src/shared/switch-root.c

   1 /* SPDX-License-Identifier: LGPL-2.1-or-later */
   2
   3 #include <errno.h>
   4 #include <fcntl.h>
   5 #include <limits.h>
   6 #include <stdbool.h>
   7 #include <sys/mount.h>
   8 #include <sys/stat.h>
   9 #include <unistd.h>
  10
  11 #include "base-filesystem.h"
  12 #include "chase.h"
  13 #include "fd-util.h"
  14 #include "initrd-util.h"
  15 #include "log.h"
  16 #include "missing_syscall.h"
  17 #include "mkdir-label.h"
  18 #include "mount-util.h"
  19 #include "mountpoint-util.h"
  20 #include "path-util.h"
  21 #include "rm-rf.h"
  22 #include "stdio-util.h"
  23 #include "string-util.h"
  24 #include "strv.h"
  25 #include "switch-root.h"
  26 #include "user-util.h"
  27
  28 int switch_root(const char *new_root,
  29                 const char *old_root_after, /* path below the new root, where to place the old root after the transition */
  30                 bool unmount_old_root,
  31                 unsigned long mount_flags) {  /* MS_MOVE or MS_BIND */
  32
  33         _cleanup_free_ char *resolved_old_root_after = NULL;
  34         _cleanup_close_ int old_root_fd = -EBADF;
  35         int r;
  36
  37         assert(new_root);
  38         assert(old_root_after);
  39
  40         if (path_equal(new_root, "/"))
  41                 return 0;
  42
  43         /* Check if we shall remove the contents of the old root */
  44         old_root_fd = open("/", O_RDONLY | O_CLOEXEC | O_DIRECTORY);
  45         if (old_root_fd < 0)
  46                 return log_error_errno(errno, "Failed to open root directory: %m");
  47         r = fd_is_temporary_fs(old_root_fd);
  48         if (r < 0)
  49                 return log_error_errno(r, "Failed to stat root directory: %m");
  50         if (r > 0)
  51                 log_debug("Root directory is on tmpfs, will do cleanup later.");
  52         else
  53                 old_root_fd = safe_close(old_root_fd);
  54
  55         /* Determine where we shall place the old root after the transition */
  56         r = chase(old_root_after, new_root, CHASE_PREFIX_ROOT|CHASE_NONEXISTENT, &resolved_old_root_after, NULL);
  57         if (r < 0)
  58                 return log_error_errno(r, "Failed to resolve %s/%s: %m", new_root, old_root_after);
  59         if (r == 0) /* Doesn't exist yet. Let's create it */
  60                 (void) mkdir_p_label(resolved_old_root_after, 0755);
  61
  62         /* Work-around for kernel design: the kernel refuses MS_MOVE if any file systems are mounted MS_SHARED. Hence
  63          * remount them MS_PRIVATE here as a work-around.
  64          *
  65          * https://bugzilla.redhat.com/show_bug.cgi?id=847418 */
  66         if (mount(NULL, "/", NULL, MS_REC|MS_PRIVATE, NULL) < 0)
  67                 return log_error_errno(errno, "Failed to set \"/\" mount propagation to private: %m");
  68
  69         FOREACH_STRING(path, "/sys", "/dev", "/run", "/proc") {
  70                 _cleanup_free_ char *chased = NULL;
  71
  72                 r = chase(path, new_root, CHASE_PREFIX_ROOT|CHASE_NONEXISTENT, &chased, NULL);
  73                 if (r < 0)
  74                         return log_error_errno(r, "Failed to resolve %s/%s: %m", new_root, path);
  75                 if (r > 0) {
  76                         /* Already exists. Let's see if it is a mount point already. */
  77                         r = path_is_mount_point(chased, NULL, 0);
  78                         if (r < 0)
  79                                 return log_error_errno(r, "Failed to determine whether %s is a mount point: %m", chased);
  80                         if (r > 0) /* If it is already mounted, then do nothing */
  81                                 continue;
  82                 } else
  83                          /* Doesn't exist yet? */
  84                         (void) mkdir_p_label(chased, 0755);
  85
  86                 if (mount(path, chased, NULL, mount_flags, NULL) < 0)
  87                         return log_error_errno(errno, "Failed to mount %s to %s: %m", path, chased);
  88         }
  89
  90         /* Do not fail if base_filesystem_create() fails. Not all switch roots are like base_filesystem_create() wants
  91          * them to look like. They might even boot, if they are RO and don't have the FS layout. Just ignore the error
  92          * and switch_root() nevertheless. */
  93         (void) base_filesystem_create(new_root, UID_INVALID, GID_INVALID);
  94
  95         if (chdir(new_root) < 0)
  96                 return log_error_errno(errno, "Failed to change directory to %s: %m", new_root);
  97
  98         /* We first try a pivot_root() so that we can umount the old root dir. In many cases (i.e. where rootfs is /),
  99          * that's not possible however, and hence we simply overmount root */
 100         if (pivot_root(new_root, resolved_old_root_after) >= 0) {
 101
 102                 /* Immediately get rid of the old root, if detach_oldroot is set.
 103                  * Since we are running off it we need to do this lazily. */
 104                 if (unmount_old_root) {
 105                         r = umount_recursive(old_root_after, MNT_DETACH);
 106                         if (r < 0)
 107                                 log_warning_errno(r, "Failed to unmount old root directory tree, ignoring: %m");
 108                 }
 109
 110         } else if (mount(new_root, "/", NULL, MS_MOVE, NULL) < 0)
 111                 return log_error_errno(errno, "Failed to move %s to /: %m", new_root);
 112
 113         if (chroot(".") < 0)
 114                 return log_error_errno(errno, "Failed to change root: %m");
 115
 116         if (chdir("/") < 0)
 117                 return log_error_errno(errno, "Failed to change directory: %m");
 118
 119         if (old_root_fd >= 0) {
 120                 struct stat rb;
 121
 122                 if (fstat(old_root_fd, &rb) < 0)
 123                         return log_error_errno(errno, "Failed to stat old root directory: %m");
 124                 (void) rm_rf_children(TAKE_FD(old_root_fd), 0, &rb); /* takes possession of the dir fd, even on failure */
 125         }
 126
 127         return 0;
 128 }