories will be used by other mounts as a base, so sort them first
         * regardless of the prefix - they are set up in the propagate directory anyway */
        d = -CMP(a->mode == EXTENSION_IMAGES, b->mode == EXTENSION_IMAGES);
        if (d != 0)
                return d;
        d = -CMP(a->mode == EXTENSION_DIRECTORIES, b->mode == EXTENSION_DIRECTORIES);
        if (d != 0)
                return d;

        /* If the paths are not equal, then order prefixes first */
        d = path_compare(mount_entry_path(a), mount_entry_path(b));
        if (d != 0)
                return d;

        /* If the paths are equal, check the mode */
        return CMP((int) a->mode, (int) b->mode);
}

static int prefix_where_needed(MountEntry *m, size_t n, const char *root_directory) {
        /* Prefixes all paths in the bind mount table with the root directory if the entry needs that. */

        assert(m || n == 0);

        for (size_t i = 0; i < n; i++) {
                char *s;

                if (m[i].has_prefix)
                        continue;

                s = path_join(root_directory, mount_entry_path(m+i));
                if (!s)
                        return -ENOMEM;

                mount_entry_consume_prefix(&m[i], s);
        }

        return 0;
}

static void drop_duplicates(MountEntry *m, size_t *n) {
        MountEntry *f, *t, *previous;

        assert(m);
        assert(n);

        /* Drops duplicate entries. Expects that the array is properly ordered already. */

        for (f = m, t = m, previous = NULL; f < m + *n; f++) {

                /* The first one wins (which is the one with the more restrictive mode), see mount_path_compare()
                 * above. Note that we only drop duplicates that haven't been mounted yet. */
                if (previous &&
                    path_equal(mount_entry_path(f), mount_entry_path(previous)) &&
                    !f->applied && !previous->applied) {
                        log_debug("%s (%s) is duplicate.", mount_entry_path(f), mount_mode_to_string(f->mode));
                        /* Propagate the flags to the remaining entry */
                        previous->read_only = previous->read_only || mount_entry_read_only(f);
                        previous->noexec = previous->noexec || mount_entry_noexec(f);
                        previous->exec = previous->exec || mount_entry_exec(f);
                        mount_entry_done(f);
                        continue;
                }

                *t = *f;
                previous = t;
                t++;
        }

        *n = t - m;
}

static void drop_inaccessible(MountEntry *m, size_t *n) {
        MountEntry *f, *t;
        const char *clear = NULL;

        assert(m);
        assert(n);

        /* Drops all entries obstructed by another entry further up the tree. Expects that the array is properly
         * ordered already. */

        for (f = m, t = m; f < m + *n; f++) {

                /* If we found a path set for INACCESSIBLE earlier, and this entry has it as prefix we should drop
                 * it, as inaccessible paths really should drop the entire subtree. */
                if (clear && path_startswith(mount_entry_path(f), clear)) {
                        log_debug("%s is masked by %s.", mount_entry_path(f), clear);
                        mount_entry_done(f);
                        continue;
                }

                clear = f->mode == INACCESSIBLE ? mount_entry_path(f) : NULL;

                *t = *f;
                t++;
        }

        *n = t - m;
}

static void drop_nop(MountEntry *m, size_t *n) {
        MountEntry *f, *t;

        assert(m);
        assert(n);

        /* Drops all entries which have an immediate parent that has the same type, as they are redundant. Assumes the
         * list is ordered by prefixes. */

        for (f = m, t = m; f < m + *n; f++) {

                /* Only suppress such subtrees for READONLY, READWRITE and READWRITE_IMPLICIT entries */
                if (IN_SET(f->mode, READONLY, READWRITE, READWRITE_IMPLICIT)) {
                        MountEntry *found = NULL;

                        /* Now let's find the first parent of the entry we are looking at. */
                        for (MountEntry *p = PTR_SUB1(t, m); p; p = PTR_SUB1(p, m))
                                if (path_startswith(mount_entry_path(f), mount_entry_path(p))) {
                                        found = p;
                                        break;
                                }

                        /* We found it, let's see if it's the same mode, if so, we can drop this entry */
                        if (found && found->mode == f->mode) {
                                log_debug("%s (%s) is made redundant by %s (%s)",
                                          mount_entry_path(f), mount_mode_to_string(f->mode),
                                          mount_entry_path(found), mount_mode_to_string(found->mode));
                                mount_entry_done(f);
                                continue;
                        }
                }

                *t = *f;
                t++;
        }

        *n = t - m;
}

static void drop_outside_root(const char *root_directory, MountEntry *m, size_t *n) {
        MountEntry *f, *t;

        assert(m);
        assert(n);

        /* Nothing to do */
        if (!root_directory)
                return;

        /* Drops all mounts that are outside of the root directory. */

        for (f = m, t = m; f < m + *n; f++) {

                /* ExtensionImages/Directories bases are opened in /run/systemd/unit-extensions on the host */
                if (!IN_SET(f->mode, EXTENSION_IMAGES, EXTENSION_DIRECTORIES) && !path_startswith(mount_entry_path(f), root_directory)) {
                        log_debug("%s is outside of root directory.", mount_entry_path(f));
                        mount_entry_done(f);
                        continue;
                }

                *t = *f;
                t++;
        }

        *n = t - m;
}

static int clone_device_node(
                const char *d,
                const char *temporary_mount,
                bool *make_devnode) {

        _cleanup_free_ char *sl = NULL;
        const char *dn, *bn, *t;
        struct stat st;
        int r;

        if (stat(d, &st) < 0) {
                if (errno == ENOENT) {
                        log_debug_errno(errno, "Device node '%s' to clone does not exist, ignoring.", d);
                        return -ENXIO;
                }

                return log_debug_errno(errno, "Failed to stat() device node '%s' to clone, ignoring: %m", d);
        }

        if (!S_ISBLK(st.st_mode) &&
            !S_ISCHR(st.st_mode))
                return log_debug_errno(SYNTHETIC_ERRNO(EINVAL),
                                       "Device node '%s' to clone is not a device node, ignoring.",
                                       d);

        dn = strjoina(temporary_mount, d);

        /* First, try to create device node properly */
        if (*make_devnode) {
                mac_selinux_create_file_prepare(d, st.st_mode);
                r = mknod(dn, st.st_mode, st.st_rdev);
                mac_selinux_create_file_clear();
                if (r >= 0)
                        goto add_symlink;
                if (errno != EPERM)
                        return log_debug_errno(errno, "mknod failed for %s: %m", d);

                /* This didn't work, let's not try this again for the next iterations. */
                *make_devnode = false;
        }

        /* We're about to fall back to bind-mounting the device node. So create a dummy bind-mount target.
         * Do not prepare device-node SELinux label (see issue 13762) */
        r = mknod(dn, S_IFREG, 0);
        if (r < 0 && errno != EEXIST)
                return log_debug_errno(errno, "mknod() fallback failed for '%s': %m", d);

        /* Fallback to bind-mounting: The assumption here is that all used device nodes carry standard
         * properties. Specifically, the devices nodes we bind-mount should either be owned by root:root or
         * root:tty (e.g. /dev/tty, /dev/ptmx) and should not carry ACLs. */
        r = mount_nofollow_verbose(LOG_DEBUG, d, dn, NULL, MS_BIND, NULL);
        if (r < 0)
                return r;

add_symlink:
        bn = path_startswith(d, "/dev/");
        if (!bn)
                return 0;

        /* Create symlinks like /dev/char/1:9 → ../urandom */
        if (asprintf(&sl, "%s/dev/%s/" DEVNUM_FORMAT_STR,
                     temporary_mount,
                     S_ISCHR(st.st_mode) ? "char" : "block",
                     DEVNUM_FORMAT_VAL(st.st_rdev)) < 0)
                return log_oom();

        (void) mkdir_parents(sl, 0755);

        t = strjoina("../", bn);
        if (symlink(t, sl) < 0)
                log_debug_errno(errno, "Failed to symlink '%s' to '%s', ignoring: %m", t, sl);

        return 0;
}

static char *settle_runtime_dir(RuntimeScope scope) {
        char *runtime_dir;

        if (scope != RUNTIME_SCOPE_USER)
                return strdup("/run/");

        if (asprintf(&runtime_dir, "/run/user/" UID_FMT, geteuid()) < 0)
                return NULL;

        return runtime_dir;
}

static int mount_private_dev(MountEntry *m, RuntimeScope scope) {
        static const char devnodes[] =
                "/dev/null\0"
                "/dev/zero\0"
                "/dev/full\0"
                "/dev/random\0"
                "/dev/urandom\0"
                "/dev/tty\0";

        _cleanup_free_ char *runtime_dir = NULL, *temporary_mount = NULL;
        const char *dev = NULL, *devpts = NULL, *devshm = NULL, *devhugepages = NULL, *devmqueue = NULL, *devlog = NULL, *devptmx = NULL;
        bool can_mknod = true;
        int r;

        assert(m);

        runtime_dir = settle_runtime_dir(scope);
        if (!runtime_dir)
                return log_oom_debug();

        temporary_mount = path_join(runtime_dir, "systemd/namespace-dev-XXXXXX");
        if (!temporary_mount)
                return log_oom_debug();

        if (!mkdtemp(temporary_mount))
                return log_debug_errno(errno, "Failed to create temporary directory '%s': %m", temporary_mount);

        dev = strjoina(temporary_mount, "/dev");
        (void) mkdir(dev, 0755);
        r = mount_nofollow_verbose(LOG_DEBUG, "tmpfs", dev, "tmpfs", DEV_MOUNT_OPTIONS, "mode=0755" TMPFS_LIMITS_PRIVATE_DEV);
        if (r < 0)
                goto fail;

        r = label_fix_full(AT_FDCWD, dev, "/dev", 0);
        if (r < 0) {
                log_debug_errno(r, "Failed to fix label of '%s' as /dev: %m", dev);
                goto fail;
        }

        devpts = strjoina(temporary_mount, "/dev/pts");
        (void) mkdir(devpts, 0755);
        r = mount_nofollow_verbose(LOG_DEBUG, "/dev/pts", devpts, NULL, MS_BIND, NULL);
        if (r < 0)
                goto fail;

