linux-kernel - [PATCH] nlink overflow in jfs

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Message-ID: <20251028122212.290745-1-jkoolstra@xs4all.nl>
Date: Tue, 28 Oct 2025 13:22:12 +0100
From: Jori Koolstra <jkoolstra@...all.nl>
To: Dave Kleikamp <shaggy@...nel.org>,
	Jeff Layton <jlayton@...nel.org>,
	Christian Brauner <brauner@...nel.org>,
	Gabriel Krisman Bertazi <gabriel@...sman.be>,
	NeilBrown <neil@...wn.name>,
	Al Viro <viro@...iv.linux.org.uk>
Cc: jfs-discussion@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org,
	jkoolstra@...all.nl,
	syzbot+9131ddfd7870623b719f@...kaller.appspotmail.com
Subject: [PATCH] nlink overflow in jfs_rename

If nlink is maximal for a directory (-1) and inside that directory you
perform a rename for some child directory (not moving from the parent),
then the nlink of the first directory is first incremented and later
decremented. Normally this is fine, but when nlink = -1 this causes a
wrap around to 0, and then drop_nlink issues a warning.

After applying the patch syzbot no longer issues any warnings. I also
ran some basic fs tests to look for any regressions.

Signed-off-by: Jori Koolstra <jkoolstra@...all.nl>
Reported-by: syzbot+9131ddfd7870623b719f@...kaller.appspotmail.com
Closes: https://syzbot.org/bug?extid=9131ddfd7870623b719f
---
 fs/jfs/namei.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/jfs/namei.c b/fs/jfs/namei.c
index 65a218eba8fa..7879c049632b 100644
--- a/fs/jfs/namei.c
+++ b/fs/jfs/namei.c
@@ -1228,7 +1228,7 @@ static int jfs_rename(struct mnt_idmap *idmap, struct inode *old_dir,
 				jfs_err("jfs_rename: dtInsert returned -EIO");
 			goto out_tx;
 		}
-		if (S_ISDIR(old_ip->i_mode))
+		if (S_ISDIR(old_ip->i_mode) && old_dir != new_dir)
 			inc_nlink(new_dir);
 	}
 	/*
@@ -1244,7 +1244,9 @@ static int jfs_rename(struct mnt_idmap *idmap, struct inode *old_dir,
 		goto out_tx;
 	}
 	if (S_ISDIR(old_ip->i_mode)) {
-		drop_nlink(old_dir);
+		if (new_ip || old_dir != new_dir)
+			drop_nlink(old_dir);
+
 		if (old_dir != new_dir) {
 			/*
 			 * Change inode number of parent for moved directory
-- 
2.51.0