On Tue, 2020-08-04 at 12:41 +0200, Miklos Szeredi wrote:

On Mon, Aug 03, 2020 at 02:37:16PM +0100, David Howells wrote:

quoted

Add a uniquifier ID to struct mount that is effectively unique over
the
kernel lifetime to deal around mnt_id values being reused.  This
can then
be exported through fsinfo() to allow detection of replacement
mounts that
happen to end up with the same mount ID.

The normal mount handle is still used for referring to a particular
mount.

The mount notification is then changed to convey these unique mount
IDs
rather than the mount handle.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 fs/mount.h        |    3 +++
 fs/mount_notify.c |    4 ++--
 fs/namespace.c    |    3 +++
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/mount.h b/fs/mount.h
index 85456a5f5a3a..1037781be055 100644
--- a/fs/mount.h
+++ b/fs/mount.h

@@ -79,6 +79,9 @@ struct mount {
 	int mnt_expiry_mark;		/* true if marked for

expiry */
 	struct hlist_head mnt_pins;
 	struct hlist_head mnt_stuck_children;
+#ifdef CONFIG_FSINFO
+	u64	mnt_unique_id;		/* ID unique over lifetime of
kernel */
+#endif

Not sure if it's worth making conditional.

quoted

 #ifdef CONFIG_MOUNT_NOTIFICATIONS
 	struct watch_list *mnt_watchers; /* Watches on dentries within
this mount */
 #endif

diff --git a/fs/mount_notify.c b/fs/mount_notify.c
index 44f570e4cebe..d8ba66ed5f77 100644
--- a/fs/mount_notify.c
+++ b/fs/mount_notify.c

@@ -90,7 +90,7 @@ void notify_mount(struct mount *trigger,
 	n.watch.type	= WATCH_TYPE_MOUNT_NOTIFY;
 	n.watch.subtype	= subtype;
 	n.watch.info	= info_flags | watch_sizeof(n);
-	n.triggered_on	= trigger->mnt_id;
+	n.triggered_on	= trigger->mnt_unique_id;
 
 	switch (subtype) {
 	case NOTIFY_MOUNT_EXPIRY:

@@ -102,7 +102,7 @@ void notify_mount(struct mount *trigger,
 	case NOTIFY_MOUNT_UNMOUNT:
 	case NOTIFY_MOUNT_MOVE_FROM:
 	case NOTIFY_MOUNT_MOVE_TO:
-		n.auxiliary_mount	= aux->mnt_id;
+		n.auxiliary_mount = aux->mnt_unique_id;

Hmm, so we now have two ID's:

 - one can be used to look up the mount
 - one is guaranteed to be unique

With this change the mount cannot be looked up with
FSINFO_FLAGS_QUERY_MOUNT,
right?

Should we be merging the two ID's into a single one which has both
properties?

I'd been thinking we would probably need to change to 64 bit ids
for a while now and I thought that was what was going to happen.

We'll need to change libmount and current code but better early
on than later.

Ian

quoted

 		break;
 
 	default:

diff --git a/fs/namespace.c b/fs/namespace.c
index b2b9920ffd3c..1db8a64cd76f 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c

@@ -115,6 +115,9 @@ static int mnt_alloc_id(struct mount *mnt)
 	if (res < 0)
 		return res;
 	mnt->mnt_id = res;
+#ifdef CONFIG_FSINFO
+	mnt->mnt_unique_id = atomic64_inc_return(&vfs_unique_counter);
+#endif
 	return 0;
 }