        /* /dev/ptmx can either be a device node or a symlink to /dev/pts/ptmx.
         * When /dev/ptmx a device node, /dev/pts/ptmx has 000 permissions making it inaccessible.
         * Thus, in that case make a clone.
         * In nspawn and other containers it will be a symlink, in that case make it a symlink. */
        r = is_symlink("/dev/ptmx");
        if (r < 0) {
                log_debug_errno(r, "Failed to detect whether /dev/ptmx is a symlink or not: %m");
                goto fail;
        } else if (r > 0) {
                devptmx = strjoina(temporary_mount, "/dev/ptmx");
                if (symlink("pts/ptmx", devptmx) < 0) {
                        r = log_debug_errno(errno, "Failed to create a symlink '%s' to pts/ptmx: %m", devptmx);
                        goto fail;
                }
        } else {
                r = clone_device_node("/dev/ptmx", temporary_mount, &can_mknod);
                if (r < 0)
                        goto fail;
        }

        devshm = strjoina(temporary_mount, "/dev/shm");
        (void) mkdir(devshm, 0755);
        r = mount_nofollow_verbose(LOG_DEBUG, "/dev/shm", devshm, NULL, MS_BIND, NULL);
        if (r < 0)
                goto fail;

        devmqueue = strjoina(temporary_mount, "/dev/mqueue");
        (void) mkdir(devmqueue, 0755);
        (void) mount_nofollow_verbose(LOG_DEBUG, "/dev/mqueue", devmqueue, NULL, MS_BIND, NULL);

        devhugepages = strjoina(temporary_mount, "/dev/hugepages");
        (void) mkdir(devhugepages, 0755);
        (void) mount_nofollow_verbose(LOG_DEBUG, "/dev/hugepages", devhugepages, NULL, MS_BIND, NULL);

        devlog = strjoina(temporary_mount, "/dev/log");
        if (symlink("/run/systemd/journal/dev-log", devlog) < 0)
                log_debug_errno(errno, "Failed to create a symlink '%s' to /run/systemd/journal/dev-log, ignoring: %m", devlog);

        NULSTR_FOREACH(d, devnodes) {
                r = clone_device_node(d, temporary_mount, &can_mknod);
                /* ENXIO means the *source* is not a device file, skip creation in that case */
                if (r < 0 && r != -ENXIO)
                        goto fail;
        }

        r = dev_setup(temporary_mount, UID_INVALID, GID_INVALID);
        if (r < 0)
                log_debug_errno(r, "Failed to set up basic device tree at '%s', ignoring: %m", temporary_mount);

        /* Make the bind mount read-only. */
        r = mount_nofollow_verbose(LOG_DEBUG, NULL, dev, NULL, MS_REMOUNT|MS_BIND|MS_RDONLY, NULL);
        if (r < 0)
                return r;

        /* Create the /dev directory if missing. It is more likely to be missing when the service is started
         * with RootDirectory. This is consistent with mount units creating the mount points when missing. */
        (void) mkdir_p_label(mount_entry_path(m), 0755);

        /* Unmount everything in old /dev */
        r = umount_recursive(mount_entry_path(m), 0);
        if (r < 0)
                log_debug_errno(r, "Failed to unmount directories below '%s', ignoring: %m", mount_entry_path(m));

        r = mount_nofollow_verbose(LOG_DEBUG, dev, mount_entry_path(m), NULL, MS_MOVE, NULL);
        if (r < 0)
                goto fail;

        (void) rmdir(dev);
        (void) rmdir(temporary_mount);

        return 0;

fail:
        if (devpts)
                (void) umount_verbose(LOG_DEBUG, devpts, UMOUNT_NOFOLLOW);

        if (devshm)
                (void) umount_verbose(LOG_DEBUG, devshm, UMOUNT_NOFOLLOW);

        if (devhugepages)
                (void) umount_verbose(LOG_DEBUG, devhugepages, UMOUNT_NOFOLLOW);

        if (devmqueue)
                (void) umount_verbose(LOG_DEBUG, devmqueue, UMOUNT_NOFOLLOW);

        (void) umount_verbose(LOG_DEBUG, dev, UMOUNT_NOFOLLOW);
        (void) rmdir(dev);
        (void) rmdir(temporary_mount);

        return r;
}

static int mount_bind_dev(const MountEntry *m) {
        int r;

        assert(m);

        /* Implements the little brother of mount_private_dev(): simply bind mounts the host's /dev into the
         * service's /dev. This is only used when RootDirectory= is set. */

        (void) mkdir_p_label(mount_entry_path(m), 0755);

        r = path_is_mount_point(mount_entry_path(m), NULL, 0);
        if (r < 0)
                return log_debug_errno(r, "Unable to determine whether /dev is already mounted: %m");
        if (r > 0) /* make this a NOP if /dev is already a mount point */
                return 0;

        return mount_nofollow_verbose(LOG_DEBUG, "/dev", mount_entry_path(m), NULL, MS_BIND|MS_REC, NULL);
}

static int mount_bind_sysfs(const MountEntry *m) {
        int r;

        assert(m);

        (void) mkdir_p_label(mount_entry_path(m), 0755);

        r = path_is_mount_point(mount_entry_path(m), NULL, 0);
        if (r < 0)
                return log_debug_errno(r, "Unable to determine whether /sys is already mounted: %m");
        if (r > 0) /* make this a NOP if /sys is already a mount point */
                return 0;

        /* Bind mount the host's version so that we get all child mounts of it, too. */
        return mount_nofollow_verbose(LOG_DEBUG, "/sys", mount_entry_path(m), NULL, MS_BIND|MS_REC, NULL);
}

static int mount_private_sysfs(const MountEntry *m) {
        const char *entry_path = mount_entry_path(ASSERT_PTR(m));
        int r, n;

        (void) mkdir_p_label(entry_path, 0755);

        n = umount_recursive(entry_path, 0);

        r = mount_nofollow_verbose(LOG_DEBUG, "sysfs", entry_path, "sysfs", MS_NOSUID|MS_NOEXEC|MS_NODEV, NULL);
        if (ERRNO_IS_NEG_PRIVILEGE(r)) {
                /* When we do not have enough privileges to mount sysfs, fall back to use existing /sys. */

                if (n > 0)
                        /* /sys or some of sub-mounts are umounted in the above. Refuse incomplete tree.
                         * Propagate the original error code returned by mount() in the above. */
                        return r;

                return mount_bind_sysfs(m);

        } else if (r < 0)
                return r;

        /* We mounted a new instance now. Let's bind mount the children over now. */
        (void) bind_mount_submounts("/sys", entry_path);
        return 0;
}

static int mount_procfs(const MountEntry *m, const NamespaceInfo *ns_info) {
        _cleanup_free_ char *opts = NULL;
        const char *entry_path;
        int r, n;

        assert(m);
        assert(ns_info);

        if (ns_info->protect_proc != PROTECT_PROC_DEFAULT ||
            ns_info->proc_subset != PROC_SUBSET_ALL) {

                /* Starting with kernel 5.8 procfs' hidepid= logic is truly per-instance (previously it
                 * pretended to be per-instance but actually was per-namespace), hence let's make use of it
                 * if requested. To make sure this logic succeeds only on kernels where hidepid= is
                 * per-instance, we'll exclusively use the textual value for hidepid=, since support was
                 * added in the same commit: if it's supported it is thus also per-instance. */

                const char *hpv = ns_info->protect_proc == PROTECT_PROC_DEFAULT ?
                                  "off" :
                                  protect_proc_to_string(ns_info->protect_proc);

                /* hidepid= support was added in 5.8, so we can use fsconfig()/fsopen() (which were added in
                 * 5.2) to check if hidepid= is supported. This avoids a noisy dmesg log by the kernel when
                 * trying to use hidepid= on systems where it isn't supported. The same applies for subset=.
                 * fsopen()/fsconfig() was also backported on some distros which allows us to detect
                 * hidepid=/subset= support in even more scenarios. */

                if (mount_option_supported("proc", "hidepid", hpv) != 0) {
                        opts = strjoin("hidepid=", hpv);
                        if (!opts)
                                return -ENOMEM;
                }

                if (ns_info->proc_subset == PROC_SUBSET_PID &&
                    mount_option_supported("proc", "subset", "pid") != 0)
                        if (!strextend_with_separator(&opts, ",", "subset=pid"))
                                return -ENOMEM;
        }

        entry_path = mount_entry_path(m);
        (void) mkdir_p_label(entry_path, 0755);

        /* Mount a new instance, so that we get the one that matches our user namespace, if we are running in
         * one. i.e we don't reuse existing mounts here under any condition, we want a new instance owned by
         * our user namespace and with our hidepid= settings applied. Hence, let's get rid of everything
         * mounted on /proc/ first. */

        n = umount_recursive(entry_path, 0);

        r = mount_nofollow_verbose(LOG_DEBUG, "proc", entry_path, "proc", MS_NOSUID|MS_NOEXEC|MS_NODEV, opts);
        if (r == -EINVAL && opts)
                /* If this failed with EINVAL then this likely means the textual hidepid= stuff is
                 * not supported by the kernel, and thus the per-instance hidepid= neither, which
                 * means we really don't want to use it, since it would affect our host's /proc
                 * mount. Hence let's gracefully fallback to a classic, unrestricted version. */
                r = mount_nofollow_verbose(LOG_DEBUG, "proc", entry_path, "proc", MS_NOSUID|MS_NOEXEC|MS_NODEV, NULL);
        if (ERRNO_IS_NEG_PRIVILEGE(r)) {
                /* When we do not have enough privileges to mount /proc, fall back to use existing /proc. */

                if (n > 0)
                        /* /proc or some of sub-mounts are umounted in the above. Refuse incomplete tree.
                         * Propagate the original error code returned by mount() in the above. */
                        return r;

                r = path_is_mount_point(entry_path, NULL, 0);
                if (r < 0)
                        return log_debug_errno(r, "Unable to determine whether /proc is already mounted: %m");
                if (r > 0)
                        return 0;

                /* We lack permissions to mount a new instance of /proc, and it is not already mounted. But
                 * we can access the host's, so as a final fallback bind-mount it to the destination, as most
                 * likely we are inside a user manager in an unprivileged user namespace. */
                return mount_nofollow_verbose(LOG_DEBUG, "/proc", entry_path, NULL, MS_BIND|MS_REC, NULL);

        } else if (r < 0)
                return r;

        /* We mounted a new instance now. Let's bind mount the children over now. This matters for nspawn
         * where a bunch of files are overmounted, in particular the boot id */
        (void) bind_mount_submounts("/proc", entry_path);
        return 0;
}

static int mount_tmpfs(const MountEntry *m) {
        const char *entry_path, *inner_path;
        int r;

        assert(m);

        entry_path = mount_entry_path(m);
        inner_path = mount_entry_unprefixed_path(m);

        /* First, get rid of everything that is below if there is anything. Then, overmount with our new
         * tmpfs */

