linux-kernel - [PATCH AUTOSEL 6.1 6/6] io_uring/fdinfo: annotate racy sq/cq head/tail reads

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Message-Id: <20250506213714.2983569-6-sashal@kernel.org>
Date: Tue,  6 May 2025 17:37:14 -0400
From: Sasha Levin <sashal@...nel.org>
To: patches@...ts.linux.dev,
	stable@...r.kernel.org
Cc: Jens Axboe <axboe@...nel.dk>,
	syzbot+3e77fd302e99f5af9394@...kaller.appspotmail.com,
	Sasha Levin <sashal@...nel.org>,
	asml.silence@...il.com,
	io-uring@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH AUTOSEL 6.1 6/6] io_uring/fdinfo: annotate racy sq/cq head/tail reads

From: Jens Axboe <axboe@...nel.dk>

[ Upstream commit f024d3a8ded0d8d2129ae123d7a5305c29ca44ce ]

syzbot complains about the cached sq head read, and it's totally right.
But we don't need to care, it's just reading fdinfo, and reading the
CQ or SQ tail/head entries are known racy in that they are just a view
into that very instant and may of course be outdated by the time they
are reported.

Annotate both the SQ head and CQ tail read with data_race() to avoid
this syzbot complaint.

Link: https://lore.kernel.org/io-uring/6811f6dc.050a0220.39e3a1.0d0e.GAE@google.com/
Reported-by: syzbot+3e77fd302e99f5af9394@...kaller.appspotmail.com
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 io_uring/fdinfo.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/io_uring/fdinfo.c b/io_uring/fdinfo.c
index ea2c2ded4e412..4a50531699777 100644
--- a/io_uring/fdinfo.c
+++ b/io_uring/fdinfo.c
@@ -79,11 +79,11 @@ static __cold void __io_uring_show_fdinfo(struct io_ring_ctx *ctx,
 	seq_printf(m, "SqMask:\t0x%x\n", sq_mask);
 	seq_printf(m, "SqHead:\t%u\n", sq_head);
 	seq_printf(m, "SqTail:\t%u\n", sq_tail);
-	seq_printf(m, "CachedSqHead:\t%u\n", ctx->cached_sq_head);
+	seq_printf(m, "CachedSqHead:\t%u\n", data_race(ctx->cached_sq_head));
 	seq_printf(m, "CqMask:\t0x%x\n", cq_mask);
 	seq_printf(m, "CqHead:\t%u\n", cq_head);
 	seq_printf(m, "CqTail:\t%u\n", cq_tail);
-	seq_printf(m, "CachedCqTail:\t%u\n", ctx->cached_cq_tail);
+	seq_printf(m, "CachedCqTail:\t%u\n", data_race(ctx->cached_cq_tail));
 	seq_printf(m, "SQEs:\t%u\n", sq_tail - sq_head);
 	sq_entries = min(sq_tail - sq_head, ctx->sq_entries);
 	for (i = 0; i < sq_entries; i++) {
-- 
2.39.5