[thirdparty/systemd.git] / src / basic / chase-symlinks.c

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

#include <linux/magic.h>

#include "alloc-util.h"
#include "chase-symlinks.h"
#include "fd-util.h"
#include "fileio.h"
#include "fs-util.h"
#include "glyph-util.h"
#include "log.h"
#include "path-util.h"
#include "string-util.h"
#include "user-util.h"

bool unsafe_transition(const struct stat *a, const struct stat *b) {
        /* Returns true if the transition from a to b is safe, i.e. that we never transition from unprivileged to
         * privileged files or directories. Why bother? So that unprivileged code can't symlink to privileged files
         * making us believe we read something safe even though it isn't safe in the specific context we open it in. */

        if (a->st_uid == 0) /* Transitioning from privileged to unprivileged is always fine */
                return false;

        return a->st_uid != b->st_uid; /* Otherwise we need to stay within the same UID */
}

static int log_unsafe_transition(int a, int b, const char *path, ChaseSymlinksFlags flags) {
        _cleanup_free_ char *n1 = NULL, *n2 = NULL, *user_a = NULL, *user_b = NULL;
        struct stat st;

        if (!FLAGS_SET(flags, CHASE_WARN))
                return -ENOLINK;

        (void) fd_get_path(a, &n1);
        (void) fd_get_path(b, &n2);

        if (fstat(a, &st) == 0)
                user_a = uid_to_name(st.st_uid);
        if (fstat(b, &st) == 0)
                user_b = uid_to_name(st.st_uid);

        return log_warning_errno(SYNTHETIC_ERRNO(ENOLINK),
                                 "Detected unsafe path transition %s (owned by %s) %s %s (owned by %s) during canonicalization of %s.",
                                 strna(n1), strna(user_a), special_glyph(SPECIAL_GLYPH_ARROW_RIGHT), strna(n2), strna(user_b), path);
}

static int log_autofs_mount_point(int fd, const char *path, ChaseSymlinksFlags flags) {
        _cleanup_free_ char *n1 = NULL;

        if (!FLAGS_SET(flags, CHASE_WARN))
                return -EREMOTE;

        (void) fd_get_path(fd, &n1);

        return log_warning_errno(SYNTHETIC_ERRNO(EREMOTE),
                                 "Detected autofs mount point %s during canonicalization of %s.",
                                 strna(n1), path);
}

int chase_symlinks(
                const char *path,
                const char *original_root,
                ChaseSymlinksFlags flags,
                char **ret_path,
                int *ret_fd) {

        _cleanup_free_ char *buffer = NULL, *done = NULL, *root = NULL;
        _cleanup_close_ int fd = -1;
        unsigned max_follow = CHASE_SYMLINKS_MAX; /* how many symlinks to follow before giving up and returning ELOOP */
        bool exists = true, append_trail_slash = false;
        struct stat previous_stat;
        const char *todo;
        int r;

        assert(path);

        /* Either the file may be missing, or we return an fd to the final object, but both make no sense */
        if ((flags & CHASE_NONEXISTENT) && ret_fd)
                return -EINVAL;

        if ((flags & CHASE_STEP) && ret_fd)
                return -EINVAL;

        if (isempty(path))
                return -EINVAL;

        /* This is a lot like canonicalize_file_name(), but takes an additional "root" parameter, that allows following
         * symlinks relative to a root directory, instead of the root of the host.
         *
         * Note that "root" primarily matters if we encounter an absolute symlink. It is also used when following
         * relative symlinks to ensure they cannot be used to "escape" the root directory. The path parameter passed is
         * assumed to be already prefixed by it, except if the CHASE_PREFIX_ROOT flag is set, in which case it is first
         * prefixed accordingly.
         *
         * Algorithmically this operates on two path buffers: "done" are the components of the path we already
         * processed and resolved symlinks, "." and ".." of. "todo" are the components of the path we still need to
         * process. On each iteration, we move one component from "todo" to "done", processing it's special meaning
         * each time. The "todo" path always starts with at least one slash, the "done" path always ends in no
         * slash. We always keep an O_PATH fd to the component we are currently processing, thus keeping lookup races
         * to a minimum.
         *
         * Suggested usage: whenever you want to canonicalize a path, use this function. Pass the absolute path you got
         * as-is: fully qualified and relative to your host's root. Optionally, specify the root parameter to tell this
         * function what to do when encountering a symlink with an absolute path as directory: prefix it by the
         * specified path.
         *
         * There are five ways to invoke this function:
         *
         * 1. Without CHASE_STEP or ret_fd: in this case the path is resolved and the normalized path is
         *    returned in `ret_path`. The return value is < 0 on error. If CHASE_NONEXISTENT is also set, 0
         *    is returned if the file doesn't exist, > 0 otherwise. If CHASE_NONEXISTENT is not set, >= 0 is
         *    returned if the destination was found, -ENOENT if it wasn't.
         *
         * 2. With ret_fd: in this case the destination is opened after chasing it as O_PATH and this file
         *    descriptor is returned as return value. This is useful to open files relative to some root
         *    directory. Note that the returned O_PATH file descriptors must be converted into a regular one (using
         *    fd_reopen() or such) before it can be used for reading/writing. ret_fd may not be combined with
         *    CHASE_NONEXISTENT.
         *
         * 3. With CHASE_STEP: in this case only a single step of the normalization is executed, i.e. only the first
         *    symlink or ".." component of the path is resolved, and the resulting path is returned. This is useful if
         *    a caller wants to trace the path through the file system verbosely. Returns < 0 on error, > 0 if the
         *    path is fully normalized, and == 0 for each normalization step. This may be combined with
         *    CHASE_NONEXISTENT, in which case 1 is returned when a component is not found.
         *
         * 4. With CHASE_SAFE: in this case the path must not contain unsafe transitions, i.e. transitions from
         *    unprivileged to privileged files or directories. In such cases the return value is -ENOLINK. If
         *    CHASE_WARN is also set, a warning describing the unsafe transition is emitted.
         *
         * 5. With CHASE_NO_AUTOFS: in this case if an autofs mount point is encountered, path normalization
         *    is aborted and -EREMOTE is returned. If CHASE_WARN is also set, a warning showing the path of
         *    the mount point is emitted.
         */

        /* A root directory of "/" or "" is identical to none */
        if (empty_or_root(original_root))
                original_root = NULL;

        if (!original_root && !ret_path && !(flags & (CHASE_NONEXISTENT|CHASE_NO_AUTOFS|CHASE_SAFE|CHASE_STEP)) && ret_fd) {
                /* Shortcut the ret_fd case if the caller isn't interested in the actual path and has no root set
                 * and doesn't care about any of the other special features we provide either. */
                r = open(path, O_PATH|O_CLOEXEC|((flags & CHASE_NOFOLLOW) ? O_NOFOLLOW : 0));
                if (r < 0)
                        return -errno;

                *ret_fd = r;
                return 0;
        }

        if (original_root) {
                r = path_make_absolute_cwd(original_root, &root);
                if (r < 0)
                        return r;

                /* Simplify the root directory, so that it has no duplicate slashes and nothing at the
                 * end. While we won't resolve the root path we still simplify it. Note that dropping the
                 * trailing slash should not change behaviour, since when opening it we specify O_DIRECTORY
                 * anyway. Moreover at the end of this function after processing everything we'll always turn
                 * the empty string back to "/". */
                delete_trailing_chars(root, "/");
                path_simplify(root);

                if (flags & CHASE_PREFIX_ROOT) {
                        buffer = path_join(root, path);
                        if (!buffer)
                                return -ENOMEM;
                }
        }

        if (!buffer) {
                r = path_make_absolute_cwd(path, &buffer);
                if (r < 0)
                        return r;
        }

        fd = open(empty_to_root(root), O_CLOEXEC|O_DIRECTORY|O_PATH);
        if (fd < 0)
                return -errno;

        if (flags & CHASE_SAFE)
                if (fstat(fd, &previous_stat) < 0)
                        return -errno;

        if (flags & CHASE_TRAIL_SLASH)
                append_trail_slash = endswith(buffer, "/") || endswith(buffer, "/.");

        if (root) {
                /* If we are operating on a root directory, let's take the root directory as it is. */

                todo = path_startswith(buffer, root);
                if (!todo)
                        return log_full_errno(flags & CHASE_WARN ? LOG_WARNING : LOG_DEBUG,
                                              SYNTHETIC_ERRNO(ECHRNG),
                                              "Specified path '%s' is outside of specified root directory '%s', refusing to resolve.",
                                              path, root);

                done = strdup(root);
        } else {
                todo = buffer;
                done = strdup("/");
        }
        if (!done)
                return -ENOMEM;

        for (;;) {
                _cleanup_free_ char *first = NULL;
                _cleanup_close_ int child = -1;
                struct stat st;
                const char *e;

                r = path_find_first_component(&todo, /* accept_dot_dot= */ true, &e);
                if (r < 0)
                        return r;
                if (r == 0) { /* We reached the end. */
                        if (append_trail_slash)
                                if (!strextend(&done, "/"))
                                        return -ENOMEM;
                        break;
                }

                first = strndup(e, r);
                if (!first)
                        return -ENOMEM;

                /* Two dots? Then chop off the last bit of what we already found out. */
                if (path_equal(first, "..")) {
                        _cleanup_free_ char *parent = NULL;
                        _cleanup_close_ int fd_parent = -1;

                        /* If we already are at the top, then going up will not change anything. This is in-line with
                         * how the kernel handles this. */
                        if (empty_or_root(done))
                                continue;

                        r = path_extract_directory(done, &parent);
                        if (r < 0)
                                return r;

                        /* Don't allow this to leave the root dir.  */
                        if (root &&
                            path_startswith(done, root) &&
                            !path_startswith(parent, root))
                                continue;

                        free_and_replace(done, parent);

                        if (flags & CHASE_STEP)
                                goto chased_one;

                        fd_parent = openat(fd, "..", O_CLOEXEC|O_NOFOLLOW|O_PATH);
                        if (fd_parent < 0)
                                return -errno;

                        if (flags & CHASE_SAFE) {
                                if (fstat(fd_parent, &st) < 0)
                                        return -errno;

                                if (unsafe_transition(&previous_stat, &st))
                                        return log_unsafe_transition(fd, fd_parent, path, flags);

                                previous_stat = st;
                        }

                        close_and_replace(fd, fd_parent);

                        continue;
                }

                /* Otherwise let's see what this is. */
                child = openat(fd, first, O_CLOEXEC|O_NOFOLLOW|O_PATH);
                if (child < 0) {
                        if (errno == ENOENT &&
                            (flags & CHASE_NONEXISTENT) &&
                            (isempty(todo) || path_is_safe(todo))) {
                                /* If CHASE_NONEXISTENT is set, and the path does not exist, then
                                 * that's OK, return what we got so far. But don't allow this if the
                                 * remaining path contains "../" or something else weird. */