        (void) mkdir_p_label(entry_path, 0755);
        (void) umount_recursive(entry_path, 0);

        r = mount_nofollow_verbose(LOG_DEBUG, "tmpfs", entry_path, "tmpfs", m->flags, mount_entry_options(m));
        if (r < 0)
                return r;

        r = label_fix_full(AT_FDCWD, entry_path, inner_path, 0);
        if (r < 0)
                return log_debug_errno(r, "Failed to fix label of '%s' as '%s': %m", entry_path, inner_path);

        return 0;
}

static int mount_run(const MountEntry *m) {
        int r;

        assert(m);

        r = path_is_mount_point(mount_entry_path(m), NULL, 0);
        if (r < 0 && r != -ENOENT)
                return log_debug_errno(r, "Unable to determine whether /run is already mounted: %m");
        if (r > 0) /* make this a NOP if /run is already a mount point */
                return 0;

        return mount_tmpfs(m);
}

static int mount_mqueuefs(const MountEntry *m) {
        int r;
        const char *entry_path;

        assert(m);

        entry_path = mount_entry_path(m);

        (void) mkdir_p_label(entry_path, 0755);
        (void) umount_recursive(entry_path, 0);

        r = mount_nofollow_verbose(LOG_DEBUG, "mqueue", entry_path, "mqueue", m->flags, mount_entry_options(m));
        if (r < 0)
                return r;

        return 0;
}

static int mount_image(
                const MountEntry *m,
                const char *root_directory,
                const ImagePolicy *image_policy) {

        _cleanup_free_ char *host_os_release_id = NULL, *host_os_release_version_id = NULL,
                            *host_os_release_level = NULL, *extension_name = NULL;
        _cleanup_strv_free_ char **extension_release = NULL;
        ImageClass class = IMAGE_SYSEXT;
        int r;

        assert(m);

        r = path_extract_filename(mount_entry_source(m), &extension_name);
        if (r < 0)
                return log_debug_errno(r, "Failed to extract extension name from %s: %m", mount_entry_source(m));

        if (m->mode == EXTENSION_IMAGES) {
                r = load_extension_release_pairs(mount_entry_source(m), IMAGE_SYSEXT, extension_name, /* relax_extension_release_check= */ false, &extension_release);
                if (r == -ENOENT) {
                        r = load_extension_release_pairs(mount_entry_source(m), IMAGE_CONFEXT, extension_name, /* relax_extension_release_check= */ false, &extension_release);
                        if (r >= 0)
                                class = IMAGE_CONFEXT;
                }
                if (r == -ENOENT)
                        return r;

                r = parse_os_release(
                                empty_to_root(root_directory),
                                "ID", &host_os_release_id,
                                "VERSION_ID", &host_os_release_version_id,
                                image_class_info[class].level_env, &host_os_release_level,
                                NULL);
                if (r < 0)
                        return log_debug_errno(r, "Failed to acquire 'os-release' data of OS tree '%s': %m", empty_to_root(root_directory));
                if (isempty(host_os_release_id))
                        return log_debug_errno(SYNTHETIC_ERRNO(EINVAL), "'ID' field not found or empty in 'os-release' data of OS tree '%s': %m", empty_to_root(root_directory));
        }

        r = verity_dissect_and_mount(
                        /* src_fd= */ -1,
                        mount_entry_source(m),
                        mount_entry_path(m),
                        m->image_options,
                        image_policy,
                        host_os_release_id,
                        host_os_release_version_id,
                        host_os_release_level,
                        /* required_sysext_scope= */ NULL,
                        /* ret_image= */ NULL);
        if (r == -ENOENT && m->ignore)
                return 0;
        if (r == -ESTALE && host_os_release_id)
                return log_error_errno(r,
                                       "Failed to mount image %s, extension-release metadata does not match the lower layer's: ID=%s%s%s%s%s",
                                       mount_entry_source(m),
                                       host_os_release_id,
                                       host_os_release_version_id ? " VERSION_ID=" : "",
                                       strempty(host_os_release_version_id),
                                       host_os_release_level ? image_class_info[class].level_env_print : "",
                                       strempty(host_os_release_level));
        if (r < 0)
                return log_debug_errno(r, "Failed to mount image %s on %s: %m", mount_entry_source(m), mount_entry_path(m));

        return 0;
}

static int mount_overlay(const MountEntry *m) {
        const char *options;
        int r;

        assert(m);

        options = strjoina("lowerdir=", mount_entry_options(m));

        (void) mkdir_p_label(mount_entry_path(m), 0755);

        r = mount_nofollow_verbose(LOG_DEBUG, "overlay", mount_entry_path(m), "overlay", MS_RDONLY, options);
        if (r == -ENOENT && m->ignore)
                return 0;

        return r;
}

static int follow_symlink(
                const char *root_directory,
                MountEntry *m) {

        _cleanup_free_ char *target = NULL;
        int r;

        /* Let's chase symlinks, but only one step at a time. That's because depending where the symlink points we
         * might need to change the order in which we mount stuff. Hence: let's normalize piecemeal, and do one step at
         * a time by specifying CHASE_STEP. This function returns 0 if we resolved one step, and > 0 if we reached the
         * end and already have a fully normalized name. */

        r = chase(mount_entry_path(m), root_directory, CHASE_STEP|CHASE_NONEXISTENT, &target, NULL);
        if (r < 0)
                return log_debug_errno(r, "Failed to chase symlinks '%s': %m", mount_entry_path(m));
        if (r > 0) /* Reached the end, nothing more to resolve */
                return 1;

        if (m->n_followed >= CHASE_MAX) /* put a boundary on things */
                return log_debug_errno(SYNTHETIC_ERRNO(ELOOP),
                                       "Symlink loop on '%s'.",
                                       mount_entry_path(m));

        log_debug("Followed mount entry path symlink %s %s %s.",
                  mount_entry_path(m), special_glyph(SPECIAL_GLYPH_ARROW_RIGHT), target);

        mount_entry_consume_prefix(m, TAKE_PTR(target));

        m->n_followed ++;

        return 0;
}

static int apply_one_mount(
                const char *root_directory,
                MountEntry *m,
                const ImagePolicy *mount_image_policy,
                const ImagePolicy *extension_image_policy,
                const NamespaceInfo *ns_info,
                RuntimeScope scope) {

        _cleanup_free_ char *inaccessible = NULL;
        bool rbind = true, make = false;
        const char *what;
        int r;

        assert(m);
        assert(ns_info);

        log_debug("Applying namespace mount on %s", mount_entry_path(m));

        switch (m->mode) {

        case INACCESSIBLE: {
                _cleanup_free_ char *runtime_dir = NULL;
                struct stat target;

                /* First, get rid of everything that is below if there
                 * is anything... Then, overmount it with an
                 * inaccessible path. */
                (void) umount_recursive(mount_entry_path(m), 0);

                if (lstat(mount_entry_path(m), &target) < 0) {
                        if (errno == ENOENT && m->ignore)
                                return 0;

                        return log_debug_errno(errno, "Failed to lstat() %s to determine what to mount over it: %m",
                                               mount_entry_path(m));
                }

                /* We don't pass the literal runtime scope through here but one based purely on our UID. This
                 * means that the root user's --user services will use the host's inaccessible inodes rather
                 * then root's private ones. This is preferable since it means device nodes that are
                 * overmounted to make them inaccessible will be overmounted with a device node, rather than
                 * an AF_UNIX socket inode. */
                runtime_dir = settle_runtime_dir(geteuid() == 0 ? RUNTIME_SCOPE_SYSTEM : RUNTIME_SCOPE_USER);
                if (!runtime_dir)
                        return log_oom_debug();

                r = mode_to_inaccessible_node(runtime_dir, target.st_mode, &inaccessible);
                if (r < 0)
                        return log_debug_errno(SYNTHETIC_ERRNO(ELOOP),
                                               "File type not supported for inaccessible mounts. Note that symlinks are not allowed");
                what = inaccessible;
                break;
        }

        case READONLY:
        case READWRITE:
        case READWRITE_IMPLICIT:
        case EXEC:
        case NOEXEC:
                r = path_is_mount_point(mount_entry_path(m), root_directory, 0);
                if (r == -ENOENT && m->ignore)
                        return 0;
                if (r < 0)
                        return log_debug_errno(r, "Failed to determine whether %s is already a mount point: %m",
                                               mount_entry_path(m));
                if (r > 0) /* Nothing to do here, it is already a mount. We just later toggle the MS_RDONLY
                            * and MS_NOEXEC bits for the mount point if needed. */
                        return 0;
                /* This isn't a mount point yet, let's make it one. */
                what = mount_entry_path(m);
                break;

        case EXTENSION_DIRECTORIES: {
                _cleanup_free_ char *host_os_release_id = NULL, *host_os_release_version_id = NULL,
                                *host_os_release_level = NULL, *extension_name = NULL;
                _cleanup_strv_free_ char **extension_release = NULL;
                ImageClass class = IMAGE_SYSEXT;

                r = path_extract_filename(mount_entry_source(m), &extension_name);
                if (r < 0)
                        return log_debug_errno(r, "Failed to extract extension name from %s: %m", mount_entry_source(m));

                r = load_extension_release_pairs(mount_entry_source(m), IMAGE_SYSEXT, extension_name, /* relax_extension_release_check= */ false, &extension_release);
                if (r == -ENOENT) {
                        r = load_extension_release_pairs(mount_entry_source(m), IMAGE_CONFEXT, extension_name, /* relax_extension_release_check= */ false, &extension_release);
                        if (r >= 0)
                                class = IMAGE_CONFEXT;
                }
                if (r == -ENOENT)
                        return r;

                r = parse_os_release(
                                empty_to_root(root_directory),
                                "ID", &host_os_release_id,
                                "VERSION_ID", &host_os_release_version_id,
                                image_class_info[class].level_env, &host_os_release_level,
                                NULL);
                if (r < 0)
                        return log_debug_errno(r, "Failed to acquire 'os-release' data of OS tree '%s': %m", empty_to_root(root_directory));
                if (isempty(host_os_release_id))
                        return log_debug_errno(SYNTHETIC_ERRNO(EINVAL), "'ID' field not found or empty in 'os-release' data of OS tree '%s': %m", empty_to_root(root_directory));

                r = load_extension_release_pairs(mount_entry_source(m), class, extension_name, /* relax_extension_release_check= */ false, &extension_release);
                if (r == -ENOENT && m->ignore)
                        return 0;
                if (r < 0)
                        return log_debug_errno(r, "Failed to parse directory %s extension-release metadata: %m", extension_name);

                r = extension_release_validate(
                                extension_name,
                                host_os_release_id,
                                host_os_release_version_id,
                                host_os_release_level,
                                /* host_extension_scope */ NULL, /* Leave empty, we need to accept both system and portable */
                                extension_release,
                                class);
                if (r == 0)
                        return log_debug_errno(SYNTHETIC_ERRNO(ESTALE), "Directory %s extension-release metadata does not match the root's", extension_name);
                if (r < 0)
                        return log_debug_errno(r, "Failed to compare directory %s extension-release metadata with the root's os-release: %m", extension_name);

