linux-kernel - [PATCH] btrfs: fix NULL dereference on root when tracing inode eviction

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Message-ID: <20251021091125.259500-1-mssola@mssola.com>
Date: Tue, 21 Oct 2025 11:11:25 +0200
From: Miquel Sabaté Solà <mssola@...ola.com>
To: linux-btrfs@...r.kernel.org
Cc: clm@...com,
	dsterba@...e.com,
	rostedt@...dmis.org,
	mhiramat@...nel.org,
	mathieu.desnoyers@...icios.com,
	linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	Miquel Sabaté Solà <mssola@...ola.com>,
	syzbot+d991fea1b4b23b1f6bf8@...kaller.appspotmail.com
Subject: [PATCH] btrfs: fix NULL dereference on root when tracing inode eviction

When evicting an inode the first thing we do is to setup tracing for it,
which implies fetching the root's id. But in btrfs_evict_inode() the
root might be NULL, as implied in the next check that we do in
btrfs_evict_inode().

Hence, we either should set the ->root_objectid to 0 in case the root is
NULL, or we move tracing setup after checking that the root is not
NULL. Setting the rootid to 0 at least gives us the possibility to trace
this call even in the case when the root is NULL, so that's the solution
taken here.

Fixes: 1abe9b8a138c ("Btrfs: add initial tracepoint support for btrfs")
Reported-by: syzbot+d991fea1b4b23b1f6bf8@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d991fea1b4b23b1f6bf8
Signed-off-by: Miquel Sabaté Solà <mssola@...ola.com>
---
 include/trace/events/btrfs.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/trace/events/btrfs.h b/include/trace/events/btrfs.h
index 7e418f065b94..f17ee9ba2cf6 100644
--- a/include/trace/events/btrfs.h
+++ b/include/trace/events/btrfs.h
@@ -224,7 +224,8 @@ DECLARE_EVENT_CLASS(btrfs__inode,
 		__entry->generation = BTRFS_I(inode)->generation;
 		__entry->last_trans = BTRFS_I(inode)->last_trans;
 		__entry->logged_trans = BTRFS_I(inode)->logged_trans;
-		__entry->root_objectid = btrfs_root_id(BTRFS_I(inode)->root);
+		__entry->root_objectid =
+			     BTRFS_I(inode)->root ? btrfs_root_id(BTRFS_I(inode)->root) : 0;
 	),

 	TP_printk_btrfs("root=%llu(%s) gen=%llu ino=%llu blocks=%llu "
--
2.51.1