src/shared/switch-root.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2 /***
   3   Copyright 2012 Harald Hoyer, Lennart Poettering
   4 ***/
   5
   6 #include <errno.h>
   7 #include <fcntl.h>
   8 #include <limits.h>
   9 #include <stdbool.h>
  10 #include <stdio.h>
  11 #include <sys/mount.h>
  12 #include <sys/stat.h>
  13 #include <unistd.h>
  14
  15 #include "base-filesystem.h"
  16 #include "fd-util.h"
  17 #include "fs-util.h"
  18 #include "log.h"
  19 #include "missing.h"
  20 #include "mkdir.h"
  21 #include "mount-util.h"
  22 #include "path-util.h"
  23 #include "rm-rf.h"
  24 #include "stdio-util.h"
  25 #include "string-util.h"
  26 #include "strv.h"
  27 #include "switch-root.h"
  28 #include "user-util.h"
  29 #include "util.h"
  30
  31 int switch_root(const char *new_root,
  32                 const char *old_root_after, /* path below the new root, where to place the old root after the transition */
  33                 bool unmount_old_root,
  34                 unsigned long mount_flags) {  /* MS_MOVE or MS_BIND */
  35
  36         _cleanup_free_ char *resolved_old_root_after = NULL;
  37         _cleanup_close_ int old_root_fd = -1;
  38         bool old_root_remove;
  39         const char *i;
  40         int r;
  41
  42         assert(new_root);
  43         assert(old_root_after);
  44
  45         if (path_equal(new_root, "/"))
  46                 return 0;
  47
  48         /* Check if we shall remove the contents of the old root */
  49         old_root_remove = in_initrd();
  50         if (old_root_remove) {
  51                 old_root_fd = open("/", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_NOCTTY|O_DIRECTORY);
  52                 if (old_root_fd < 0)
  53                         return log_error_errno(errno, "Failed to open root directory: %m");
  54         }
  55
  56         /* Determine where we shall place the old root after the transition */
  57         r = chase_symlinks(old_root_after, new_root, CHASE_PREFIX_ROOT|CHASE_NONEXISTENT, &resolved_old_root_after);
  58         if (r < 0)
  59                 return log_error_errno(r, "Failed to resolve %s/%s: %m", new_root, old_root_after);
  60         if (r == 0) /* Doesn't exist yet. Let's create it */
  61                 (void) mkdir_p_label(resolved_old_root_after, 0755);
  62
  63         /* Work-around for kernel design: the kernel refuses MS_MOVE if any file systems are mounted MS_SHARED. Hence
  64          * remount them MS_PRIVATE here as a work-around.
  65          *
  66          * https://bugzilla.redhat.com/show_bug.cgi?id=847418 */
  67         if (mount(NULL, "/", NULL, MS_REC|MS_PRIVATE, NULL) < 0)
  68                 return log_error_errno(errno, "Failed to set \"/\" mount propagation to private: %m");
  69
  70         FOREACH_STRING(i, "/sys", "/dev", "/run", "/proc") {
  71                 _cleanup_free_ char *chased = NULL;
  72
  73                 r = chase_symlinks(i, new_root, CHASE_PREFIX_ROOT|CHASE_NONEXISTENT, &chased);
  74                 if (r < 0)
  75                         return log_error_errno(r, "Failed to resolve %s/%s: %m", new_root, i);
  76                 if (r > 0) {
  77                         /* Already exists. Let's see if it is a mount point already. */
  78                         r = path_is_mount_point(chased, NULL, 0);
  79                         if (r < 0)
  80                                 return log_error_errno(r, "Failed to determine whether %s is a mount point: %m", chased);
  81                         if (r > 0) /* If it is already mounted, then do nothing */
  82                                 continue;
  83                 } else
  84                          /* Doesn't exist yet? */
  85                         (void) mkdir_p_label(chased, 0755);
  86
  87                 if (mount(i, chased, NULL, mount_flags, NULL) < 0)
  88                         return log_error_errno(r, "Failed to mount %s to %s: %m", i, chased);
  89         }
  90
  91         /* Do not fail if base_filesystem_create() fails. Not all switch roots are like base_filesystem_create() wants
  92          * them to look like. They might even boot, if they are RO and don't have the FS layout. Just ignore the error
  93          * and switch_root() nevertheless. */
  94         (void) base_filesystem_create(new_root, UID_INVALID, GID_INVALID);
  95
  96         if (chdir(new_root) < 0)
  97                 return log_error_errno(errno, "Failed to change directory to %s: %m", new_root);
  98
  99         /* We first try a pivot_root() so that we can umount the old root dir. In many cases (i.e. where rootfs is /),
 100          * that's not possible however, and hence we simply overmount root */
 101         if (pivot_root(new_root, resolved_old_root_after) >= 0) {
 102
 103                 /* Immediately get rid of the old root, if detach_oldroot is set.
 104                  * Since we are running off it we need to do this lazily. */
 105                 if (unmount_old_root) {
 106                         r = umount_recursive(old_root_after, MNT_DETACH);
 107                         if (r < 0)
 108                                 log_warning_errno(r, "Failed to unmount old root directory tree, ignoring: %m");
 109                 }
 110
 111         } else if (mount(new_root, "/", NULL, MS_MOVE, NULL) < 0)
 112                 return log_error_errno(errno, "Failed to move %s to /: %m", new_root);
 113
 114         if (chroot(".") < 0)
 115                 return log_error_errno(errno, "Failed to change root: %m");
 116
 117         if (chdir("/") < 0)
 118                 return log_error_errno(errno, "Failed to change directory: %m");
 119
 120         if (old_root_fd >= 0) {
 121                 struct stat rb;
 122
 123                 if (fstat(old_root_fd, &rb) < 0)
 124                         log_warning_errno(errno, "Failed to stat old root directory, leaving: %m");
 125                 else {
 126                         (void) rm_rf_children(old_root_fd, 0, &rb);
 127                         old_root_fd = -1;
 128                 }
 129         }
 130
 131         return 0;
 132 }