                                if (!path_extend(&done, first, todo))
                                        return -ENOMEM;

                                exists = false;
                                break;
                        }

                        return -errno;
                }

                if (fstat(child, &st) < 0)
                        return -errno;
                if ((flags & CHASE_SAFE) &&
                    unsafe_transition(&previous_stat, &st))
                        return log_unsafe_transition(fd, child, path, flags);

                previous_stat = st;

                if ((flags & CHASE_NO_AUTOFS) &&
                    fd_is_fs_type(child, AUTOFS_SUPER_MAGIC) > 0)
                        return log_autofs_mount_point(child, path, flags);

                if (S_ISLNK(st.st_mode) && !((flags & CHASE_NOFOLLOW) && isempty(todo))) {
                        _cleanup_free_ char *destination = NULL;

                        /* This is a symlink, in this case read the destination. But let's make sure we
                         * don't follow symlinks without bounds. */
                        if (--max_follow <= 0)
                                return -ELOOP;

                        r = readlinkat_malloc(fd, first, &destination);
                        if (r < 0)
                                return r;
                        if (isempty(destination))
                                return -EINVAL;

                        if (path_is_absolute(destination)) {

                                /* An absolute destination. Start the loop from the beginning, but use the root
                                 * directory as base. */

                                safe_close(fd);
                                fd = open(empty_to_root(root), O_CLOEXEC|O_DIRECTORY|O_PATH);
                                if (fd < 0)
                                        return -errno;

                                if (flags & CHASE_SAFE) {
                                        if (fstat(fd, &st) < 0)
                                                return -errno;

                                        if (unsafe_transition(&previous_stat, &st))
                                                return log_unsafe_transition(child, fd, path, flags);

                                        previous_stat = st;
                                }

                                /* Note that we do not revalidate the root, we take it as is. */
                                r = free_and_strdup(&done, empty_to_root(root));
                                if (r < 0)
                                        return r;
                        }

                        /* Prefix what's left to do with what we just read, and start the loop again, but
                         * remain in the current directory. */
                        if (!path_extend(&destination, todo))
                                return -ENOMEM;

                        free_and_replace(buffer, destination);
                        todo = buffer;

                        if (flags & CHASE_STEP)
                                goto chased_one;

                        continue;
                }

                /* If this is not a symlink, then let's just add the name we read to what we already verified. */
                if (!path_extend(&done, first))
                        return -ENOMEM;

                /* And iterate again, but go one directory further down. */
                close_and_replace(fd, child);
        }

        if (ret_path)
                *ret_path = TAKE_PTR(done);

        if (ret_fd) {
                /* Return the O_PATH fd we currently are looking to the caller. It can translate it to a
                 * proper fd by opening /proc/self/fd/xyz. */

                assert(fd >= 0);
                *ret_fd = TAKE_FD(fd);
        }

        if (flags & CHASE_STEP)
                return 1;

        return exists;

chased_one:
        if (ret_path) {
                const char *e;

                /* todo may contain slashes at the beginning. */
                r = path_find_first_component(&todo, /* accept_dot_dot= */ true, &e);
                if (r < 0)
                        return r;
                if (r == 0)
                        *ret_path = TAKE_PTR(done);
                else {
                        char *c;

                        c = path_join(done, e);
                        if (!c)
                                return -ENOMEM;

                        *ret_path = c;
                }
        }

        return 0;
}

int chase_symlinks_and_open(
                const char *path,
                const char *root,
                ChaseSymlinksFlags chase_flags,
                int open_flags,
                char **ret_path) {

        _cleanup_close_ int path_fd = -1;
        _cleanup_free_ char *p = NULL;
        int r;

        if (chase_flags & (CHASE_NONEXISTENT|CHASE_STEP))
                return -EINVAL;

        if (empty_or_root(root) && !ret_path && (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE)) == 0) {
                /* Shortcut this call if none of the special features of this call are requested */
                r = open(path, open_flags | (FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? O_NOFOLLOW : 0));
                if (r < 0)
                        return -errno;

                return r;
        }

        r = chase_symlinks(path, root, chase_flags, ret_path ? &p : NULL, &path_fd);
        if (r < 0)
                return r;
        assert(path_fd >= 0);

        r = fd_reopen(path_fd, open_flags);
        if (r < 0)
                return r;

        if (ret_path)
                *ret_path = TAKE_PTR(p);

        return r;
}

int chase_symlinks_and_opendir(
                const char *path,
                const char *root,
                ChaseSymlinksFlags chase_flags,
                char **ret_path,
                DIR **ret_dir) {

        _cleanup_close_ int path_fd = -1;
        _cleanup_free_ char *p = NULL;
        DIR *d;
        int r;

        if (!ret_dir)
                return -EINVAL;
        if (chase_flags & (CHASE_NONEXISTENT|CHASE_STEP))
                return -EINVAL;

        if (empty_or_root(root) && !ret_path && (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE)) == 0) {
                /* Shortcut this call if none of the special features of this call are requested */
                d = opendir(path);
                if (!d)
                        return -errno;

                *ret_dir = d;
                return 0;
        }

        r = chase_symlinks(path, root, chase_flags, ret_path ? &p : NULL, &path_fd);
        if (r < 0)
                return r;
        assert(path_fd >= 0);

        d = opendir(FORMAT_PROC_FD_PATH(path_fd));
        if (!d)
                return -errno;

        if (ret_path)
                *ret_path = TAKE_PTR(p);

