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switch-root: don't require /mnt/ when switching root into host OS

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So far, we invoked pivot_root() specifying /mnt/ as second argument,
which then unmounted right-after. We'd create /mnt/ if needed. This
sucks, because it means /mnt/ must strictly be pre-created on immutable
images.

Remove this limitation, by using pivot_root() with "." as source and
target, which will result in two stacked mounts afterwards: the new one
underneath, the old one ontop. We can then simply unmount the top one,
and have what we want without needing any extra /mnt/ dir.

Since we don't need /mnt/ anymore we can get rid of the extra
unmount_old_root parameter and simply specify it as NULL if we don't
want the old mount to stick around.
This commit is contained in:
Lennart Poettering 2023-04-28 18:35:31 +02:00
parent c4039ceb9e
commit f2c1d491a5
4 changed files with 49 additions and 40 deletions
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2
src/core/main.c
View File

@ -1797,7 +1797,7 @@ static int do_reexecute(
broadcast_signal(SIGTERM, false, true, arg_default_timeout_stop_usec);
/* And switch root with MS_MOVE, because we remove the old directory afterwards and detach it. */
r = switch_root(switch_root_dir, "/mnt", true, MS_MOVE);
r = switch_root(switch_root_dir, /* old_root_after= */ NULL, MS_MOVE);
if (r < 0)
log_error_errno(r, "Failed to switch root, trying to continue: %m");
}

83
src/shared/switch-root.c
View File

@ -26,41 +26,45 @@
#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 */
bool unmount_old_root,
unsigned long mount_flags) { /* MS_MOVE or MS_BIND */
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 */
unsigned long mount_flags) { /* MS_MOVE or MS_BIND used for /proc/, /dev/, /run/, /sys/ */
_cleanup_close_ int old_root_fd = -EBADF, new_root_fd = -EBADF;
_cleanup_free_ char *resolved_old_root_after = NULL;
_cleanup_close_ int old_root_fd = -EBADF;
int r;
int r, istmp;
assert(new_root);
assert(old_root_after);
assert(IN_SET(mount_flags, MS_MOVE, MS_BIND));
if (path_equal(new_root, "/"))
return 0;
/* Check if we shall remove the contents of the old root */
old_root_fd = open("/", O_RDONLY | O_CLOEXEC | O_DIRECTORY);
old_root_fd = open("/", O_DIRECTORY|O_CLOEXEC);
if (old_root_fd < 0)
return log_error_errno(errno, "Failed to open root directory: %m");
r = fd_is_temporary_fs(old_root_fd);
if (r < 0)
return log_error_errno(r, "Failed to stat root directory: %m");
if (r > 0)
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
old_root_fd = safe_close(old_root_fd);
/* 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);
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);
/* 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.
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);
}
/* 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)
@ -92,35 +96,40 @@ int switch_root(const char *new_root,
* and switch_root() nevertheless. */
(void) base_filesystem_create(new_root, UID_INVALID, GID_INVALID);
if (chdir(new_root) < 0)
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 (pivot_root(new_root, resolved_old_root_after) >= 0) {
/* Immediately get rid of the old root, if detach_oldroot is set.
* Since we are running off it we need to do this lazily. */
if (unmount_old_root) {
r = umount_recursive(old_root_after, MNT_DETACH);
if (r < 0)
log_warning_errno(r, "Failed to unmount old root directory tree, ignoring: %m");
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");
} else if (mount(new_root, "/", NULL, MS_MOVE, NULL) < 0)
return log_error_errno(errno, "Failed to move %s to /: %m", 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 (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 (chdir(".") < 0)
return log_error_errno(errno, "Failed to change directory: %m");
}
if (old_root_fd >= 0) {
if (istmp) {
struct stat rb;
if (fstat(old_root_fd, &rb) < 0)
return log_error_errno(errno, "Failed to stat old root directory: %m");
(void) rm_rf_children(TAKE_FD(old_root_fd), 0, &rb); /* takes possession of the dir fd, even on failure */
}

2
src/shared/switch-root.h
View File

@ -3,4 +3,4 @@
#include <stdbool.h>
int switch_root(const char *new_root, const char *oldroot, bool detach_oldroot, unsigned long mountflags);
int switch_root(const char *new_root, const char *old_root_after, unsigned long mount_flags);

2
src/shutdown/shutdown.c
View File

@ -169,7 +169,7 @@ static int switch_root_initramfs(void) {
* /run/initramfs/shutdown will take care of these.
* Also do not detach the old root, because /run/initramfs/shutdown needs to access it.
*/
return switch_root("/run/initramfs", "/oldroot", false, MS_BIND);
return switch_root("/run/initramfs", "/oldroot", MS_BIND);
}
/* Read the following fields from /proc/meminfo: