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[86.153.201.25]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-493f2a2e499sm8261895e9.0.2026.07.09.11.43.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 09 Jul 2026 11:43:57 -0700 (PDT) Date: Thu, 9 Jul 2026 19:43:56 +0100 From: Nnamdi Onyeyiri To: nnamdio@gmail.com, nnamdi.onyeyiri@squarepoint-capital.com, john.fastabend@gmail.com, jakub@cloudflare.com, jiayuan.chen@linux.dev, edumazet@google.com, ncardwell@google.com, kuniyu@google.com, davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org Cc: netdev@vger.kernel.org, bpf@vger.kernel.org Subject: [BUG] bpf, sockmap: spurious wakeup by tcp_msg_wait_data() causing unexpected EAGAIN in recvfrom() Message-ID: Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Hi, We've encounted what appears to be a bug with bpf when invoking recvfrom() on an ipv4 tcp socket that has been added to a sockmap. It results in unexpected EAGAIN errors, that we've diagnosed as the result of spurious wakeups from tcp_msg_wait_data(). This has been confirmed to still be present on the mainline kernel, and I have written a reproducer at: https://github.com/Nnamdi/recvfrom_sockmap_eagain/ Attched is a patch that we found resolved this using kpatch locally. It just causes spurious wakups to go round the loop again. I'd like to get a sense of whether this is expected behaviour, or really is a bug, in which case, is this the correct fix? Thanks, Nnamdi. ----8<---- >From 0e0c342363b2e435297ab1feda402cde6ad54525 Mon Sep 17 00:00:00 2001 From: Nnamdi Onyeyiri Date: Thu, 9 Jul 2026 13:06:33 +0100 Subject: [PATCH] bpf, sockmap: handle spurious tcp_msg_wait_data() wakeup recvfrom()/recv() are documented as only returning EAGAIN for blocking sockets when they have a receive timeout configured. however, adding a blocking ipv4 tcp socket without a receive timeout to a sockmap will cause EAGAIN errors sporadically. this happens when tcp_msg_wait_data() wakes spuriously (returning 0) in which case, if no receive timeout is configured, we loop again instead of returning -EAGAIN. Signed-off-by: Nnamdi Onyeyiri --- net/ipv4/tcp_bpf.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c index cc0bd73f3..38fd391ff 100644 --- a/net/ipv4/tcp_bpf.c +++ b/net/ipv4/tcp_bpf.c @@ -317,6 +317,8 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk, } if (data && !sk_psock_queue_empty(psock)) goto msg_bytes_ready; + if (!data && timeo == MAX_SCHEDULE_TIMEOUT) + goto msg_bytes_ready; copied = -EAGAIN; } out: @@ -390,6 +392,8 @@ static int tcp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, sk_psock_put(sk, psock); return tcp_recvmsg(sk, msg, len, flags); } + if (!data && timeo == MAX_SCHEDULE_TIMEOUT) + goto msg_bytes_ready; copied = -EAGAIN; } ret = copied; -- 2.52.0