Re: Apparent mount behaviour change in 6.15

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, May 23, 2025 at 10:37:35PM +0100, Al Viro wrote:
> On Fri, May 23, 2025 at 10:29:58PM +0100, Al Viro wrote:
> 
> > This is bogus, IMO.  I'm perfectly fine with propagate_one() returning 0
> > on anon_ns(m->mnt); that would refuse to propagate into *any* anon ns,
> > but won't screw the propagation between the mounts that are in normal, non-anon
> > namespaces.
> 
> IOW, I mean this variant - the only difference from what you've posted is
> the location of is_anon_ns() test; you do it in IS_MNT_NEW(), this variant
> has it in propagate_one().  Does the variant below fix regression?

AFAICS, it does suffice to revert the behaviour change on the reproducer
upthread.

I've replaced the top of viro/vfs.git#fixes with that; commit message there
is tentative - if nothing else, that's a patch from Christian with slight
modifications from me.  It also needs reported-by, etc.

Said that, could somebody (original reporter) confirm that the variant
in git.kernel.org:/pub/scm/linux/kernel/git/viro/vfs.git #fixes (head at
63e90fcc1807) is OK with them?

And yes, it will need a proper commit message.  Christian?

commit 63e90fcc18072638a62196caae93de66fc6cbc37
Author: Al Viro <viro@zeniv.linux.org.uk>
Date:   Fri May 23 19:20:36 2025 -0400

    Don't propagate mounts into detached trees
    
    That reverts to behaviour of 6.14 and earlier, with
    fix from "fix IS_MNT_PROPAGATING uses" preserved.
    
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/mount.h b/fs/mount.h
index 7aecf2a60472..ad7173037924 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -7,10 +7,6 @@
 
 extern struct list_head notify_list;
 
-typedef __u32 __bitwise mntns_flags_t;
-
-#define MNTNS_PROPAGATING	((__force mntns_flags_t)(1 << 0))
-
 struct mnt_namespace {
 	struct ns_common	ns;
 	struct mount *	root;
@@ -37,7 +33,6 @@ struct mnt_namespace {
 	struct rb_node		mnt_ns_tree_node; /* node in the mnt_ns_tree */
 	struct list_head	mnt_ns_list; /* entry in the sequential list of mounts namespace */
 	refcount_t		passive; /* number references not pinning @mounts */
-	mntns_flags_t		mntns_flags;
 } __randomize_layout;
 
 struct mnt_pcp {
diff --git a/fs/namespace.c b/fs/namespace.c
index 1b466c54a357..623cd110076d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3648,7 +3648,7 @@ static int do_move_mount(struct path *old_path,
 	if (!(attached ? check_mnt(old) : is_anon_ns(ns)))
 		goto out;
 
-	if (is_anon_ns(ns)) {
+	if (is_anon_ns(ns) && ns == p->mnt_ns) {
 		/*
 		 * Ending up with two files referring to the root of the
 		 * same anonymous mount namespace would cause an error
@@ -3656,16 +3656,7 @@ static int do_move_mount(struct path *old_path,
 		 * twice into the mount tree which would be rejected
 		 * later. But be explicit about it right here.
 		 */
-		if ((is_anon_ns(p->mnt_ns) && ns == p->mnt_ns))
-			goto out;
-
-		/*
-		 * If this is an anonymous mount tree ensure that mount
-		 * propagation can detect mounts that were just
-		 * propagated to the target mount tree so we don't
-		 * propagate onto them.
-		 */
-		ns->mntns_flags |= MNTNS_PROPAGATING;
+		goto out;
 	} else if (is_anon_ns(p->mnt_ns)) {
 		/*
 		 * Don't allow moving an attached mount tree to an
@@ -3722,8 +3713,6 @@ static int do_move_mount(struct path *old_path,
 	if (attached)
 		put_mountpoint(old_mp);
 out:
-	if (is_anon_ns(ns))
-		ns->mntns_flags &= ~MNTNS_PROPAGATING;
 	unlock_mount(mp);
 	if (!err) {
 		if (attached) {
diff --git a/fs/pnode.c b/fs/pnode.c
index fb77427df39e..ffd429b760d5 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -231,8 +231,8 @@ static int propagate_one(struct mount *m, struct mountpoint *dest_mp)
 	/* skip if mountpoint isn't visible in m */
 	if (!is_subdir(dest_mp->m_dentry, m->mnt.mnt_root))
 		return 0;
-	/* skip if m is in the anon_ns we are emptying */
-	if (m->mnt_ns->mntns_flags & MNTNS_PROPAGATING)
+	/* skip if m is in the anon_ns */
+	if (is_anon_ns(m->mnt_ns))
 		return 0;
 
 	if (peers(m, last_dest)) {