        *ret_dir = d;
        return 0;
}

int chase_symlinks_and_stat(
                const char *path,
                const char *root,
                ChaseSymlinksFlags chase_flags,
                char **ret_path,
                struct stat *ret_stat,
                int *ret_fd) {

        _cleanup_close_ int path_fd = -1;
        _cleanup_free_ char *p = NULL;
        int r;

        assert(path);
        assert(ret_stat);

        if (chase_flags & (CHASE_NONEXISTENT|CHASE_STEP))
                return -EINVAL;

        if (empty_or_root(root) && !ret_path && (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE)) == 0 && !ret_fd) {
                /* Shortcut this call if none of the special features of this call are requested */

                if (fstatat(AT_FDCWD, path, ret_stat, FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? AT_SYMLINK_NOFOLLOW : 0) < 0)
                        return -errno;

                return 1;
        }

        r = chase_symlinks(path, root, chase_flags, ret_path ? &p : NULL, &path_fd);
        if (r < 0)
                return r;
        assert(path_fd >= 0);

        if (fstat(path_fd, ret_stat) < 0)
                return -errno;

        if (ret_path)
                *ret_path = TAKE_PTR(p);
        if (ret_fd)
                *ret_fd = TAKE_FD(path_fd);

        return 1;
}

int chase_symlinks_and_access(
                const char *path,
                const char *root,
                ChaseSymlinksFlags chase_flags,
                int access_mode,
                char **ret_path,
                int *ret_fd) {

        _cleanup_close_ int path_fd = -1;
        _cleanup_free_ char *p = NULL;
        int r;

        assert(path);

        if (chase_flags & (CHASE_NONEXISTENT|CHASE_STEP))
                return -EINVAL;

        if (empty_or_root(root) && !ret_path && (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE)) == 0 && !ret_fd) {
                /* Shortcut this call if none of the special features of this call are requested */

                if (faccessat(AT_FDCWD, path, access_mode, FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? AT_SYMLINK_NOFOLLOW : 0) < 0)
                        return -errno;

                return 1;
        }

        r = chase_symlinks(path, root, chase_flags, ret_path ? &p : NULL, &path_fd);
        if (r < 0)
                return r;
        assert(path_fd >= 0);

        r = access_fd(path_fd, access_mode);
        if (r < 0)
                return r;

        if (ret_path)
                *ret_path = TAKE_PTR(p);
        if (ret_fd)
                *ret_fd = TAKE_FD(path_fd);

        return 1;
}

int chase_symlinks_and_fopen_unlocked(
                const char *path,
                const char *root,
                ChaseSymlinksFlags chase_flags,
                const char *open_flags,
                char **ret_path,
                FILE **ret_file) {

        _cleanup_free_ char *final_path = NULL;
        _cleanup_close_ int fd = -1;
        int mode_flags, r;

        assert(path);
        assert(open_flags);
        assert(ret_file);

        mode_flags = fopen_mode_to_flags(open_flags);
        if (mode_flags < 0)
                return mode_flags;

        fd = chase_symlinks_and_open(path, root, chase_flags, mode_flags, ret_path ? &final_path : NULL);
        if (fd < 0)
                return fd;

        r = take_fdopen_unlocked(&fd, open_flags, ret_file);
        if (r < 0)
                return r;

        if (ret_path)
                *ret_path = TAKE_PTR(final_path);

