From 7bdf2b5d5fbabfc8749c4ff6e618c3e843b14de0 Mon Sep 17 00:00:00 2001
From: Konstantin Belousov <kib@FreeBSD.org>
Date: Sun, 17 May 2026 01:16:45 +0300
Subject: [PATCH] nullfs: do not allow to mount a vnode over itself

This causes recursion in VFS that is not worth handling.

PR:	275570
Reported by:	Alex S <iwtcex@gmail.com>
Reviewed by:	markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D57043
---
 sys/fs/nullfs/null_vfsops.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/sys/fs/nullfs/null_vfsops.c b/sys/fs/nullfs/null_vfsops.c
index c3eec727aee..0ec4f9c8729 100644
--- a/sys/fs/nullfs/null_vfsops.c
+++ b/sys/fs/nullfs/null_vfsops.c
@@ -151,6 +151,14 @@ nullfs_mount(struct mount *mp)
 	 */
 	lowerrootvp = ndp->ni_vp;
 
+	/*
+	 * Do not allow to mount a vnode over itself.
+	 */
+	if (mp->mnt_vnodecovered == lowerrootvp) {
+		vput(lowerrootvp);
+		return (EDEADLK);
+	}
+
 	/*
 	 * Check multi null mount to avoid `lock against myself' panic.
 	 */