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From: David Wei <dw@davidwei.uk>
To: io-uring@vger.kernel.org
Cc: Jens Axboe <axboe@kernel.dk>,
Pavel Begunkov <asml.silence@gmail.com>
Subject: [PATCH v2] io_uring/zcrx: return early from io_zcrx_recv_skb if readlen is 0
Date: Tue, 1 Apr 2025 12:53:55 -0700
Message-ID: <20250401195355.1613813-1-dw@davidwei.uk>
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When readlen is set for a recvzc request, tcp_read_sock() will call
io_zcrx_recv_skb() one final time with len == desc->count == 0. This is
caused by the !desc->count check happening too late. The offset + 1 !=
skb->len happens earlier and causes the while loop to continue.
Fix this in io_zcrx_recv_skb() instead of tcp_read_sock(). Return early
if len is 0 i.e. the read is done.
Changes in v2:
--------------
* Add Fixes tag
* Return 0 directly
* Add comment explaining why the !len check is needed
Fixes: 6699ec9a23f8 ("io_uring/zcrx: add a read limit to recvzc requests")
Signed-off-by: David Wei <dw@davidwei.uk>
---
io_uring/zcrx.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
index 9c95b5b6ec4e..2c8b29c745c5 100644
--- a/io_uring/zcrx.c
+++ b/io_uring/zcrx.c
@@ -818,6 +818,13 @@ io_zcrx_recv_skb(read_descriptor_t *desc, struct sk_buff *skb,
int ret = 0;
len = min_t(size_t, len, desc->count);
+ /* __tcp_read_sock() always calls io_zcrx_recv_skb one last time, even
+ * if desc->count is already 0. This is caused by the if (offset + 1 !=
+ * skb->len) check. Return early in this case to break out of
+ * __tcp_read_sock().
+ */
+ if (!len)
+ return 0;
if (unlikely(args->nr_skbs++ > IO_SKBS_PER_CALL_LIMIT))
return -EAGAIN;
--
2.47.1