        return 0;
}
Commit	Line	Data
f4351959 LP	1	/* SPDX-License-Identifier: LGPL-2.1-or-later */
	2
	3	#include <linux/magic.h>
	4
	5	#include "alloc-util.h"
	6	#include "chase-symlinks.h"
	7	#include "fd-util.h"
01bebba3	8	#include "fileio.h"
f4351959 LP	9	#include "fs-util.h"
	10	#include "glyph-util.h"
	11	#include "log.h"
	12	#include "path-util.h"
	13	#include "string-util.h"
	14	#include "user-util.h"
	15
	16	bool unsafe_transition(const struct stat a, const struct stat b) {
	17	/* Returns true if the transition from a to b is safe, i.e. that we never transition from unprivileged to
	18	* privileged files or directories. Why bother? So that unprivileged code can't symlink to privileged files
	19	* making us believe we read something safe even though it isn't safe in the specific context we open it in. */
	20
	21	if (a->st_uid == 0) /* Transitioning from privileged to unprivileged is always fine */
	22	return false;
	23
	24	return a->st_uid != b->st_uid; /* Otherwise we need to stay within the same UID */
	25	}
	26
8f47f880	27	static int log_unsafe_transition(int a, int b, const char *path, ChaseSymlinksFlags flags) {
f4351959 LP	28	_cleanup_free_ char n1 = NULL, n2 = NULL, user_a = NULL, user_b = NULL;
	29	struct stat st;
	30
	31	if (!FLAGS_SET(flags, CHASE_WARN))
	32	return -ENOLINK;
	33
	34	(void) fd_get_path(a, &n1);
	35	(void) fd_get_path(b, &n2);
	36
	37	if (fstat(a, &st) == 0)
	38	user_a = uid_to_name(st.st_uid);
	39	if (fstat(b, &st) == 0)
	40	user_b = uid_to_name(st.st_uid);
	41
	42	return log_warning_errno(SYNTHETIC_ERRNO(ENOLINK),
	43	"Detected unsafe path transition %s (owned by %s) %s %s (owned by %s) during canonicalization of %s.",
fc03e80c	44	strna(n1), strna(user_a), special_glyph(SPECIAL_GLYPH_ARROW_RIGHT), strna(n2), strna(user_b), path);
f4351959 LP	45	}
f4351959 LP	46
8f47f880	47	static int log_autofs_mount_point(int fd, const char *path, ChaseSymlinksFlags flags) {
f4351959 LP	48	_cleanup_free_ char *n1 = NULL;
	49
	50	if (!FLAGS_SET(flags, CHASE_WARN))
	51	return -EREMOTE;
	52
	53	(void) fd_get_path(fd, &n1);
	54
	55	return log_warning_errno(SYNTHETIC_ERRNO(EREMOTE),
	56	"Detected autofs mount point %s during canonicalization of %s.",
	57	strna(n1), path);
	58	}
	59
8f47f880 LP	60	int chase_symlinks(
	61	const char *path,
	62	const char *original_root,
	63	ChaseSymlinksFlags flags,
	64	char **ret_path,
	65	int *ret_fd) {
	66
f4351959 LP	67	_cleanup_free_ char buffer = NULL, done = NULL, *root = NULL;
	68	_cleanup_close_ int fd = -1;
	69	unsigned max_follow = CHASE_SYMLINKS_MAX; /* how many symlinks to follow before giving up and returning ELOOP */
	70	bool exists = true, append_trail_slash = false;
	71	struct stat previous_stat;
	72	const char *todo;
	73	int r;
	74
	75	assert(path);
	76
	77	/* Either the file may be missing, or we return an fd to the final object, but both make no sense */
	78	if ((flags & CHASE_NONEXISTENT) && ret_fd)
	79	return -EINVAL;
	80
	81	if ((flags & CHASE_STEP) && ret_fd)
	82	return -EINVAL;
	83
	84	if (isempty(path))
	85	return -EINVAL;
	86
	87	/* This is a lot like canonicalize_file_name(), but takes an additional "root" parameter, that allows following
	88	* symlinks relative to a root directory, instead of the root of the host.
	89	*
	90	* Note that "root" primarily matters if we encounter an absolute symlink. It is also used when following
	91	* relative symlinks to ensure they cannot be used to "escape" the root directory. The path parameter passed is
	92	* assumed to be already prefixed by it, except if the CHASE_PREFIX_ROOT flag is set, in which case it is first
	93	* prefixed accordingly.
	94	*
	95	* Algorithmically this operates on two path buffers: "done" are the components of the path we already
	96	* processed and resolved symlinks, "." and ".." of. "todo" are the components of the path we still need to
	97	* process. On each iteration, we move one component from "todo" to "done", processing it's special meaning
	98	* each time. The "todo" path always starts with at least one slash, the "done" path always ends in no
	99	* slash. We always keep an O_PATH fd to the component we are currently processing, thus keeping lookup races
	100	* to a minimum.
	101	*
	102	* Suggested usage: whenever you want to canonicalize a path, use this function. Pass the absolute path you got
	103	* as-is: fully qualified and relative to your host's root. Optionally, specify the root parameter to tell this
	104	* function what to do when encountering a symlink with an absolute path as directory: prefix it by the
	105	* specified path.
	106	*
	107	* There are five ways to invoke this function:
	108	*
	109	* 1. Without CHASE_STEP or ret_fd: in this case the path is resolved and the normalized path is
	110	* returned in `ret_path`. The return value is < 0 on error. If CHASE_NONEXISTENT is also set, 0
	111	* is returned if the file doesn't exist, > 0 otherwise. If CHASE_NONEXISTENT is not set, >= 0 is