                _fallthrough_;
        }

        case BIND_MOUNT:
                rbind = false;

                _fallthrough_;
        case BIND_MOUNT_RECURSIVE: {
                _cleanup_free_ char *chased = NULL;

                /* Since mount() will always follow symlinks we chase the symlinks on our own first. Note
                 * that bind mount source paths are always relative to the host root, hence we pass NULL as
                 * root directory to chase() here. */

                r = chase(mount_entry_source(m), NULL, CHASE_TRAIL_SLASH, &chased, NULL);
                if (r == -ENOENT && m->ignore) {
                        log_debug_errno(r, "Path %s does not exist, ignoring.", mount_entry_source(m));
                        return 0;
                }
                if (r < 0)
                        return log_debug_errno(r, "Failed to follow symlinks on %s: %m", mount_entry_source(m));

                log_debug("Followed source symlinks %s %s %s.",
                          mount_entry_source(m), special_glyph(SPECIAL_GLYPH_ARROW_RIGHT), chased);

                free_and_replace(m->source_malloc, chased);

                what = mount_entry_source(m);
                make = true;
                break;
        }

        case EMPTY_DIR:
        case TMPFS:
                return mount_tmpfs(m);

        case PRIVATE_TMP:
        case PRIVATE_TMP_READONLY:
                what = mount_entry_source(m);
                make = true;
                break;

        case PRIVATE_DEV:
                return mount_private_dev(m, scope);

        case BIND_DEV:
                return mount_bind_dev(m);

        case PRIVATE_SYSFS:
                return mount_private_sysfs(m);

        case BIND_SYSFS:
                return mount_bind_sysfs(m);

        case PROCFS:
                return mount_procfs(m, ns_info);

        case RUN:
                return mount_run(m);

        case MQUEUEFS:
                return mount_mqueuefs(m);

        case MOUNT_IMAGES:
                return mount_image(m, NULL, mount_image_policy);

        case EXTENSION_IMAGES:
                return mount_image(m, root_directory, extension_image_policy);

        case OVERLAY_MOUNT:
                return mount_overlay(m);

        default:
                assert_not_reached();
        }

        assert(what);

        r = mount_nofollow_verbose(LOG_DEBUG, what, mount_entry_path(m), NULL, MS_BIND|(rbind ? MS_REC : 0), NULL);
        if (r < 0) {
                bool try_again = false;

                if (r == -ENOENT && make) {
                        int q;

                        /* Hmm, either the source or the destination are missing. Let's see if we can create
                           the destination, then try again. */

                        (void) mkdir_parents(mount_entry_path(m), 0755);

                        q = make_mount_point_inode_from_path(what, mount_entry_path(m), 0755);
                        if (q < 0 && q != -EEXIST)
                                log_error_errno(q, "Failed to create destination mount point node '%s': %m",
                                                mount_entry_path(m));
                        else
                                try_again = true;
                }

                if (try_again)
                        r = mount_nofollow_verbose(LOG_DEBUG, what, mount_entry_path(m), NULL, MS_BIND|(rbind ? MS_REC : 0), NULL);
                if (r < 0)
                        return log_error_errno(r, "Failed to mount %s to %s: %m", what, mount_entry_path(m));
        }

        log_debug("Successfully mounted %s to %s", what, mount_entry_path(m));
        return 0;
}

static int make_read_only(const MountEntry *m, char **deny_list, FILE *proc_self_mountinfo) {
        unsigned long new_flags = 0, flags_mask = 0;
        bool submounts;
        int r;

        assert(m);
        assert(proc_self_mountinfo);

        if (mount_entry_read_only(m) || m->mode == PRIVATE_DEV) {
                new_flags |= MS_RDONLY;
                flags_mask |= MS_RDONLY;
        }

        if (m->nosuid) {
                new_flags |= MS_NOSUID;
                flags_mask |= MS_NOSUID;
        }

        if (flags_mask == 0) /* No Change? */
                return 0;

        /* We generally apply these changes recursively, except for /dev, and the cases we know there's
         * nothing further down.  Set /dev readonly, but not submounts like /dev/shm. Also, we only set the
         * per-mount read-only flag.  We can't set it on the superblock, if we are inside a user namespace
         * and running Linux <= 4.17. */
        submounts =
                mount_entry_read_only(m) &&
                !IN_SET(m->mode, EMPTY_DIR, TMPFS);
        if (submounts)
                r = bind_remount_recursive_with_mountinfo(mount_entry_path(m), new_flags, flags_mask, deny_list, proc_self_mountinfo);
        else
                r = bind_remount_one_with_mountinfo(mount_entry_path(m), new_flags, flags_mask, proc_self_mountinfo);

        /* Note that we only turn on the MS_RDONLY flag here, we never turn it off. Something that was marked
         * read-only already stays this way. This improves compatibility with container managers, where we
         * won't attempt to undo read-only mounts already applied. */

        if (r == -ENOENT && m->ignore)
                return 0;
        if (r < 0)
                return log_debug_errno(r, "Failed to re-mount '%s'%s: %m", mount_entry_path(m),
                                       submounts ? " and its submounts" : "");
        return 0;
}

static int make_noexec(const MountEntry *m, char **deny_list, FILE *proc_self_mountinfo) {
        unsigned long new_flags = 0, flags_mask = 0;
        bool submounts;
        int r;

        assert(m);
        assert(proc_self_mountinfo);

        if (mount_entry_noexec(m)) {
                new_flags |= MS_NOEXEC;
                flags_mask |= MS_NOEXEC;
        } else if (mount_entry_exec(m)) {
                new_flags &= ~MS_NOEXEC;
                flags_mask |= MS_NOEXEC;
        }

        if (flags_mask == 0) /* No Change? */
                return 0;

        submounts = !IN_SET(m->mode, EMPTY_DIR, TMPFS);

        if (submounts)
                r = bind_remount_recursive_with_mountinfo(mount_entry_path(m), new_flags, flags_mask, deny_list, proc_self_mountinfo);
        else
                r = bind_remount_one_with_mountinfo(mount_entry_path(m), new_flags, flags_mask, proc_self_mountinfo);

        if (r == -ENOENT && m->ignore)
                return 0;
        if (r < 0)
                return log_debug_errno(r, "Failed to re-mount '%s'%s: %m", mount_entry_path(m),
                                       submounts ? " and its submounts" : "");
        return 0;
}

static int make_nosuid(const MountEntry *m, FILE *proc_self_mountinfo) {
        bool submounts;
        int r;

        assert(m);
        assert(proc_self_mountinfo);

        submounts = !IN_SET(m->mode, EMPTY_DIR, TMPFS);

        if (submounts)
                r = bind_remount_recursive_with_mountinfo(mount_entry_path(m), MS_NOSUID, MS_NOSUID, NULL, proc_self_mountinfo);
        else
                r = bind_remount_one_with_mountinfo(mount_entry_path(m), MS_NOSUID, MS_NOSUID, proc_self_mountinfo);
        if (r == -ENOENT && m->ignore)
                return 0;
        if (r < 0)
                return log_debug_errno(r, "Failed to re-mount '%s'%s: %m", mount_entry_path(m),
                                       submounts ? " and its submounts" : "");
        return 0;
}

static bool namespace_info_mount_apivfs(const NamespaceInfo *ns_info) {
        assert(ns_info);

        /*
         * ProtectControlGroups= and ProtectKernelTunables= imply MountAPIVFS=,
         * since to protect the API VFS mounts, they need to be around in the
         * first place...
         */

        return ns_info->mount_apivfs ||
                ns_info->protect_control_groups ||
                ns_info->protect_kernel_tunables ||
                ns_info->protect_proc != PROTECT_PROC_DEFAULT ||
                ns_info->proc_subset != PROC_SUBSET_ALL;
}

static size_t namespace_calculate_mounts(
                const NamespaceInfo *ns_info,
                char** read_write_paths,
                char** read_only_paths,
                char** inaccessible_paths,
                char** exec_paths,
                char** no_exec_paths,
                char** empty_directories,
                size_t n_bind_mounts,
                size_t n_temporary_filesystems,
                size_t n_mount_images,
                size_t n_extension_images,
                size_t n_extension_directories,
                size_t n_hierarchies,
                const char* tmp_dir,
                const char* var_tmp_dir,
                const char *creds_path,
                const char* log_namespace,
                bool setup_propagate,
                const char* notify_socket,
                const char* host_os_release) {

        size_t protect_home_cnt;
        size_t protect_system_cnt =
                (ns_info->protect_system == PROTECT_SYSTEM_STRICT ?
                 ELEMENTSOF(protect_system_strict_table) :
                 ((ns_info->protect_system == PROTECT_SYSTEM_FULL) ?
                  ELEMENTSOF(protect_system_full_table) :
                  ((ns_info->protect_system == PROTECT_SYSTEM_YES) ?
                   ELEMENTSOF(protect_system_yes_table) : 0)));

        protect_home_cnt =
                (ns_info->protect_home == PROTECT_HOME_YES ?
                 ELEMENTSOF(protect_home_yes_table) :
                 ((ns_info->protect_home == PROTECT_HOME_READ_ONLY) ?
                  ELEMENTSOF(protect_home_read_only_table) :
                  ((ns_info->protect_home == PROTECT_HOME_TMPFS) ?
                   ELEMENTSOF(protect_home_tmpfs_table) : 0)));

        return !!tmp_dir + !!var_tmp_dir +
                strv_length(read_write_paths) +
                strv_length(read_only_paths) +
                strv_length(inaccessible_paths) +
                strv_length(exec_paths) +
                strv_length(no_exec_paths) +
                strv_length(empty_directories) +
                n_bind_mounts +
                n_mount_images +
                (n_extension_images > 0 || n_extension_directories > 0 ? /* Mount each image and directory plus an overlay per hierarchy */
                 n_hierarchies + n_extension_images + n_extension_directories: 0) +
                n_temporary_filesystems +
                ns_info->private_dev +
                (ns_info->protect_kernel_tunables ?
                 ELEMENTSOF(protect_kernel_tunables_proc_table) + ELEMENTSOF(protect_kernel_tunables_sys_table) : 0) +
                (ns_info->protect_kernel_modules ? ELEMENTSOF(protect_kernel_modules_table) : 0) +
                (ns_info->protect_kernel_logs ?
                 ELEMENTSOF(protect_kernel_logs_proc_table) + ELEMENTSOF(protect_kernel_logs_dev_table) : 0) +
                (ns_info->protect_control_groups ? 1 : 0) +
                protect_home_cnt + protect_system_cnt +
                (ns_info->protect_hostname ? 2 : 0) +
                (namespace_info_mount_apivfs(ns_info) ? ELEMENTSOF(apivfs_table) : 0) +
                (creds_path ? 2 : 1) +
                !!log_namespace +
                setup_propagate + /* /run/systemd/incoming */
                !!notify_socket +
                !!host_os_release +
                ns_info->private_network + /* /sys */
                ns_info->private_ipc; /* /dev/mqueue */
}

