Re: [BUG] bpf, sockmap: spurious wakeup by tcp_msg_wait_data() causing unexpected EAGAIN in recvfrom()
From: John Fastabend <john.fastabend@gmail.com>
Date: 2026-07-14 18:16:31
Also in:
bpf
On Thu, Jul 09, 2026 at 07:43:56PM +0100, Nnamdi Onyeyiri wrote:
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?
If its breaking/changing applications I think its a bug. Can you submit the patch below? We had something similar with ioctl FIONREAD behavior as well that was recently fixed. Thanks, John
quoted hunk ↗ jump to hunk
Thanks, Nnamdi. ----8<---- From 0e0c342363b2e435297ab1feda402cde6ad54525 Mon Sep 17 00:00:00 2001 From: Nnamdi Onyeyiri <redacted> 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 <redacted> --- 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