	112	* returned if the destination was found, -ENOENT if it wasn't.
	113	*
	114	* 2. With ret_fd: in this case the destination is opened after chasing it as O_PATH and this file
	115	* descriptor is returned as return value. This is useful to open files relative to some root
	116	* directory. Note that the returned O_PATH file descriptors must be converted into a regular one (using
	117	* fd_reopen() or such) before it can be used for reading/writing. ret_fd may not be combined with
	118	* CHASE_NONEXISTENT.
	119	*
	120	* 3. With CHASE_STEP: in this case only a single step of the normalization is executed, i.e. only the first
	121	* symlink or ".." component of the path is resolved, and the resulting path is returned. This is useful if
	122	* a caller wants to trace the path through the file system verbosely. Returns < 0 on error, > 0 if the
	123	* path is fully normalized, and == 0 for each normalization step. This may be combined with
	124	* CHASE_NONEXISTENT, in which case 1 is returned when a component is not found.
	125	*
	126	* 4. With CHASE_SAFE: in this case the path must not contain unsafe transitions, i.e. transitions from
	127	* unprivileged to privileged files or directories. In such cases the return value is -ENOLINK. If
	128	* CHASE_WARN is also set, a warning describing the unsafe transition is emitted.
	129	*
	130	* 5. With CHASE_NO_AUTOFS: in this case if an autofs mount point is encountered, path normalization
131	* is aborted and -EREMOTE is returned. If CHASE_WARN is also set, a warning showing the path of
132	* the mount point is emitted.
133	*/
134
135	/* A root directory of "/" or "" is identical to none */
136	if (empty_or_root(original_root))
137	original_root = NULL;
138
139	if (!original_root && !ret_path && !(flags & (CHASE_NONEXISTENT\|CHASE_NO_AUTOFS\|CHASE_SAFE\|CHASE_STEP)) && ret_fd) {
140	/* Shortcut the ret_fd case if the caller isn't interested in the actual path and has no root set
141	* and doesn't care about any of the other special features we provide either. */
142	r = open(path, O_PATH\|O_CLOEXEC\|((flags & CHASE_NOFOLLOW) ? O_NOFOLLOW : 0));
143	if (r < 0)
144	return -errno;
145
146	*ret_fd = r;
147	return 0;
148	}
149
150	if (original_root) {
151	r = path_make_absolute_cwd(original_root, &root);
152	if (r < 0)
153	return r;
154
155	/* Simplify the root directory, so that it has no duplicate slashes and nothing at the
156	* end. While we won't resolve the root path we still simplify it. Note that dropping the
157	* trailing slash should not change behaviour, since when opening it we specify O_DIRECTORY
158	* anyway. Moreover at the end of this function after processing everything we'll always turn
159	* the empty string back to "/". */
160	delete_trailing_chars(root, "/");
161	path_simplify(root);
162
163	if (flags & CHASE_PREFIX_ROOT) {
7b9be862 LP	164	buffer = path_join(root, path);
	165	if (!buffer)
	166	return -ENOMEM;
f4351959 LP	167	}
	168	}
	169
7b9be862 LP	170	if (!buffer) {
	171	r = path_make_absolute_cwd(path, &buffer);
	172	if (r < 0)
	173	return r;
	174	}
f4351959	175
69cf392f	176	fd = open(empty_to_root(root), O_CLOEXEC\|O_DIRECTORY\|O_PATH);
f4351959 LP	177	if (fd < 0)
	178	return -errno;
	179
	180	if (flags & CHASE_SAFE)
	181	if (fstat(fd, &previous_stat) < 0)
	182	return -errno;
	183
	184	if (flags & CHASE_TRAIL_SLASH)
	185	append_trail_slash = endswith(buffer, "/") \|\| endswith(buffer, "/.");
	186
	187	if (root) {
	188	/* If we are operating on a root directory, let's take the root directory as it is. */
	189
	190	todo = path_startswith(buffer, root);
	191	if (!todo)
	192	return log_full_errno(flags & CHASE_WARN ? LOG_WARNING : LOG_DEBUG,
	193	SYNTHETIC_ERRNO(ECHRNG),
	194	"Specified path '%s' is outside of specified root directory '%s', refusing to resolve.",
	195	path, root);
	196
	197	done = strdup(root);
	198	} else {
	199	todo = buffer;
	200	done = strdup("/");
	201	}
0ac6cdd6 LP	202	if (!done)
0ac6cdd6 LP	203	return -ENOMEM;
f4351959 LP	204
	205	for (;;) {
	206	_cleanup_free_ char *first = NULL;
	207	_cleanup_close_ int child = -1;
	208	struct stat st;
	209	const char *e;
	210
860f4c6a	211	r = path_find_first_component(&todo, /* accept_dot_dot= */ true, &e);
f4351959 LP	212	if (r < 0)
	213	return r;
	214	if (r == 0) { /* We reached the end. */
	215	if (append_trail_slash)
	216	if (!strextend(&done, "/"))
	217	return -ENOMEM;
	218	break;
	219	}
	220
	221	first = strndup(e, r);
	222	if (!first)
	223	return -ENOMEM;
	224
	225	/* Two dots? Then chop off the last bit of what we already found out. */
	226	if (path_equal(first, "..")) {
	227	_cleanup_free_ char *parent = NULL;
	228	_cleanup_close_ int fd_parent = -1;
	229
	230	/* If we already are at the top, then going up will not change anything. This is in-line with
	231	* how the kernel handles this. */
	232	if (empty_or_root(done))
	233	continue;
	234