/* Walk all mount entries and dropping any unused mounts. This affects all
 * mounts:
 * - that are implicitly protected by a path that has been rendered inaccessible
 * - whose immediate parent requests the same protection mode as the mount itself
 * - that are outside of the relevant root directory
 * - which are duplicates
 */
static void drop_unused_mounts(const char *root_directory, MountEntry *mounts, size_t *n_mounts) {
        assert(root_directory);
        assert(n_mounts);
        assert(mounts || *n_mounts == 0);

        typesafe_qsort(mounts, *n_mounts, mount_path_compare);

        drop_duplicates(mounts, n_mounts);
        drop_outside_root(root_directory, mounts, n_mounts);
        drop_inaccessible(mounts, n_mounts);
        drop_nop(mounts, n_mounts);
}

static int create_symlinks_from_tuples(const char *root, char **strv_symlinks) {
        int r;

        STRV_FOREACH_PAIR(src, dst, strv_symlinks) {
                _cleanup_free_ char *src_abs = NULL, *dst_abs = NULL;

                src_abs = path_join(root, *src);
                dst_abs = path_join(root, *dst);
                if (!src_abs || !dst_abs)
                        return -ENOMEM;

                r = mkdir_parents_label(dst_abs, 0755);
                if (r < 0)
                        return r;

                r = symlink_idempotent(src_abs, dst_abs, true);
                if (r < 0)
                        return r;
        }

        return 0;
}

static void mount_entry_path_debug_string(const char *root, MountEntry *m, char **error_path) {
        assert(m);

        /* Create a string suitable for debugging logs, stripping for example the local working directory.
         * For example, with a BindPaths=/var/bar that does not exist on the host:
         *
         * Before:
         *  foo.service: Failed to set up mount namespacing: /run/systemd/unit-root/var/bar: No such file or directory
         * After:
         *  foo.service: Failed to set up mount namespacing: /var/bar: No such file or directory
         *
         * Note that this is an error path, so no OOM check is done on purpose. */

        if (!error_path)
                return;

        if (!mount_entry_path(m)) {
                *error_path = NULL;
                return;
        }

        if (root) {
                const char *e = startswith(mount_entry_path(m), root);
                if (e) {
                        *error_path = strdup(e);
                        return;
                }
        }

        *error_path = strdup(mount_entry_path(m));
        return;
}

static int apply_mounts(
                const char *root,
                const ImagePolicy *mount_image_policy,
                const ImagePolicy *extension_image_policy,
                const NamespaceInfo *ns_info,
                MountEntry *mounts,
                size_t *n_mounts,
                RuntimeScope scope,
                char **symlinks,
                char **error_path) {

        _cleanup_fclose_ FILE *proc_self_mountinfo = NULL;
        _cleanup_free_ char **deny_list = NULL;
        int r;

        if (n_mounts == 0) /* Shortcut: nothing to do */
                return 0;

        assert(root);
        assert(mounts);
        assert(n_mounts);

        /* Open /proc/self/mountinfo now as it may become unavailable if we mount anything on top of
         * /proc. For example, this is the case with the option: 'InaccessiblePaths=/proc'. */
        proc_self_mountinfo = fopen("/proc/self/mountinfo", "re");
        if (!proc_self_mountinfo) {
                r = -errno;

                if (error_path)
                        *error_path = strdup("/proc/self/mountinfo");

                return log_debug_errno(r, "Failed to open /proc/self/mountinfo: %m");
        }

        /* First round, establish all mounts we need */
        for (;;) {
                bool again = false;

                for (MountEntry *m = mounts; m < mounts + *n_mounts; ++m) {

                        if (m->applied)
                                continue;

                        /* ExtensionImages/Directories are first opened in the propagate directory, not in the root_directory */
                        r = follow_symlink(!IN_SET(m->mode, EXTENSION_IMAGES, EXTENSION_DIRECTORIES) ? root : NULL, m);
                        if (r < 0) {
                                mount_entry_path_debug_string(root, m, error_path);
                                return r;
                        }
                        if (r == 0) {
                                /* We hit a symlinked mount point. The entry got rewritten and might
                                 * point to a very different place now. Let's normalize the changed
                                 * list, and start from the beginning. After all to mount the entry
                                 * at the new location we might need some other mounts first */
                                again = true;
                                break;
                        }

                        r = apply_one_mount(root, m, mount_image_policy, extension_image_policy, ns_info, scope);
                        if (r < 0) {
                                mount_entry_path_debug_string(root, m, error_path);
                                return r;
                        }

                        m->applied = true;
                }

                if (!again)
                        break;

                drop_unused_mounts(root, mounts, n_mounts);
        }

        /* Now that all filesystems have been set up, but before the
         * read-only switches are flipped, create the exec dirs and other symlinks.
         * Note that when /var/lib is not empty/tmpfs, these symlinks will already
         * exist, which means this will be a no-op. */
        r = create_symlinks_from_tuples(root, symlinks);
        if (r < 0)
                return log_debug_errno(r, "Failed to set up symlinks inside mount namespace: %m");

        /* Create a deny list we can pass to bind_mount_recursive() */
        deny_list = new(char*, (*n_mounts)+1);
        if (!deny_list)
                return -ENOMEM;
        for (size_t j = 0; j < *n_mounts; j++)
                deny_list[j] = (char*) mount_entry_path(mounts+j);
        deny_list[*n_mounts] = NULL;

        /* Second round, flip the ro bits if necessary. */
        for (MountEntry *m = mounts; m < mounts + *n_mounts; ++m) {
                r = make_read_only(m, deny_list, proc_self_mountinfo);
                if (r < 0) {
                        mount_entry_path_debug_string(root, m, error_path);
                        return r;
                }
        }

        /* Third round, flip the noexec bits with a simplified deny list. */
        for (size_t j = 0; j < *n_mounts; j++)
                if (IN_SET((mounts+j)->mode, EXEC, NOEXEC))
                        deny_list[j] = (char*) mount_entry_path(mounts+j);
        deny_list[*n_mounts] = NULL;

        for (MountEntry *m = mounts; m < mounts + *n_mounts; ++m) {
                r = make_noexec(m, deny_list, proc_self_mountinfo);
                if (r < 0) {
                        mount_entry_path_debug_string(root, m, error_path);
                        return r;
                }
        }

        /* Fourth round, flip the nosuid bits without a deny list. */
        if (ns_info->mount_nosuid)
                for (MountEntry *m = mounts; m < mounts + *n_mounts; ++m) {
                        r = make_nosuid(m, proc_self_mountinfo);
                        if (r < 0) {
                                mount_entry_path_debug_string(root, m, error_path);
                                return r;
                        }
                }

        return 1;
}

static bool root_read_only(
                char **read_only_paths,
                ProtectSystem protect_system) {

        /* Determine whether the root directory is going to be read-only given the configured settings. */

        if (protect_system == PROTECT_SYSTEM_STRICT)
                return true;

        if (prefixed_path_strv_contains(read_only_paths, "/"))
                return true;

        return false;
}

static bool home_read_only(
                char** read_only_paths,
                char** inaccessible_paths,
                char** empty_directories,
                const BindMount *bind_mounts,
                size_t n_bind_mounts,
                const TemporaryFileSystem *temporary_filesystems,
                size_t n_temporary_filesystems,
                ProtectHome protect_home) {

        /* Determine whether the /home directory is going to be read-only given the configured settings. Yes,
         * this is a bit sloppy, since we don't bother checking for cases where / is affected by multiple
         * settings. */

        if (protect_home != PROTECT_HOME_NO)
                return true;

        if (prefixed_path_strv_contains(read_only_paths, "/home") ||
            prefixed_path_strv_contains(inaccessible_paths, "/home") ||
            prefixed_path_strv_contains(empty_directories, "/home"))
                return true;

        for (size_t i = 0; i < n_temporary_filesystems; i++)
                if (path_equal(temporary_filesystems[i].path, "/home"))
                        return true;

        /* If /home is overmounted with some dir from the host it's not writable. */
        for (size_t i = 0; i < n_bind_mounts; i++)
                if (path_equal(bind_mounts[i].destination, "/home"))
                        return true;

        return false;
}

int setup_namespace(
                const char* root_directory,
                const char* root_image,
                const MountOptions *root_image_mount_options,
                const ImagePolicy *root_image_policy,
                const NamespaceInfo *ns_info,
                char** read_write_paths,
                char** read_only_paths,
                char** inaccessible_paths,
                char** exec_paths,
                char** no_exec_paths,
                char** empty_directories,
                char** symlinks,
                const BindMount *bind_mounts,
                size_t n_bind_mounts,
                const TemporaryFileSystem *temporary_filesystems,
                size_t n_temporary_filesystems,
                const MountImage *mount_images,
                size_t n_mount_images,
                const ImagePolicy *mount_image_policy,
                const char* tmp_dir,
                const char* var_tmp_dir,
                const char *creds_path,
                const char *log_namespace,
                unsigned long mount_propagation_flag,
                VeritySettings *verity,
                const MountImage *extension_images,
                size_t n_extension_images,
                const ImagePolicy *extension_image_policy,
                char **extension_directories,
                const char *propagate_dir,
                const char *incoming_dir,
                const char *extension_dir,
                const char *notify_socket,
                const char *host_os_release_stage,
                RuntimeScope scope,
                char **error_path) {

