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Revert "shutdown: do not umount recursively before MS_MOVE"

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This reverts commit 6b219b74de53729249956221a971047aab7c96e0.

This commit doesn't look right to me. We have to unmount everything
recursively *before* we MS_MOVE because the MS_MOVE will not get rid of
it for us, and we simply cannot access these mounts after the MS_MOVE is
complete anymore.

This is a fundamental difference between MS_MOVE and pivot_root(). The
latter repivots the entire mount table getting rid of anything outside
of the new root. MS_MOVE otoh just mounts a bunch of mount points to the
top, leaving in place whatever might be underneath it.

Thus, if we go through the MS_MOVE codepath we must unmount everything
explicitly before doing so because otherwise the mounts will be pinned
forever, but be entirely invisble to userspace.
This commit is contained in:
Lennart Poettering 2023-08-11 12:15:25 +02:00 committed by Mike Yuan
parent 95648f9ed0
commit c6c5d20de5
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GPG Key ID: 417471C0A40F58B3
3 changed files with 3 additions and 6 deletions
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3
src/shared/switch-root.c
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@ -185,8 +185,7 @@ int switch_root(const char *new_root,
* 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. */
if (!FLAGS_SET(flags, SWITCH_ROOT_SKIP_RECURSIVE_UMOUNT))
(void) umount_recursive_full(NULL, MNT_DETACH, STRV_MAKE(new_root));
(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);

1
src/shared/switch-root.h
View File

@ -8,7 +8,6 @@ typedef enum SwitchRootFlags {
* that it is backed by non-persistent tmpfs/ramfs/… */
SWITCH_ROOT_DONT_SYNC = 1 << 1, /* don't call sync() immediately before switching root */
SWITCH_ROOT_RECURSIVE_RUN = 1 << 2, /* move /run/ with MS_REC from old to new root */
SWITCH_ROOT_SKIP_RECURSIVE_UMOUNT = 1 << 3, /* do not umount recursively on move */
} SwitchRootFlags;
int switch_root(const char *new_root, const char *old_root_after, SwitchRootFlags flags);

5
src/shutdown/shutdown.c
View File

@ -169,12 +169,11 @@ static int switch_root_initramfs(void) {
* Disable sync() during switch-root, we after all sync'ed here plenty, and a dumb sync (as opposed
* to the "smart" sync() we did here that looks at progress parameters) would defeat much of our
* efforts here. As the new root will be /run/initramfs/, it is not necessary to mount /run/
* recursively. Also, do not umount filesystems before MS_MOVE, as that should be done by ourself. */
* recursively. */
return switch_root(
/* new_root= */ "/run/initramfs",
/* old_root_after= */ "/oldroot",
/* flags= */ SWITCH_ROOT_DONT_SYNC |
SWITCH_ROOT_SKIP_RECURSIVE_UMOUNT);
/* flags= */ SWITCH_ROOT_DONT_SYNC);
}
/* Read the following fields from /proc/meminfo: