Date: Sun, 19 Jul 2026 15:13:58 +0000 From: Alexander Leidinger <netchild@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: git: d6915bffb7b6 - main - nullfs: close a race when syncing inotify flags from the lower vnode Message-ID: <6a5ce9b6.21080.28e0c812@gitrepo.freebsd.org>
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The branch main has been updated by netchild: URL: https://cgit.FreeBSD.org/src/commit/?id=d6915bffb7b68d9b55fa3db4e5709463549c379e commit d6915bffb7b68d9b55fa3db4e5709463549c379e Author: Alexander Leidinger <netchild@FreeBSD.org> AuthorDate: 2026-07-19 07:38:52 +0000 Commit: Alexander Leidinger <netchild@FreeBSD.org> CommitDate: 2026-07-19 15:13:40 +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) --- 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 50d359c9b909..41823d0c6202 100644 --- a/sys/fs/nullfs/null_vnops.c +++ b/sys/fs/nullfs/null_vnops.c @@ -200,17 +200,26 @@ 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); } /*home | help
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