        _cleanup_(loop_device_unrefp) LoopDevice *loop_device = NULL;
        _cleanup_(dissected_image_unrefp) DissectedImage *dissected_image = NULL;
        _cleanup_strv_free_ char **hierarchies = NULL;
        MountEntry *m = NULL, *mounts = NULL;
        bool require_prefix = false, setup_propagate = false;
        const char *root;
        DissectImageFlags dissect_image_flags =
                DISSECT_IMAGE_GENERIC_ROOT |
                DISSECT_IMAGE_REQUIRE_ROOT |
                DISSECT_IMAGE_DISCARD_ON_LOOP |
                DISSECT_IMAGE_RELAX_VAR_CHECK |
                DISSECT_IMAGE_FSCK |
                DISSECT_IMAGE_USR_NO_ROOT |
                DISSECT_IMAGE_GROWFS |
                DISSECT_IMAGE_ADD_PARTITION_DEVICES |
                DISSECT_IMAGE_PIN_PARTITION_DEVICES;
        size_t n_mounts;
        int r;

        assert(ns_info);

        /* Make sure that all mknod(), mkdir() calls we do are unaffected by the umask, and the access modes
         * we configure take effect */
        BLOCK_WITH_UMASK(0000);

        if (!isempty(propagate_dir) && !isempty(incoming_dir))
                setup_propagate = true;

        if (mount_propagation_flag == 0)
                mount_propagation_flag = MS_SHARED;

        if (root_image) {
                /* Make the whole image read-only if we can determine that we only access it in a read-only fashion. */
                if (root_read_only(read_only_paths,
                                   ns_info->protect_system) &&
                    home_read_only(read_only_paths, inaccessible_paths, empty_directories,
                                   bind_mounts, n_bind_mounts, temporary_filesystems, n_temporary_filesystems,
                                   ns_info->protect_home) &&
                    strv_isempty(read_write_paths))
                        dissect_image_flags |= DISSECT_IMAGE_READ_ONLY;

                SET_FLAG(dissect_image_flags, DISSECT_IMAGE_NO_PARTITION_TABLE, verity && verity->data_path);

                r = loop_device_make_by_path(
                                root_image,
                                FLAGS_SET(dissect_image_flags, DISSECT_IMAGE_DEVICE_READ_ONLY) ? O_RDONLY : -1 /* < 0 means writable if possible, read-only as fallback */,
                                /* sector_size= */ UINT32_MAX,
                                FLAGS_SET(dissect_image_flags, DISSECT_IMAGE_NO_PARTITION_TABLE) ? 0 : LO_FLAGS_PARTSCAN,
                                LOCK_SH,
                                &loop_device);
                if (r < 0)
                        return log_debug_errno(r, "Failed to create loop device for root image: %m");

                r = dissect_loop_device(
                                loop_device,
                                verity,
                                root_image_mount_options,
                                root_image_policy,
                                dissect_image_flags,
                                &dissected_image);
                if (r < 0)
                        return log_debug_errno(r, "Failed to dissect image: %m");

                r = dissected_image_load_verity_sig_partition(
                                dissected_image,
                                loop_device->fd,
                                verity);
                if (r < 0)
                        return r;

                r = dissected_image_decrypt(
                                dissected_image,
                                NULL,
                                verity,
                                dissect_image_flags);
                if (r < 0)
                        return log_debug_errno(r, "Failed to decrypt dissected image: %m");
        }

        if (root_directory)
                root = root_directory;
        else {
                /* /run/systemd should have been created by PID 1 early on already, but in some cases, like
                 * when running tests (test-execute), it might not have been created yet so let's make sure
                 * we create it if it doesn't already exist. */
                (void) mkdir_p_label("/run/systemd", 0755);

                /* Always create the mount namespace in a temporary directory, instead of operating directly
                 * in the root. The temporary directory prevents any mounts from being potentially obscured
                 * my other mounts we already applied.  We use the same mount point for all images, which is
                 * safe, since they all live in their own namespaces after all, and hence won't see each
                 * other. (Note: this directory is also created by PID 1 early on, we create it here for
                 * similar reasons as /run/systemd/ first.) */
                root = "/run/systemd/mount-rootfs";
                (void) mkdir_label(root, 0555);

                require_prefix = true;
        }

        if (n_extension_images > 0 || !strv_isempty(extension_directories)) {
                /* Hierarchy population needs to be done for sysext and confext extension images */
                r = parse_env_extension_hierarchies(&hierarchies, "SYSTEMD_SYSEXT_AND_CONFEXT_HIERARCHIES");
                if (r < 0)
                        return r;
        }

        n_mounts = namespace_calculate_mounts(
                        ns_info,
                        read_write_paths,
                        read_only_paths,
                        inaccessible_paths,
                        exec_paths,
                        no_exec_paths,
                        empty_directories,
                        n_bind_mounts,
                        n_temporary_filesystems,
                        n_mount_images,
                        n_extension_images,
                        strv_length(extension_directories),
                        strv_length(hierarchies),
                        tmp_dir, var_tmp_dir,
                        creds_path,
                        log_namespace,
                        setup_propagate,
                        notify_socket,
                        host_os_release_stage);

        if (n_mounts > 0) {
                m = mounts = new0(MountEntry, n_mounts);
                if (!mounts)
                        return -ENOMEM;

                r = append_access_mounts(&m, read_write_paths, READWRITE, require_prefix);
                if (r < 0)
                        goto finish;

                r = append_access_mounts(&m, read_only_paths, READONLY, require_prefix);
                if (r < 0)
                        goto finish;

                r = append_access_mounts(&m, inaccessible_paths, INACCESSIBLE, require_prefix);
                if (r < 0)
                        goto finish;

                r = append_access_mounts(&m, exec_paths, EXEC, require_prefix);
                if (r < 0)
                        goto finish;

                r = append_access_mounts(&m, no_exec_paths, NOEXEC, require_prefix);
                if (r < 0)
                        goto finish;

                r = append_empty_dir_mounts(&m, empty_directories);
                if (r < 0)
                        goto finish;

                r = append_bind_mounts(&m, bind_mounts, n_bind_mounts);
                if (r < 0)
                        goto finish;

                r = append_tmpfs_mounts(&m, temporary_filesystems, n_temporary_filesystems);
                if (r < 0)
                        goto finish;

                if (tmp_dir) {
                        bool ro = streq(tmp_dir, RUN_SYSTEMD_EMPTY);

                        *(m++) = (MountEntry) {
                                .path_const = "/tmp",
                                .mode = ro ? PRIVATE_TMP_READONLY : PRIVATE_TMP,
                                .source_const = tmp_dir,
                        };
                }

                if (var_tmp_dir) {
                        bool ro = streq(var_tmp_dir, RUN_SYSTEMD_EMPTY);

                        *(m++) = (MountEntry) {
                                .path_const = "/var/tmp",
                                .mode = ro ? PRIVATE_TMP_READONLY : PRIVATE_TMP,
                                .source_const = var_tmp_dir,
                        };
                }

                r = append_mount_images(&m, mount_images, n_mount_images);
                if (r < 0)
                        goto finish;

                r = append_extensions(&m, root, extension_dir, hierarchies, extension_images, n_extension_images, extension_directories);
                if (r < 0)
                        goto finish;

                if (ns_info->private_dev)
                        *(m++) = (MountEntry) {
                                .path_const = "/dev",
                                .mode = PRIVATE_DEV,
                                .flags = DEV_MOUNT_OPTIONS,
                        };

                /* In case /proc is successfully mounted with pid tree subset only (ProcSubset=pid), the
                   protective mounts to non-pid /proc paths would fail. But the pid only option may have
                   failed gracefully, so let's try the mounts but it's not fatal if they don't succeed. */
                bool ignore_protect_proc = ns_info->ignore_protect_paths || ns_info->proc_subset == PROC_SUBSET_PID;
                if (ns_info->protect_kernel_tunables) {
                        r = append_static_mounts(&m,
                                                 protect_kernel_tunables_proc_table,
                                                 ELEMENTSOF(protect_kernel_tunables_proc_table),
                                                 ignore_protect_proc);
                        if (r < 0)
                                goto finish;

                        r = append_static_mounts(&m,
                                                 protect_kernel_tunables_sys_table,
                                                 ELEMENTSOF(protect_kernel_tunables_sys_table),
                                                 ns_info->ignore_protect_paths);
                        if (r < 0)
                                goto finish;
                }

                if (ns_info->protect_kernel_modules) {
                        r = append_static_mounts(&m,
                                                 protect_kernel_modules_table,
                                                 ELEMENTSOF(protect_kernel_modules_table),
                                                 ns_info->ignore_protect_paths);
                        if (r < 0)
                                goto finish;
                }

                if (ns_info->protect_kernel_logs) {
                        r = append_static_mounts(&m,
                                                 protect_kernel_logs_proc_table,
                                                 ELEMENTSOF(protect_kernel_logs_proc_table),
                                                 ignore_protect_proc);
                        if (r < 0)
                                goto finish;

                        r = append_static_mounts(&m,
                                                 protect_kernel_logs_dev_table,
                                                 ELEMENTSOF(protect_kernel_logs_dev_table),
                                                 ns_info->ignore_protect_paths);
                        if (r < 0)
                                goto finish;
                }

                if (ns_info->protect_control_groups)
                        *(m++) = (MountEntry) {
                                .path_const = "/sys/fs/cgroup",
                                .mode = READONLY,
                        };

                r = append_protect_home(&m, ns_info->protect_home, ns_info->ignore_protect_paths);
                if (r < 0)
                        goto finish;

                r = append_protect_system(&m, ns_info->protect_system, false);
                if (r < 0)
                        goto finish;

                if (namespace_info_mount_apivfs(ns_info)) {
                        r = append_static_mounts(&m,
                                                 apivfs_table,
                                                 ELEMENTSOF(apivfs_table),
                                                 ns_info->ignore_protect_paths);
                        if (r < 0)
                                goto finish;
                }

                /* Note, if proc is mounted with subset=pid then neither of the
                 * two paths will exist, i.e. they are implicitly protected by
                 * the mount option. */
                if (ns_info->protect_hostname) {
                        *(m++) = (MountEntry) {
                                .path_const = "/proc/sys/kernel/hostname",
                                .mode = READONLY,
                                .ignore = ignore_protect_proc,
                        };
                        *(m++) = (MountEntry) {
                                .path_const = "/proc/sys/kernel/domainname",
                                .mode = READONLY,
                                .ignore = ignore_protect_proc,
                        };
                }

                if (ns_info->private_network)
                        *(m++) = (MountEntry) {
                                .path_const = "/sys",
                                .mode = PRIVATE_SYSFS,
                        };

                if (ns_info->private_ipc)
                        *(m++) = (MountEntry) {
                                .path_const = "/dev/mqueue",
                                .mode = MQUEUEFS,
                                .flags = MS_NOSUID | MS_NODEV | MS_NOEXEC | MS_RELATIME,
                        };

                if (creds_path) {
                        /* If our service has a credentials store configured, then bind that one in, but hide
                         * everything else. */