57f9ca3a LP	235	r = path_extract_directory(done, &parent);
	236	if (r < 0)
	237	return r;
f4351959 LP	238
	239	/* Don't allow this to leave the root dir. */
	240	if (root &&
	241	path_startswith(done, root) &&
	242	!path_startswith(parent, root))
	243	continue;
	244
	245	free_and_replace(done, parent);
	246
	247	if (flags & CHASE_STEP)
	248	goto chased_one;
	249
	250	fd_parent = openat(fd, "..", O_CLOEXEC\|O_NOFOLLOW\|O_PATH);
	251	if (fd_parent < 0)
	252	return -errno;
	253
	254	if (flags & CHASE_SAFE) {
	255	if (fstat(fd_parent, &st) < 0)
	256	return -errno;
	257
	258	if (unsafe_transition(&previous_stat, &st))
	259	return log_unsafe_transition(fd, fd_parent, path, flags);
	260
	261	previous_stat = st;
	262	}
	263
ee3455cf	264	close_and_replace(fd, fd_parent);
f4351959 LP	265
	266	continue;
	267	}
	268
	269	/* Otherwise let's see what this is. */
	270	child = openat(fd, first, O_CLOEXEC\|O_NOFOLLOW\|O_PATH);
	271	if (child < 0) {
	272	if (errno == ENOENT &&
	273	(flags & CHASE_NONEXISTENT) &&
	274	(isempty(todo) \|\| path_is_safe(todo))) {
	275	/* If CHASE_NONEXISTENT is set, and the path does not exist, then
	276	* that's OK, return what we got so far. But don't allow this if the
	277	* remaining path contains "../" or something else weird. */
	278
	279	if (!path_extend(&done, first, todo))
	280	return -ENOMEM;
	281
	282	exists = false;
	283	break;
	284	}
	285
	286	return -errno;
	287	}
	288
	289	if (fstat(child, &st) < 0)
	290	return -errno;
	291	if ((flags & CHASE_SAFE) &&
	292	unsafe_transition(&previous_stat, &st))
	293	return log_unsafe_transition(fd, child, path, flags);
	294
	295	previous_stat = st;
	296
	297	if ((flags & CHASE_NO_AUTOFS) &&
	298	fd_is_fs_type(child, AUTOFS_SUPER_MAGIC) > 0)
	299	return log_autofs_mount_point(child, path, flags);
	300
	301	if (S_ISLNK(st.st_mode) && !((flags & CHASE_NOFOLLOW) && isempty(todo))) {
	302	_cleanup_free_ char *destination = NULL;
	303
	304	/* This is a symlink, in this case read the destination. But let's make sure we
	305	* don't follow symlinks without bounds. */
	306	if (--max_follow <= 0)
	307	return -ELOOP;
	308
	309	r = readlinkat_malloc(fd, first, &destination);
	310	if (r < 0)
	311	return r;
	312	if (isempty(destination))
	313	return -EINVAL;
	314
	315	if (path_is_absolute(destination)) {
	316
	317	/* An absolute destination. Start the loop from the beginning, but use the root
	318	* directory as base. */
	319
	320	safe_close(fd);
69cf392f	321	fd = open(empty_to_root(root), O_CLOEXEC\|O_DIRECTORY\|O_PATH);
f4351959 LP	322	if (fd < 0)
	323	return -errno;
	324
	325	if (flags & CHASE_SAFE) {
	326	if (fstat(fd, &st) < 0)
	327	return -errno;
	328
	329	if (unsafe_transition(&previous_stat, &st))
	330	return log_unsafe_transition(child, fd, path, flags);
	331
	332	previous_stat = st;
	333	}
	334
	335	/* Note that we do not revalidate the root, we take it as is. */
	336	r = free_and_strdup(&done, empty_to_root(root));
	337	if (r < 0)
	338	return r;
	339	}
	340
	341	/* Prefix what's left to do with what we just read, and start the loop again, but
	342	* remain in the current directory. */
	343	if (!path_extend(&destination, todo))
	344	return -ENOMEM;
	345
	346	free_and_replace(buffer, destination);
	347	todo = buffer;
	348
	349	if (flags & CHASE_STEP)
	350	goto chased_one;
	351
	352	continue;
	353	}
	354
	355	/* If this is not a symlink, then let's just add the name we read to what we already verified. */
	356	if (!path_extend(&done, first))
	357	return -ENOMEM;
	358
	359	/* And iterate again, but go one directory further down. */
ee3455cf	360	close_and_replace(fd, child);
f4351959 LP	361	}
	362
	363	if (ret_path)
	364	*ret_path = TAKE_PTR(done);
	365
	366	if (ret_fd) {
	367	/* Return the O_PATH fd we currently are looking to the caller. It can translate it to a
	368	* proper fd by opening /proc/self/fd/xyz. */
	369
	370	assert(fd >= 0);
	371	*ret_fd = TAKE_FD(fd);
	372	}
	373
	374	if (flags & CHASE_STEP)
	375	return 1;
	376
	377	return exists;
	378
	379	chased_one:
	380	if (ret_path) {
	381	const char *e;
	382
	383	/* todo may contain slashes at the beginning. */
860f4c6a	384	r = path_find_first_component(&todo, /* accept_dot_dot= */ true, &e);
f4351959 LP	385	if (r < 0)
	386	return r;
	387	if (r == 0)
	388	*ret_path = TAKE_PTR(done);
	389	else {
	390	char *c;
	391
	392	c = path_join(done, e);
	393	if (!c)
	394	return -ENOMEM;
	395
	396	*ret_path = c;
	397	}
	398	}
	399
	400	return 0;
	401	}
	402
	403	int chase_symlinks_and_open(
	404	const char *path,
	405	const char *root,
8f47f880	406	ChaseSymlinksFlags chase_flags,
f4351959 LP	407	int open_flags,
	408	char **ret_path) {
	409
	410	_cleanup_close_ int path_fd = -1;
	411	_cleanup_free_ char *p = NULL;
	412	int r;
	413
81a7eac1	414	if (chase_flags & (CHASE_NONEXISTENT\|CHASE_STEP))
f4351959 LP	415	return -EINVAL;
	416
	417	if (empty_or_root(root) && !ret_path && (chase_flags & (CHASE_NO_AUTOFS\|CHASE_SAFE)) == 0) {
