src/core/switch-root.c

   1 /*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
   2
   3 /***
   4   This file is part of systemd.
   5
   6   Copyright 2012 Harald Hoyer, Lennart Poettering
   7
   8   systemd is free software; you can redistribute it and/or modify it
   9   under the terms of the GNU Lesser General Public License as published by
  10   the Free Software Foundation; either version 2.1 of the License, or
  11   (at your option) any later version.
  12
  13   systemd is distributed in the hope that it will be useful, but
  14   WITHOUT ANY WARRANTY; without even the implied warranty of
  15   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  16   Lesser General Public License for more details.
  17
  18   You should have received a copy of the GNU Lesser General Public License
  19   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  20 ***/
  21
  22 #include <sys/stat.h>
  23 #include <stdbool.h>
  24 #include <errno.h>
  25 #include <string.h>
  26 #include <sys/mount.h>
  27 #include <unistd.h>
  28 #include <fcntl.h>
  29
  30 #include "util.h"
  31 #include "path-util.h"
  32 #include "switch-root.h"
  33 #include "missing.h"
  34
  35 int switch_root(const char *new_root) {
  36
  37         /*  Don't try to unmount/move the old "/", there's no way to do it. */
  38         static const char move_mounts[] =
  39                 "/dev\0"
  40                 "/proc\0"
  41                 "/sys\0"
  42                 "/run\0";
  43
  44         int r, old_root_fd = -1;
  45         struct stat new_root_stat;
  46         bool old_root_remove;
  47         const char *i;
  48         _cleanup_free_ char *temporary_old_root = NULL;
  49
  50         if (path_equal(new_root, "/"))
  51                 return 0;
  52
  53         /* When using pivot_root() we assume that /mnt exists as place
  54          * we can temporarily move the old root to. As we immediately
  55          * unmount it from there it doesn't matter much which
  56          * directory we choose for this, but it should be more likely
  57          * than not that /mnt exists and is suitable as mount point
  58          * and is on the same fs as the old root dir */
  59         temporary_old_root = strappend(new_root, "/mnt");
  60         if (!temporary_old_root)
  61                 return -ENOMEM;
  62
  63         old_root_remove = in_initrd();
  64
  65         if (stat(new_root, &new_root_stat) < 0) {
  66                 r = -errno;
  67                 log_error("Failed to stat directory %s: %m", new_root);
  68                 goto fail;
  69         }
  70
  71         /* Work-around for a kernel bug: for some reason the kernel
  72          * refuses switching root if any file systems are mounted
  73          * MS_SHARED. Hence remount them MS_PRIVATE here as a
  74          * work-around.
  75          *
  76          * https://bugzilla.redhat.com/show_bug.cgi?id=847418 */
  77         if (mount(NULL, "/", NULL, MS_REC|MS_PRIVATE, NULL) < 0)
  78                 log_warning("Failed to make \"/\" private mount: %m");
  79
  80         NULSTR_FOREACH(i, move_mounts) {
  81                 char new_mount[PATH_MAX];
  82                 struct stat sb;
  83
  84                 snprintf(new_mount, sizeof(new_mount), "%s%s", new_root, i);
  85                 char_array_0(new_mount);
  86
  87                 if ((stat(new_mount, &sb) < 0) ||
  88                     sb.st_dev != new_root_stat.st_dev) {
  89
  90                         /* Mount point seems to be mounted already or
  91                          * stat failed. Unmount the old mount
  92                          * point. */
  93                         if (umount2(i, MNT_DETACH) < 0)
  94                                 log_warning("Failed to unmount %s: %m", i);
  95                         continue;
  96                 }
  97
  98                 if (mount(i, new_mount, NULL, MS_MOVE, NULL) < 0) {
  99                         log_error("Failed to move mount %s to %s, forcing unmount: %m", i, new_mount);
 100
 101                         if (umount2(i, MNT_FORCE) < 0)
 102                                 log_warning("Failed to unmount %s: %m", i);
 103                 }
 104         }
 105
 106         if (chdir(new_root) < 0) {
 107                 r = -errno;
 108                 log_error("Failed to change directory to %s: %m", new_root);
 109                 goto fail;
 110         }
 111
 112         if (old_root_remove) {
 113                 old_root_fd = open("/", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_NOCTTY|O_DIRECTORY);
 114                 if (old_root_fd < 0)
 115                         log_warning("Failed to open root directory: %m");
 116         }
 117
 118         /* We first try a pivot_root() so that we can umount the old
 119          * root dir. In many cases (i.e. where rootfs is /), that's
 120          * not possible however, and hence we simply overmount root */
 121         if (pivot_root(new_root, temporary_old_root) >= 0) {
 122
 123                 /* Immediately get rid of the old root. Since we are
 124                  * running off it we need to do this lazily. */
 125                 if (umount2(temporary_old_root, MNT_DETACH) < 0) {
 126                         r = -errno;
 127                         log_error("Failed to umount old root dir %s: %m", temporary_old_root);
 128                         goto fail;
 129                 }
 130
 131         } else if (mount(new_root, "/", NULL, MS_MOVE, NULL) < 0) {
 132                 r = -errno;
 133                 log_error("Failed to mount moving %s to /: %m", new_root);
 134                 goto fail;
 135         }
 136
 137         if (chroot(".") < 0) {
 138                 r = -errno;
 139                 log_error("Failed to change root: %m");
 140                 goto fail;
 141         }
 142
 143         if (chdir("/") < 0) {
 144                 r = -errno;
 145                 log_error("Failed to change directory: %m");
 146                 goto fail;
 147         }
 148
 149         if (old_root_fd >= 0) {
 150                 struct stat rb;
 151
 152                 if (fstat(old_root_fd, &rb) < 0)
 153                         log_warning("Failed to stat old root directory, leaving: %m");
 154                 else {
 155                         rm_rf_children(old_root_fd, false, false, &rb);
 156                         old_root_fd = -1;
 157                 }
 158         }
 159
 160         r = 0;
 161
 162 fail:
 163         if (old_root_fd >= 0)
 164                 close_nointr_nofail(old_root_fd);
 165
 166         return r;
 167 }