src/shared/switch-root.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2
   3 #include <errno.h>
   4 #include <fcntl.h>
   5 #include <limits.h>
   6 #include <stdbool.h>
   7 #include <stdio.h>
   8 #include <sys/mount.h>
   9 #include <sys/stat.h>
  10 #include <unistd.h>
  11
  12 #include "base-filesystem.h"
  13 #include "fd-util.h"
  14 #include "fs-util.h"
  15 #include "log.h"
  16 #include "missing.h"
  17 #include "mkdir.h"
  18 #include "mount-util.h"
  19 #include "path-util.h"
  20 #include "rm-rf.h"
  21 #include "stdio-util.h"
  22 #include "string-util.h"
  23 #include "strv.h"
  24 #include "switch-root.h"
  25 #include "user-util.h"
  26 #include "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 = -1;
  35         bool old_root_remove;
  36         const char *i;
  37         int r;
  38
  39         assert(new_root);
  40         assert(old_root_after);
  41
  42         if (path_equal(new_root, "/"))
  43                 return 0;
  44
  45         /* Check if we shall remove the contents of the old root */
  46         old_root_remove = in_initrd();
  47         if (old_root_remove) {
  48                 old_root_fd = open("/", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_NOCTTY|O_DIRECTORY);
  49                 if (old_root_fd < 0)
  50                         return log_error_errno(errno, "Failed to open root directory: %m");
  51         }
  52
  53         /* Determine where we shall place the old root after the transition */
  54         r = chase_symlinks(old_root_after, new_root, CHASE_PREFIX_ROOT|CHASE_NONEXISTENT, &resolved_old_root_after);
  55         if (r < 0)
  56                 return log_error_errno(r, "Failed to resolve %s/%s: %m", new_root, old_root_after);
  57         if (r == 0) /* Doesn't exist yet. Let's create it */
  58                 (void) mkdir_p_label(resolved_old_root_after, 0755);
  59
  60         /* Work-around for kernel design: the kernel refuses MS_MOVE if any file systems are mounted MS_SHARED. Hence
  61          * remount them MS_PRIVATE here as a work-around.
  62          *
  63          * https://bugzilla.redhat.com/show_bug.cgi?id=847418 */
  64         if (mount(NULL, "/", NULL, MS_REC|MS_PRIVATE, NULL) < 0)
  65                 return log_error_errno(errno, "Failed to set \"/\" mount propagation to private: %m");
  66
  67         FOREACH_STRING(i, "/sys", "/dev", "/run", "/proc") {
  68                 _cleanup_free_ char *chased = NULL;
  69
  70                 r = chase_symlinks(i, new_root, CHASE_PREFIX_ROOT|CHASE_NONEXISTENT, &chased);
  71                 if (r < 0)
  72                         return log_error_errno(r, "Failed to resolve %s/%s: %m", new_root, i);
  73                 if (r > 0) {
  74                         /* Already exists. Let's see if it is a mount point already. */
  75                         r = path_is_mount_point(chased, NULL, 0);
  76                         if (r < 0)
  77                                 return log_error_errno(r, "Failed to determine whether %s is a mount point: %m", chased);
  78                         if (r > 0) /* If it is already mounted, then do nothing */
  79                                 continue;
  80                 } else
  81                          /* Doesn't exist yet? */
  82                         (void) mkdir_p_label(chased, 0755);
  83
  84                 if (mount(i, chased, NULL, mount_flags, NULL) < 0)
  85                         return log_error_errno(r, "Failed to mount %s to %s: %m", i, chased);
  86         }
  87
  88         /* Do not fail if base_filesystem_create() fails. Not all switch roots are like base_filesystem_create() wants
  89          * them to look like. They might even boot, if they are RO and don't have the FS layout. Just ignore the error
  90          * and switch_root() nevertheless. */
  91         (void) base_filesystem_create(new_root, UID_INVALID, GID_INVALID);
  92
  93         if (chdir(new_root) < 0)
  94                 return log_error_errno(errno, "Failed to change directory to %s: %m", new_root);
  95
  96         /* We first try a pivot_root() so that we can umount the old root dir. In many cases (i.e. where rootfs is /),
  97          * that's not possible however, and hence we simply overmount root */
  98         if (pivot_root(new_root, resolved_old_root_after) >= 0) {
  99
 100                 /* Immediately get rid of the old root, if detach_oldroot is set.
 101                  * Since we are running off it we need to do this lazily. */
 102                 if (unmount_old_root) {
 103                         r = umount_recursive(old_root_after, MNT_DETACH);
 104                         if (r < 0)
 105                                 log_warning_errno(r, "Failed to unmount old root directory tree, ignoring: %m");
 106                 }
 107
 108         } else if (mount(new_root, "/", NULL, MS_MOVE, NULL) < 0)
 109                 return log_error_errno(errno, "Failed to move %s to /: %m", new_root);
 110
 111         if (chroot(".") < 0)
 112                 return log_error_errno(errno, "Failed to change root: %m");
 113
 114         if (chdir("/") < 0)
 115                 return log_error_errno(errno, "Failed to change directory: %m");
 116
 117         if (old_root_fd >= 0) {
 118                 struct stat rb;
 119
 120                 if (fstat(old_root_fd, &rb) < 0)
 121                         log_warning_errno(errno, "Failed to stat old root directory, leaving: %m");
 122                 else {
 123                         (void) rm_rf_children(old_root_fd, 0, &rb);
 124                         old_root_fd = -1;
 125                 }
 126         }
 127
 128         return 0;
 129 }