                        *(m++) = (MountEntry) {
                                .path_const = "/run/credentials",
                                .mode = TMPFS,
                                .read_only = true,
                                .options_const = "mode=0755" TMPFS_LIMITS_EMPTY_OR_ALMOST,
                                .flags = MS_NODEV|MS_STRICTATIME|MS_NOSUID|MS_NOEXEC,
                        };

                        *(m++) = (MountEntry) {
                                .path_const = creds_path,
                                .mode = BIND_MOUNT,
                                .read_only = true,
                                .source_const = creds_path,
                                .ignore = true,
                        };
                } else {
                        /* If our service has no credentials store configured, then make the whole
                         * credentials tree inaccessible wholesale. */

                        *(m++) = (MountEntry) {
                                .path_const = "/run/credentials",
                                .mode = INACCESSIBLE,
                                .ignore = true,
                        };
                }

                if (log_namespace) {
                        _cleanup_free_ char *q = NULL;

                        q = strjoin("/run/systemd/journal.", log_namespace);
                        if (!q) {
                                r = -ENOMEM;
                                goto finish;
                        }

                        *(m++) = (MountEntry) {
                                .path_const = "/run/systemd/journal",
                                .mode = BIND_MOUNT_RECURSIVE,
                                .read_only = true,
                                .source_malloc = TAKE_PTR(q),
                        };
                }

                /* Will be used to add bind mounts at runtime */
                if (setup_propagate)
                        *(m++) = (MountEntry) {
                                .source_const = propagate_dir,
                                .path_const = incoming_dir,
                                .mode = BIND_MOUNT,
                                .read_only = true,
                        };

                if (notify_socket)
                        *(m++) = (MountEntry) {
                                .path_const = notify_socket,
                                .source_const = notify_socket,
                                .mode = BIND_MOUNT,
                                .read_only = true,
                        };

                if (host_os_release_stage)
                        *(m++) = (MountEntry) {
                                .path_const = "/run/host/.os-release-stage/",
                                .source_const = host_os_release_stage,
                                .mode = BIND_MOUNT,
                                .read_only = true,
                                .ignore = true, /* Live copy, don't hard-fail if it goes missing */
                        };

                assert(mounts + n_mounts == m);

                /* Prepend the root directory where that's necessary */
                r = prefix_where_needed(mounts, n_mounts, root);
                if (r < 0)
                        goto finish;

                drop_unused_mounts(root, mounts, &n_mounts);
        }

        /* All above is just preparation, figuring out what to do. Let's now actually start doing something. */

        if (unshare(CLONE_NEWNS) < 0) {
                r = log_debug_errno(errno, "Failed to unshare the mount namespace: %m");
                if (ERRNO_IS_PRIVILEGE(r) ||
                    ERRNO_IS_NOT_SUPPORTED(r))
                        /* If the kernel doesn't support namespaces, or when there's a MAC or seccomp filter
                         * in place that doesn't allow us to create namespaces (or a missing cap), then
                         * propagate a recognizable error back, which the caller can use to detect this case
                         * (and only this) and optionally continue without namespacing applied. */
                        r = -ENOANO;

                goto finish;
        }

        /* Create the source directory to allow runtime propagation of mounts */
        if (setup_propagate)
                (void) mkdir_p(propagate_dir, 0600);

        if (n_extension_images > 0 || !strv_isempty(extension_directories))
                /* ExtensionImages/Directories mountpoint directories will be created while parsing the
                 * mounts to create, so have the parent ready */
                (void) mkdir_p(extension_dir, 0600);

        /* Remount / as SLAVE so that nothing now mounted in the namespace
         * shows up in the parent */
        if (mount(NULL, "/", NULL, MS_SLAVE|MS_REC, NULL) < 0) {
                r = log_debug_errno(errno, "Failed to remount '/' as SLAVE: %m");
                goto finish;
        }

        if (root_image) {
                /* A root image is specified, mount it to the right place */
                r = dissected_image_mount(
                                dissected_image,
                                root,
                                /* uid_shift= */ UID_INVALID,
                                /* uid_range= */ UID_INVALID,
                                /* userns_fd= */ -EBADF,
                                dissect_image_flags);
                if (r < 0) {
                        log_debug_errno(r, "Failed to mount root image: %m");
                        goto finish;
                }

                /* Now release the block device lock, so that udevd is free to call BLKRRPART on the device
                 * if it likes. */
                r = loop_device_flock(loop_device, LOCK_UN);
                if (r < 0) {
                        log_debug_errno(r, "Failed to release lock on loopback block device: %m");
                        goto finish;
                }

                r = dissected_image_relinquish(dissected_image);
                if (r < 0) {
                        log_debug_errno(r, "Failed to relinquish dissected image: %m");
                        goto finish;
                }

        } else if (root_directory) {

                /* A root directory is specified. Turn its directory into bind mount, if it isn't one yet. */
                r = path_is_mount_point(root, NULL, AT_SYMLINK_FOLLOW);
                if (r < 0) {
                        log_debug_errno(r, "Failed to detect that %s is a mount point or not: %m", root);
                        goto finish;
                }
                if (r == 0) {
                        r = mount_nofollow_verbose(LOG_DEBUG, root, root, NULL, MS_BIND|MS_REC, NULL);
                        if (r < 0)
                                goto finish;
                }

        } else {
                /* Let's mount the main root directory to the root directory to use */
                r = mount_nofollow_verbose(LOG_DEBUG, "/", root, NULL, MS_BIND|MS_REC, NULL);
                if (r < 0)
                        goto finish;
        }

        /* Try to set up the new root directory before mounting anything else there. */
        if (root_image || root_directory)
                (void) base_filesystem_create(root, UID_INVALID, GID_INVALID);

        /* Now make the magic happen */
        r = apply_mounts(root,
                         mount_image_policy,
                         extension_image_policy,
                         ns_info,
                         mounts, &n_mounts,
                         scope,
                         symlinks,
                         error_path);
        if (r < 0)
                goto finish;

        /* MS_MOVE does not work on MS_SHARED so the remount MS_SHARED will be done later */
        r = mount_switch_root(root, /* mount_propagation_flag = */ 0);
        if (r == -EINVAL && root_directory) {
                /* If we are using root_directory and we don't have privileges (ie: user manager in a user
                 * namespace) and the root_directory is already a mount point in the parent namespace,
                 * MS_MOVE will fail as we don't have permission to change it (with EINVAL rather than
                 * EPERM). Attempt to bind-mount it over itself (like we do above if it's not already a
                 * mount point) and try again. */
                r = mount_nofollow_verbose(LOG_DEBUG, root, root, NULL, MS_BIND|MS_REC, NULL);
                if (r < 0)
                        goto finish;
                r = mount_switch_root(root, /* mount_propagation_flag = */ 0);
        }
        if (r < 0) {
                log_debug_errno(r, "Failed to mount root with MS_MOVE: %m");
                goto finish;
        }

        /* Remount / as the desired mode. Note that this will not reestablish propagation from our side to
         * the host, since what's disconnected is disconnected. */
        if (mount(NULL, "/", NULL, mount_propagation_flag | MS_REC, NULL) < 0) {
                r = log_debug_errno(errno, "Failed to remount '/' with desired mount flags: %m");
                goto finish;
        }

        /* bind_mount_in_namespace() will MS_MOVE into that directory, and that's only
         * supported for non-shared mounts. This needs to happen after remounting / or it will fail. */
        if (setup_propagate) {
                r = mount(NULL, incoming_dir, NULL, MS_SLAVE, NULL);
                if (r < 0) {
                        log_error_errno(r, "Failed to remount %s with MS_SLAVE: %m", incoming_dir);
                        goto finish;
                }
        }

        r = 0;

finish:
        if (n_mounts > 0)
                for (m = mounts; m < mounts + n_mounts; m++)
                        mount_entry_done(m);

        free(mounts);

        return r;
}

void bind_mount_free_many(BindMount *b, size_t n) {
        assert(b || n == 0);

        for (size_t i = 0; i < n; i++) {
                free(b[i].source);
                free(b[i].destination);
        }

        free(b);
}

int bind_mount_add(BindMount **b, size_t *n, const BindMount *item) {
        _cleanup_free_ char *s = NULL, *d = NULL;
        BindMount *c;

        assert(b);
        assert(n);
        assert(item);

        s = strdup(item->source);
        if (!s)
                return -ENOMEM;

        d = strdup(item->destination);
        if (!d)
                return -ENOMEM;

        c = reallocarray(*b, *n + 1, sizeof(BindMount));
        if (!c)
                return -ENOMEM;

        *b = c;

        c[(*n) ++] = (BindMount) {
                .source = TAKE_PTR(s),
                .destination = TAKE_PTR(d),
                .read_only = item->read_only,
                .nosuid = item->nosuid,
                .recursive = item->recursive,
                .ignore_enoent = item->ignore_enoent,
        };

        return 0;
}

MountImage* mount_image_free_many(MountImage *m, size_t *n) {
        assert(n);
        assert(m || *n == 0);

        for (size_t i = 0; i < *n; i++) {
                free(m[i].source);
                free(m[i].destination);
                mount_options_free_all(m[i].mount_options);
        }

        free(m);
        *n = 0;
        return NULL;
}

int mount_image_add(MountImage **m, size_t *n, const MountImage *item) {
        _cleanup_free_ char *s = NULL, *d = NULL;
        _cleanup_(mount_options_free_allp) MountOptions *options = NULL;
        MountImage *c;

        assert(m);
        assert(n);
        assert(item);

        s = strdup(item->source);
        if (!s)
                return -ENOMEM;

        if (item->destination) {
                d = strdup(item->destination);
                if (!d)
                        return -ENOMEM;
        }

        LIST_FOREACH(mount_options, i, item->mount_options) {
                _cleanup_(mount_options_free_allp) MountOptions *o = NULL;

                o = new(MountOptions, 1);
                if (!o)
                        return -ENOMEM;

                *o = (MountOptions) {
                        .partition_designator = i->partition_designator,
                        .options = strdup(i->options),
                };
                if (!o->options)
                        return -ENOMEM;

                LIST_APPEND(mount_options, options, TAKE_PTR(o));
        }

        c = reallocarray(*m, *n + 1, sizeof(MountImage));
        if (!c)
                return -ENOMEM;

        *m = c;

        c[(*n) ++] = (MountImage) {
                .source = TAKE_PTR(s),
                .destination = TAKE_PTR(d),
                .mount_options = TAKE_PTR(options),
                .ignore_enoent = item->ignore_enoent,
                .type = item->type,
        };

        return 0;
}

void temporary_filesystem_free_many(TemporaryFileSystem *t, size_t n) {
        assert(t || n == 0);

        for (size_t i = 0; i < n; i++) {
                free(t[i].path);
                free(t[i].options);
        }

        free(t);
}

int temporary_filesystem_add(
                TemporaryFileSystem **t,
                size_t *n,
                const char *path,
                const char *options) {

