Unverified Commit a68cb186 authored by Christian Brauner's avatar Christian Brauner
Browse files

mount: add a comment about concurrent changes with statmount()/listmount() 

Add some comments in there highlighting a few non-obvious assumptions.

Link: https://lore.kernel.org/20250416-zerknirschen-aluminium-14a55639076f@brauner


Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent 0af2f6be

fs/namespace.c

+23 −3
Original line number Diff line number Diff line
@@ -5831,13 +5831,29 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id, 
		return err;



	s->root = root;

	s->idmap = mnt_idmap(s->mnt);

	if (s->mask & STATMOUNT_SB_BASIC)

		statmount_sb_basic(s);



	/*

	 * Note that mount properties in mnt->mnt_flags, mnt->mnt_idmap

	 * can change concurrently as we only hold the read-side of the

	 * namespace semaphore and mount properties may change with only

	 * the mount lock held.

	 *

	 * We could sample the mount lock sequence counter to detect

	 * those changes and retry. But it's not worth it. Worst that

	 * happens is that the mnt->mnt_idmap pointer is already changed

	 * while mnt->mnt_flags isn't or vica versa. So what.

	 *

	 * Both mnt->mnt_flags and mnt->mnt_idmap are set and retrieved

	 * via READ_ONCE()/WRITE_ONCE() and guard against theoretical

	 * torn read/write. That's all we care about right now.

	 */

	s->idmap = mnt_idmap(s->mnt);

	if (s->mask & STATMOUNT_MNT_BASIC)

		statmount_mnt_basic(s);



	if (s->mask & STATMOUNT_SB_BASIC)

		statmount_sb_basic(s);



	if (s->mask & STATMOUNT_PROPAGATE_FROM)

		statmount_propagate_from(s);
@@ -6149,6 +6165,10 @@ SYSCALL_DEFINE4(listmount, const struct mnt_id_req __user *, req, 
	    !ns_capable_noaudit(ns->user_ns, CAP_SYS_ADMIN))

		return -ENOENT;



	/*

	 * We only need to guard against mount topology changes as

	 * listmount() doesn't care about any mount properties.

	 */

	scoped_guard(rwsem_read, &namespace_sem)

		ret = do_listmount(ns, kreq.mnt_id, last_mnt_id, kmnt_ids,

				   nr_mnt_ids, (flags & LISTMOUNT_REVERSE));