	418	/* Shortcut this call if none of the special features of this call are requested */
69570232	419	r = open(path, open_flags \| (FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? O_NOFOLLOW : 0));
f4351959 LP	420	if (r < 0)
	421	return -errno;
	422
	423	return r;
	424	}
	425
	426	r = chase_symlinks(path, root, chase_flags, ret_path ? &p : NULL, &path_fd);
	427	if (r < 0)
	428	return r;
	429	assert(path_fd >= 0);
	430
	431	r = fd_reopen(path_fd, open_flags);
	432	if (r < 0)
	433	return r;
	434
	435	if (ret_path)
	436	*ret_path = TAKE_PTR(p);
	437
	438	return r;
	439	}
	440
	441	int chase_symlinks_and_opendir(
	442	const char *path,
	443	const char *root,
8f47f880	444	ChaseSymlinksFlags chase_flags,
f4351959 LP	445	char **ret_path,
	446	DIR **ret_dir) {
	447
	448	_cleanup_close_ int path_fd = -1;
	449	_cleanup_free_ char *p = NULL;
	450	DIR *d;
	451	int r;
	452
	453	if (!ret_dir)
	454	return -EINVAL;
81a7eac1	455	if (chase_flags & (CHASE_NONEXISTENT\|CHASE_STEP))
f4351959 LP	456	return -EINVAL;
	457
	458	if (empty_or_root(root) && !ret_path && (chase_flags & (CHASE_NO_AUTOFS\|CHASE_SAFE)) == 0) {
	459	/* Shortcut this call if none of the special features of this call are requested */
	460	d = opendir(path);
	461	if (!d)
	462	return -errno;
	463
	464	*ret_dir = d;
	465	return 0;
	466	}
	467
	468	r = chase_symlinks(path, root, chase_flags, ret_path ? &p : NULL, &path_fd);
	469	if (r < 0)
	470	return r;
	471	assert(path_fd >= 0);
	472
	473	d = opendir(FORMAT_PROC_FD_PATH(path_fd));
	474	if (!d)
	475	return -errno;
	476
	477	if (ret_path)
	478	*ret_path = TAKE_PTR(p);
	479
	480	*ret_dir = d;
	481	return 0;
	482	}
	483
	484	int chase_symlinks_and_stat(
	485	const char *path,
	486	const char *root,
8f47f880	487	ChaseSymlinksFlags chase_flags,
f4351959 LP	488	char **ret_path,
	489	struct stat *ret_stat,
	490	int *ret_fd) {
	491
	492	_cleanup_close_ int path_fd = -1;
	493	_cleanup_free_ char *p = NULL;
	494	int r;
	495
	496	assert(path);
	497	assert(ret_stat);
	498
81a7eac1	499	if (chase_flags & (CHASE_NONEXISTENT\|CHASE_STEP))
f4351959 LP	500	return -EINVAL;
f4351959 LP	501
37b9bc56	502	if (empty_or_root(root) && !ret_path && (chase_flags & (CHASE_NO_AUTOFS\|CHASE_SAFE)) == 0 && !ret_fd) {
f4351959	503	/* Shortcut this call if none of the special features of this call are requested */
69570232 LP	504
69570232 LP	505	if (fstatat(AT_FDCWD, path, ret_stat, FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? AT_SYMLINK_NOFOLLOW : 0) < 0)
f4351959 LP	506	return -errno;
	507
	508	return 1;
	509	}
	510
	511	r = chase_symlinks(path, root, chase_flags, ret_path ? &p : NULL, &path_fd);
	512	if (r < 0)
	513	return r;
	514	assert(path_fd >= 0);
	515
	516	if (fstat(path_fd, ret_stat) < 0)
	517	return -errno;
	518
	519	if (ret_path)
	520	*ret_path = TAKE_PTR(p);
2b2caea2 LP	521	if (ret_fd)
	522	*ret_fd = TAKE_FD(path_fd);
	523
	524	return 1;
	525	}
	526
	527	int chase_symlinks_and_access(
	528	const char *path,
	529	const char *root,
	530	ChaseSymlinksFlags chase_flags,
	531	int access_mode,
	532	char **ret_path,
	533	int *ret_fd) {
	534
	535	_cleanup_close_ int path_fd = -1;
	536	_cleanup_free_ char *p = NULL;
	537	int r;
	538
	539	assert(path);
	540
	541	if (chase_flags & (CHASE_NONEXISTENT\|CHASE_STEP))
	542	return -EINVAL;
	543
	544	if (empty_or_root(root) && !ret_path && (chase_flags & (CHASE_NO_AUTOFS\|CHASE_SAFE)) == 0 && !ret_fd) {
	545	/* Shortcut this call if none of the special features of this call are requested */
	546
	547	if (faccessat(AT_FDCWD, path, access_mode, FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? AT_SYMLINK_NOFOLLOW : 0) < 0)
	548	return -errno;
	549
	550	return 1;
	551	}
	552
	553	r = chase_symlinks(path, root, chase_flags, ret_path ? &p : NULL, &path_fd);
	554	if (r < 0)
	555	return r;
	556	assert(path_fd >= 0);
	557
	558	r = access_fd(path_fd, access_mode);
	559	if (r < 0)
	560	return r;
	561
	562	if (ret_path)
	563	*ret_path = TAKE_PTR(p);
f4351959 LP	564	if (ret_fd)
	565	*ret_fd = TAKE_FD(path_fd);
	566
	567	return 1;
	568	}
01bebba3 LP	569
	570	int chase_symlinks_and_fopen_unlocked(
	571	const char *path,
	572	const char *root,
8f47f880	573	ChaseSymlinksFlags chase_flags,
01bebba3 LP	574	const char *open_flags,
	575	char **ret_path,
	576	FILE **ret_file) {
	577
	578	_cleanup_free_ char *final_path = NULL;
	579	_cleanup_close_ int fd = -1;
	580	int mode_flags, r;
	581
	582	assert(path);
	583	assert(open_flags);
	584	assert(ret_file);
	585
	586	mode_flags = fopen_mode_to_flags(open_flags);
	587	if (mode_flags < 0)
	588	return mode_flags;
	589
	590	fd = chase_symlinks_and_open(path, root, chase_flags, mode_flags, ret_path ? &final_path : NULL);
	591	if (fd < 0)
	592	return fd;
	593
	594	r = take_fdopen_unlocked(&fd, open_flags, ret_file);
	595	if (r < 0)
	596	return r;
	597
	598	if (ret_path)
	599	*ret_path = TAKE_PTR(final_path);
	600
	601	return 0;
	602	}