Commit 49acacdc authored by Al Viro's avatar Al Viro
Browse files

clone_mnt(): simplify the propagation-related logics 



The underlying rules are simple:
	* MNT_SHARED should be set iff ->mnt_group_id of new mount ends up
non-zero.
	* mounts should be on the same ->mnt_share cyclic list iff they have
the same non-zero ->mnt_group_id value.
	* CL_PRIVATE is mutually exclusive with MNT_SHARED, MNT_SLAVE,
MNT_SHARED_TO_SLAVE and MNT_EXPIRE; the whole point of that thing is to
get a clone of old mount that would *not* be on any namespace-related
lists.

The above allows to make the logics more straightforward; what's more,
it makes the proof that invariants are maintained much simpler.
The variant in mainline is safe (aside of a very narrow race with
unsafe modification of mnt_flags right after we had the mount exposed
in superblock's ->s_mounts; theoretically it can race with ro remount
of the original, but it's not easy to hit), but proof of its correctness
is really unpleasant.

Reviewed-by: default avatarChristian Brauner <brauner@kernel.org>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent d08fa7f4

fs/namespace.c

+13 −13
Original line number Diff line number Diff line
@@ -1301,6 +1301,9 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root, 
	if (!mnt)

		return ERR_PTR(-ENOMEM);



	mnt->mnt.mnt_flags = READ_ONCE(old->mnt.mnt_flags) &

			     ~MNT_INTERNAL_FLAGS;



	if (flag & (CL_SLAVE | CL_PRIVATE | CL_SHARED_TO_SLAVE))

		mnt->mnt_group_id = 0; /* not a peer of original */

	else
@@ -1312,8 +1315,8 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root, 
			goto out_free;

	}



	mnt->mnt.mnt_flags = old->mnt.mnt_flags;

	mnt->mnt.mnt_flags &= ~(MNT_WRITE_HOLD|MNT_MARKED|MNT_INTERNAL|MNT_LOCKED);

	if (mnt->mnt_group_id)

		set_mnt_shared(mnt);



	atomic_inc(&sb->s_active);

	mnt->mnt.mnt_idmap = mnt_idmap_get(mnt_idmap(&old->mnt));
@@ -1326,22 +1329,20 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root, 
	list_add_tail(&mnt->mnt_instance, &sb->s_mounts);

	unlock_mount_hash();



	if (flag & CL_PRIVATE)	// we are done with it

		return mnt;



	if (peers(mnt, old))

		list_add(&mnt->mnt_share, &old->mnt_share);



	if ((flag & CL_SLAVE) ||

	    ((flag & CL_SHARED_TO_SLAVE) && IS_MNT_SHARED(old))) {

		list_add(&mnt->mnt_slave, &old->mnt_slave_list);

		mnt->mnt_master = old;

		CLEAR_MNT_SHARED(mnt);

	} else if (!(flag & CL_PRIVATE)) {

		if ((flag & CL_MAKE_SHARED) || IS_MNT_SHARED(old))

			list_add(&mnt->mnt_share, &old->mnt_share);

		if (IS_MNT_SLAVE(old))

	} else if (IS_MNT_SLAVE(old)) {

		list_add(&mnt->mnt_slave, &old->mnt_slave);

		mnt->mnt_master = old->mnt_master;

	} else {

		CLEAR_MNT_SHARED(mnt);

	}

	if (flag & CL_MAKE_SHARED)

		set_mnt_shared(mnt);



	/* stick the duplicate mount on the same expiry list

	 * as the original if that was on one */
@@ -1349,7 +1350,6 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root, 
		if (!list_empty(&old->mnt_expire))

			list_add(&mnt->mnt_expire, &old->mnt_expire);

	}



	return mnt;



 out_free: