git: a5a16ba5f179 - stable/15 - nullfs: close a race when syncing inotify flags from the lower vnode

The branch stable/15 has been updated by netchild:

URL: https://cgit.FreeBSD.org/src/commit/?id=a5a16ba5f1791e1be87fede24887b469a9a0d911

commit a5a16ba5f1791e1be87fede24887b469a9a0d911
Author:     Alexander Leidinger <netchild@FreeBSD.org>
AuthorDate: 2026-07-19 07:38:52 +0000
Commit:     Alexander Leidinger <netchild@FreeBSD.org>
CommitDate: 2026-08-02 12:12:07 +0000

    nullfs: close a race when syncing inotify flags from the lower vnode
    
    After a bypassed VOP, nullfs mirrors the lower vnode's inotify state
    onto the upper vnode.  The flags were checked with lockless reads
    before being updated with the asserting flag set/unset primitives, so
    two threads syncing the same vnode concurrently (or a sync racing a
    watch being established) could both decide to make the same change;
    the loser then trips the "flags already set" assertion on an
    INVARIANTS kernel.  On other kernels the race is harmless.
    
    Keep the lockless check as the fast path, but re-make the decision
    under the vnode interlock before actually changing the flags.
    
    Reproduced in a 4-CPU VM with one thread cycling an inotify watch on
    a lower-filesystem file while several threads stat(2) the same file
    through a nullfs mount: the unpatched INVARIANTS kernel panics under
    this load, the patched kernel runs it to completion.
    
    Fixes:                  f1f230439fa4 ("vfs: Initial revision of inotify")
    MFC after:              2 weeks
    Differential Revision:  D58344
    Reviewed by:            markj
    Assisted-by:            Claude Code (Fable 5)
    
    (cherry picked from commit d6915bffb7b68d9b55fa3db4e5709463549c379e)
---
 sys/fs/nullfs/null_vnops.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/sys/fs/nullfs/null_vnops.c b/sys/fs/nullfs/null_vnops.c
index ee7d89db6b05..27ccb54c2a4f 100644
--- a/sys/fs/nullfs/null_vnops.c
+++ b/sys/fs/nullfs/null_vnops.c
@@ -196,17 +196,26 @@ SYSCTL_INT(_debug, OID_AUTO, nullfs_bug_bypass, CTLFLAG_RW,
  *   VOP_INOTIFY.
  * - If the lower vnode is watched, then the upper vnode should go through
  *   VOP_INOTIFY, so copy the flag up.
+ *
+ * The lockless check is only a fast path: the decision to change a flag
+ * is re-made under the upper vnode's interlock, since another thread may
+ * set or clear the flag concurrently.
  */
 static void
 null_copy_inotify(struct vnode *vp, struct vnode *lvp, short flag)
 {
+	if (__predict_true((vn_irflag_read(vp) & flag) ==
+	    (vn_irflag_read(lvp) & flag)))
+		return;
+	VI_LOCK(vp);
 	if ((vn_irflag_read(vp) & flag) != 0) {
-		if (__predict_false((vn_irflag_read(lvp) & flag) == 0))
-			vn_irflag_unset(vp, flag);
-	} else if ((vn_irflag_read(lvp) & flag) != 0) {
-		if (__predict_false((vn_irflag_read(vp) & flag) == 0))
-			vn_irflag_set(vp, flag);
+		if ((vn_irflag_read(lvp) & flag) == 0)
+			vn_irflag_unset_locked(vp, flag);
+	} else {
+		if ((vn_irflag_read(lvp) & flag) != 0)
+			vn_irflag_set_locked(vp, flag);
 	}
+	VI_UNLOCK(vp);
 }
 
 /*