linux-kernel - [PATCH] fs: assert on ->i_count in iput

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Message-ID: <20251001010010.9967-1-mjguzik@gmail.com>
Date: Wed,  1 Oct 2025 03:00:10 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: brauner@...nel.org
Cc: viro@...iv.linux.org.uk,
	jack@...e.cz,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH] fs: assert on ->i_count in iput_final()

Notably make sure the count is 0 after the return from ->drop_inode(),
provided we are going to drop.

Inspired by suspicious games played by f2fs.

Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---

boots on ext4 without splats

 fs/inode.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/inode.c b/fs/inode.c
index ec9339024ac3..fa82cb810af4 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1879,6 +1879,7 @@ static void iput_final(struct inode *inode)
 	int drop;
 
 	WARN_ON(inode->i_state & I_NEW);
+	VFS_BUG_ON_INODE(atomic_read(&inode->i_count) != 0, inode);
 
 	if (op->drop_inode)
 		drop = op->drop_inode(inode);
@@ -1893,6 +1894,12 @@ static void iput_final(struct inode *inode)
 		return;
 	}
 
+	/*
+	 * Re-check ->i_count in case the ->drop_inode() hooks played games.
+	 * Note we only execute this if the verdict was to drop the inode.
+	 */
+	VFS_BUG_ON_INODE(atomic_read(&inode->i_count) != 0, inode);
+
 	state = inode->i_state;
 	if (!drop) {
 		WRITE_ONCE(inode->i_state, state | I_WILL_FREE);
-- 
2.34.1