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/* SPDX-License-Identifier: LGPL-2.1-or-later */
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <stdbool.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <unistd.h>
#include "base-filesystem.h"
#include "chase.h"
#include "creds-util.h"
#include "fd-util.h"
#include "initrd-util.h"
#include "log.h"
#include "missing_syscall.h"
#include "mkdir-label.h"
#include "mount-util.h"
#include "mountpoint-util.h"
#include "path-util.h"
#include "rm-rf.h"
#include "stdio-util.h"
#include "string-util.h"
#include "strv.h"
#include "switch-root.h"
#include "user-util.h"
int switch_root(const char *new_root,
const char *old_root_after, /* path below the new root, where to place the old root after the transition; may be NULL to unmount it */
SwitchRootFlags flags) {
/* Stuff mounted below /run/ we don't save on soft reboot, as it might have lost its relevance,
* e.g. removable media and such. We rather want that the new boot mounts this fresh. But on
* the switch from initrd we do use MS_REC, as it is expected that mounts set up in /run/ are
* maintained. */
static const struct {
const char *path;
unsigned long mount_flags; /* Flags to apply if SWITCH_ROOT_RECURSIVE_RUN is unset */
unsigned long mount_flags_recursive_run; /* Flags to apply if SWITCH_ROOT_RECURSIVE_RUN is set (0 if shall be skipped) */
} transfer_table[] = {
{ "/dev", MS_BIND|MS_REC, MS_BIND|MS_REC }, /* Recursive, because we want to save the original /dev/shm/ + /dev/pts/ and similar */
{ "/sys", MS_BIND|MS_REC, MS_BIND|MS_REC }, /* Similar, we want to retain various API VFS, or the cgroupv1 /sys/fs/cgroup/ tree */
{ "/proc", MS_BIND|MS_REC, MS_BIND|MS_REC }, /* Similar */
{ "/run", MS_BIND, MS_BIND|MS_REC }, /* Recursive except on soft reboot, see above */
{ "/run/credentials", MS_BIND|MS_REC, 0 /* skip! */ }, /* Credential mounts should survive */
{ "/run/host", MS_BIND|MS_REC, 0 /* skip! */ }, /* Host supplied hierarchy should also survive */
};
_cleanup_close_ int old_root_fd = -EBADF, new_root_fd = -EBADF;
_cleanup_free_ char *resolved_old_root_after = NULL;
int r, istmp;
assert(new_root);
/* Check if we shall remove the contents of the old root */
old_root_fd = open("/", O_DIRECTORY|O_CLOEXEC);
if (old_root_fd < 0)
return log_error_errno(errno, "Failed to open root directory: %m");
new_root_fd = open(new_root, O_DIRECTORY|O_CLOEXEC);
if (new_root_fd < 0)
return log_error_errno(errno, "Failed to open target directory '%s': %m", new_root);
r = fds_are_same_mount(old_root_fd, new_root_fd);
if (r < 0)
return log_error_errno(r, "Failed to check if old and new root directory/mount are the same: %m");
if (r > 0) {
log_debug("Skipping switch root, as old and new root directories/mounts are the same.");
return 0;
}
/* Make the new root directory a mount point if it isn't */
r = fd_make_mount_point(new_root_fd);
if (r < 0)
return log_error_errno(r, "Failed to make new root directory a mount point: %m");
if (r > 0) {
int fd;
/* When the path was not a mount point, then we need to reopen the path, otherwise, it still
* points to the underlying directory. */
fd = open(new_root, O_DIRECTORY|O_CLOEXEC);
if (fd < 0)
return log_error_errno(errno, "Failed to reopen target directory '%s': %m", new_root);
close_and_replace(new_root_fd, fd);
}
if (FLAGS_SET(flags, SWITCH_ROOT_DESTROY_OLD_ROOT)) {
istmp = fd_is_temporary_fs(old_root_fd);
if (istmp < 0)
return log_error_errno(istmp, "Failed to stat root directory: %m");
if (istmp > 0)
log_debug("Root directory is on tmpfs, will do cleanup later.");
} else
istmp = -1; /* don't know */
if (old_root_after) {
/* Determine where we shall place the old root after the transition */
r = chase(old_root_after, new_root, CHASE_PREFIX_ROOT|CHASE_NONEXISTENT, &resolved_old_root_after, NULL);
if (r < 0)
return log_error_errno(r, "Failed to resolve %s/%s: %m", new_root, old_root_after);
if (r == 0) /* Doesn't exist yet. Let's create it */
(void) mkdir_p_label(resolved_old_root_after, 0755);
}
/* We are about to unmount various file systems with MNT_DETACH (either explicitly via umount() or
* indirectly via pivot_root()), and thus do not synchronously wait for them to be fully sync'ed —
* all while making them invisible/inaccessible in the file system tree for later code. That makes
* sync'ing them then difficult. Let's hence issue a manual sync() here, so that we at least can
* guarantee all file systems are an a good state before entering this state. */
if (!FLAGS_SET(flags, SWITCH_ROOT_DONT_SYNC))
sync();
/* Work-around for kernel design: the kernel refuses MS_MOVE if any file systems are mounted
* MS_SHARED. Hence remount them MS_PRIVATE here as a work-around.
*
* https://bugzilla.redhat.com/show_bug.cgi?id=847418 */
if (mount(NULL, "/", NULL, MS_REC|MS_PRIVATE, NULL) < 0)
return log_error_errno(errno, "Failed to set \"/\" mount propagation to private: %m");
/* Do not fail if base_filesystem_create() fails. Not all switch roots are like base_filesystem_create() wants
* them to look like. They might even boot, if they are RO and don't have the FS layout. Just ignore the error
* and switch_root() nevertheless. */
(void) base_filesystem_create_fd(new_root_fd, new_root, UID_INVALID, GID_INVALID);
FOREACH_ELEMENT(transfer, transfer_table) {
_cleanup_free_ char *chased = NULL;
unsigned long mount_flags;
mount_flags = FLAGS_SET(flags, SWITCH_ROOT_RECURSIVE_RUN) ? transfer->mount_flags_recursive_run : transfer->mount_flags;
if (mount_flags == 0) /* skip if zero */
continue;
if (access(transfer->path, F_OK) < 0) {
log_debug_errno(errno, "Path '%s' to move to target root directory, not found, ignoring: %m", transfer->path);
continue;
}
r = chase(transfer->path, new_root, CHASE_PREFIX_ROOT, &chased, NULL);
if (r < 0)
return log_error_errno(r, "Failed to resolve %s/%s: %m", new_root, transfer->path);
/* Let's see if it is a mount point already. */
r = path_is_mount_point(chased);
if (r < 0)
return log_error_errno(r, "Failed to determine whether %s is a mount point: %m", chased);
if (r > 0) /* If it is already mounted, then do nothing */
continue;
r = mount_nofollow_verbose(LOG_ERR, transfer->path, chased, NULL, mount_flags, NULL);
if (r < 0)
return r;
}
if (fchdir(new_root_fd) < 0)
return log_error_errno(errno, "Failed to change directory to %s: %m", new_root);
/* We first try a pivot_root() so that we can umount the old root dir. In many cases (i.e. where rootfs is /),
* that's not possible however, and hence we simply overmount root */
if (resolved_old_root_after)
r = RET_NERRNO(pivot_root(".", resolved_old_root_after));
else {
r = RET_NERRNO(pivot_root(".", "."));
if (r >= 0) {
/* Now unmount the upper of the two stacked file systems */
if (umount2(".", MNT_DETACH) < 0)
return log_error_errno(errno, "Failed to unmount the old root: %m");
}
}
if (r < 0) {
log_debug_errno(r, "Pivoting root file system failed, moving mounts instead: %m");
if (resolved_old_root_after) {
r = mount_nofollow_verbose(LOG_ERR, "/", resolved_old_root_after, NULL, MS_BIND|MS_REC, NULL);
if (r < 0)
return r;
}
/* If we have to use MS_MOVE let's first try to get rid of *all* mounts we can, with the
* exception of the path we want to switch to, plus everything leading to it and within
* it. This is necessary because unlike pivot_root() just moving the mount to the root via
* MS_MOVE won't magically unmount anything below it. Once the chroot() succeeds the mounts
* below would still be around but invisible to us, because not accessible via
* /proc/self/mountinfo. Hence, let's clean everything up first, as long as we still can. */
(void) umount_recursive_full(NULL, MNT_DETACH, STRV_MAKE(new_root));
if (mount(".", "/", NULL, MS_MOVE, NULL) < 0)
return log_error_errno(errno, "Failed to move %s to /: %m", new_root);
if (chroot(".") < 0)
return log_error_errno(errno, "Failed to change root: %m");
if (chdir(".") < 0)
return log_error_errno(errno, "Failed to change directory: %m");
}
if (istmp > 0) {
struct stat rb;
if (fstat(old_root_fd, &rb) < 0)
return log_error_errno(errno, "Failed to stat old root directory: %m");
/* Note: the below won't operate on non-memory file systems (i.e. only on tmpfs, ramfs), and
* it will stop at mount boundaries */
(void) rm_rf_children(TAKE_FD(old_root_fd), 0, &rb); /* takes possession of the dir fd, even on failure */
}
return 0;
}
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