        _cleanup_free_ char *p = NULL, *o = NULL;
        TemporaryFileSystem *c;

        assert(t);
        assert(n);
        assert(path);

        p = strdup(path);
        if (!p)
                return -ENOMEM;

        if (!isempty(options)) {
                o = strdup(options);
                if (!o)
                        return -ENOMEM;
        }

        c = reallocarray(*t, *n + 1, sizeof(TemporaryFileSystem));
        if (!c)
                return -ENOMEM;

        *t = c;

        c[(*n) ++] = (TemporaryFileSystem) {
                .path = TAKE_PTR(p),
                .options = TAKE_PTR(o),
        };

        return 0;
}

static int make_tmp_prefix(const char *prefix) {
        _cleanup_free_ char *t = NULL;
        _cleanup_close_ int fd = -EBADF;
        int r;

        /* Don't do anything unless we know the dir is actually missing */
        r = access(prefix, F_OK);
        if (r >= 0)
                return 0;
        if (errno != ENOENT)
                return -errno;

        WITH_UMASK(000)
                r = mkdir_parents(prefix, 0755);
        if (r < 0)
                return r;

        r = tempfn_random(prefix, NULL, &t);
        if (r < 0)
                return r;

        /* umask will corrupt this access mode, but that doesn't matter, we need to call chmod() anyway for
         * the suid bit, below. */
        fd = open_mkdir_at(AT_FDCWD, t, O_EXCL|O_CLOEXEC, 0777);
        if (fd < 0)
                return fd;

        r = RET_NERRNO(fchmod(fd, 01777));
        if (r < 0) {
                (void) rmdir(t);
                return r;
        }

        r = RET_NERRNO(rename(t, prefix));
        if (r < 0) {
                (void) rmdir(t);
                return r == -EEXIST ? 0 : r; /* it's fine if someone else created the dir by now */
        }

        return 0;

}

static int setup_one_tmp_dir(const char *id, const char *prefix, char **path, char **tmp_path) {
        _cleanup_free_ char *x = NULL;
        _cleanup_free_ char *y = NULL;
        sd_id128_t boot_id;
        bool rw = true;
        int r;

        assert(id);
        assert(prefix);
        assert(path);

        /* We include the boot id in the directory so that after a
         * reboot we can easily identify obsolete directories. */

        r = sd_id128_get_boot(&boot_id);
        if (r < 0)
                return r;

        x = strjoin(prefix, "/systemd-private-", SD_ID128_TO_STRING(boot_id), "-", id, "-XXXXXX");
        if (!x)
                return -ENOMEM;

        r = make_tmp_prefix(prefix);
        if (r < 0)
                return r;

        WITH_UMASK(0077)
                if (!mkdtemp(x)) {
                        if (errno == EROFS || ERRNO_IS_DISK_SPACE(errno))
                                rw = false;
                        else
                                return -errno;
                }

        if (rw) {
                y = strjoin(x, "/tmp");
                if (!y)
                        return -ENOMEM;

                WITH_UMASK(0000)
                        if (mkdir(y, 0777 | S_ISVTX) < 0)
                                return -errno;

                r = label_fix_full(AT_FDCWD, y, prefix, 0);
                if (r < 0)
                        return r;

                if (tmp_path)
                        *tmp_path = TAKE_PTR(y);
        } else {
                /* Trouble: we failed to create the directory. Instead of failing, let's simulate /tmp being
                 * read-only. This way the service will get the EROFS result as if it was writing to the real
                 * file system. */
                WITH_UMASK(0000)
                        r = mkdir_p(RUN_SYSTEMD_EMPTY, 0500);
                if (r < 0)
                        return r;

                r = free_and_strdup(&x, RUN_SYSTEMD_EMPTY);
                if (r < 0)
                        return r;
        }

        *path = TAKE_PTR(x);
        return 0;
}

int setup_tmp_dirs(const char *id, char **tmp_dir, char **var_tmp_dir) {
        _cleanup_(namespace_cleanup_tmpdirp) char *a = NULL;
        _cleanup_(rmdir_and_freep) char *a_tmp = NULL;
        char *b;
        int r;

        assert(id);
        assert(tmp_dir);
        assert(var_tmp_dir);

        r = setup_one_tmp_dir(id, "/tmp", &a, &a_tmp);
        if (r < 0)
                return r;

        r = setup_one_tmp_dir(id, "/var/tmp", &b, NULL);
        if (r < 0)
                return r;

        a_tmp = mfree(a_tmp); /* avoid rmdir */
        *tmp_dir = TAKE_PTR(a);
        *var_tmp_dir = TAKE_PTR(b);

        return 0;
}

int setup_shareable_ns(int ns_storage_socket[static 2], unsigned long nsflag) {
        _cleanup_close_ int ns = -EBADF;
        int r;
        const char *ns_name, *ns_path;

        assert(ns_storage_socket);
        assert(ns_storage_socket[0] >= 0);
        assert(ns_storage_socket[1] >= 0);

        ns_name = namespace_single_flag_to_string(nsflag);
        assert(ns_name);

        /* We use the passed socketpair as a storage buffer for our
         * namespace reference fd. Whatever process runs this first
         * shall create a new namespace, all others should just join
         * it. To serialize that we use a file lock on the socket
         * pair.
         *
         * It's a bit crazy, but hey, works great! */

        r = posix_lock(ns_storage_socket[0], LOCK_EX);
        if (r < 0)
                return r;

        CLEANUP_POSIX_UNLOCK(ns_storage_socket[0]);

        ns = receive_one_fd(ns_storage_socket[0], MSG_PEEK|MSG_DONTWAIT);
        if (ns >= 0) {
                /* Yay, found something, so let's join the namespace */
                r = RET_NERRNO(setns(ns, nsflag));
                if (r < 0)
                        return r;

                return 0;
        }

        if (ns != -EAGAIN)
                return ns;

        /* Nothing stored yet, so let's create a new namespace. */

        if (unshare(nsflag) < 0)
                return -errno;

        (void) loopback_setup();

        ns_path = strjoina("/proc/self/ns/", ns_name);
        ns = open(ns_path, O_RDONLY|O_CLOEXEC|O_NOCTTY);
        if (ns < 0)
                return -errno;

        r = send_one_fd(ns_storage_socket[1], ns, MSG_DONTWAIT);
        if (r < 0)
                return r;

        return 1;
}

int open_shareable_ns_path(int ns_storage_socket[static 2], const char *path, unsigned long nsflag) {
        _cleanup_close_ int ns = -EBADF;
        int r;

        assert(ns_storage_socket);
        assert(ns_storage_socket[0] >= 0);
        assert(ns_storage_socket[1] >= 0);
        assert(path);

        /* If the storage socket doesn't contain a ns fd yet, open one via the file system and store it in
         * it. This is supposed to be called ahead of time, i.e. before setup_shareable_ns() which will
         * allocate a new anonymous ns if needed. */

        r = posix_lock(ns_storage_socket[0], LOCK_EX);
        if (r < 0)
                return r;

        CLEANUP_POSIX_UNLOCK(ns_storage_socket[0]);

        ns = receive_one_fd(ns_storage_socket[0], MSG_PEEK|MSG_DONTWAIT);
        if (ns >= 0)
                return 0;
        if (ns != -EAGAIN)
                return ns;

        /* Nothing stored yet. Open the file from the file system. */

        ns = open(path, O_RDONLY|O_NOCTTY|O_CLOEXEC);
        if (ns < 0)
                return -errno;

        r = fd_is_ns(ns, nsflag);
        if (r == 0)
                return -EINVAL;
        if (r < 0 && r != -EUCLEAN) /* EUCLEAN: we don't know */
                return r;

        r = send_one_fd(ns_storage_socket[1], ns, MSG_DONTWAIT);
        if (r < 0)
                return r;

        return 1;
}

bool ns_type_supported(NamespaceType type) {
        const char *t, *ns_proc;

        t = namespace_type_to_string(type);
        if (!t) /* Don't know how to translate this? Then it's not supported */
                return false;

        ns_proc = strjoina("/proc/self/ns/", t);
        return access(ns_proc, F_OK) == 0;
}

static const char *const protect_home_table[_PROTECT_HOME_MAX] = {
        [PROTECT_HOME_NO]        = "no",
        [PROTECT_HOME_YES]       = "yes",
        [PROTECT_HOME_READ_ONLY] = "read-only",
        [PROTECT_HOME_TMPFS]     = "tmpfs",
};

DEFINE_STRING_TABLE_LOOKUP_WITH_BOOLEAN(protect_home, ProtectHome, PROTECT_HOME_YES);

static const char *const protect_system_table[_PROTECT_SYSTEM_MAX] = {
        [PROTECT_SYSTEM_NO]     = "no",
        [PROTECT_SYSTEM_YES]    = "yes",
        [PROTECT_SYSTEM_FULL]   = "full",
        [PROTECT_SYSTEM_STRICT] = "strict",
};

DEFINE_STRING_TABLE_LOOKUP_WITH_BOOLEAN(protect_system, ProtectSystem, PROTECT_SYSTEM_YES);

static const char* const namespace_type_table[] = {
        [NAMESPACE_MOUNT]  = "mnt",
        [NAMESPACE_CGROUP] = "cgroup",
        [NAMESPACE_UTS]    = "uts",
        [NAMESPACE_IPC]    = "ipc",
        [NAMESPACE_USER]   = "user",
        [NAMESPACE_PID]    = "pid",
        [NAMESPACE_NET]    = "net",
        [NAMESPACE_TIME]   = "time",
};

DEFINE_STRING_TABLE_LOOKUP(namespace_type, NamespaceType);

static const char* const protect_proc_table[_PROTECT_PROC_MAX] = {
        [PROTECT_PROC_DEFAULT]    = "default",
        [PROTECT_PROC_NOACCESS]   = "noaccess",
        [PROTECT_PROC_INVISIBLE]  = "invisible",
        [PROTECT_PROC_PTRACEABLE] = "ptraceable",
};

DEFINE_STRING_TABLE_LOOKUP(protect_proc, ProtectProc);

static const char* const proc_subset_table[_PROC_SUBSET_MAX] = {
        [PROC_SUBSET_ALL] = "all",
        [PROC_SUBSET_PID] = "pid",
};

DEFINE_STRING_TABLE_LOOKUP(proc_subset, ProcSubset);