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Wed, 22 Jul 2026 04:01:10 -0700 (PDT) From: Jakub Sitnicki To: Nnamdi Onyeyiri Cc: bpf@vger.kernel.org, davem@davemloft.net, edumazet@google.com, horms@kernel.org, jiayuan.chen@linux.dev, john.fastabend@gmail.com, kuba@kernel.org, kuniyu@google.com, ncardwell@google.com, netdev@vger.kernel.org, pabeni@redhat.com, sashiko-reviews@lists.linux.dev, linux-kernel@vger.kernel.org, emil@etsalapatis.com Subject: Re: [PATCH v7 1/2] bpf, sockmap: handle spurious tcp_msg_wait_data() wakeup In-Reply-To: <20260721223807.75101-2-nnamdio@gmail.com> (Nnamdi Onyeyiri's message of "Tue, 21 Jul 2026 23:38:06 +0100") References: <20260721223807.75101-1-nnamdio@gmail.com> <20260721223807.75101-2-nnamdio@gmail.com> User-Agent: mu4e 1.14.1; emacs 30.2 Date: Wed, 22 Jul 2026 13:01:09 +0200 Message-ID: <87tsprl24q.fsf@cloudflare.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain On Tue, Jul 21, 2026 at 11:38 PM +01, Nnamdi Onyeyiri wrote: > 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. A socket with a receive timeout may return > EAGAIN before the timeout expires. > > There are 2 code paths affected by this: > > 1. tcp_bpf_recvmsg() - Used when the socket has been added to a sockmap > that has no verdict program attached. > > 2. tcp_bpf_recvmsg_parser() - Used when the socket has been added to a > sockmap that has a verdict program. To reproduce this issue, it is > enough for the verdict program to do nothing but return SK_PASS. > > In both cases this happens when tcp_msg_wait_data() wakes spuriously > (returning 0). To fix it, we now loop back to msg_bytes_ready instead > of returning -EAGAIN on spurious wakeup. > > To ensure the looping does not cause sockets with a SO_RCVTIMEO set to > wait excessively long, tcp_msg_wait_data() now takes a pointer to timeo, > allowing sk_wait_event() to update it as appropriate. > > The logic in tcp_bpf_recvmsg_parser() that allow it to handle signals, > socket errors and closuers in its loop was also added to tcp_bpf_recvmsg(). > > Signed-off-by: Nnamdi Onyeyiri > --- > net/ipv4/tcp_bpf.c | 72 ++++++++++++++++++++++++++++++++++++++++------ > 1 file changed, 63 insertions(+), 9 deletions(-) > > diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c > index cc0bd73f36b6..1755fcf726fc 100644 > --- a/net/ipv4/tcp_bpf.c > +++ b/net/ipv4/tcp_bpf.c > @@ -179,7 +179,7 @@ EXPORT_SYMBOL_GPL(tcp_bpf_sendmsg_redir); > > #ifdef CONFIG_BPF_SYSCALL > static int tcp_msg_wait_data(struct sock *sk, struct sk_psock *psock, > - long timeo) > + long *timeo) > { > DEFINE_WAIT_FUNC(wait, woken_wake_function); > int ret = 0; > @@ -187,12 +187,12 @@ static int tcp_msg_wait_data(struct sock *sk, struct sk_psock *psock, > if (sk->sk_shutdown & RCV_SHUTDOWN) > return 1; > > - if (!timeo) > + if (!*timeo) > return ret; > > add_wait_queue(sk_sleep(sk), &wait); > sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk); > - ret = sk_wait_event(sk, &timeo, > + ret = sk_wait_event(sk, timeo, > !list_empty(&psock->ingress_msg) || > !skb_queue_empty_lockless(&sk->sk_receive_queue), &wait); > sk_clear_bit(SOCKWQ_ASYNC_WAITDATA, sk); > @@ -229,6 +229,7 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk, > int copied_from_self = 0; > int copied = 0; > u32 seq; > + long timeo; > > if (unlikely(flags & MSG_ERRQUEUE)) > return inet_recv_error(sk, msg, len); > @@ -262,6 +263,8 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk, > } > } > > + timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT); > + > msg_bytes_ready: > copied = __sk_msg_recvmsg(sk, psock, msg, len, flags, &copied_from_self); > /* The typical case for EFAULT is the socket was gracefully > @@ -280,7 +283,6 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk, > } > seq += copied_from_self; > if (!copied) { > - long timeo; > int data; > > if (sock_flag(sk, SOCK_DONE)) > @@ -299,7 +301,6 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk, > goto out; > } > > - timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT); > if (!timeo) { > copied = -EAGAIN; > goto out; > @@ -310,13 +311,15 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk, > goto out; > } > > - data = tcp_msg_wait_data(sk, psock, timeo); > + data = tcp_msg_wait_data(sk, psock, &timeo); > if (data < 0) { > copied = data; > goto unlock; > } > if (data && !sk_psock_queue_empty(psock)) > goto msg_bytes_ready; > + if (!data && timeo > 0) > + goto msg_bytes_ready; > copied = -EAGAIN; > } > out: > @@ -355,6 +358,7 @@ static int tcp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, > { > struct sk_psock *psock; > int copied, ret; > + long timeo; > > if (unlikely(flags & MSG_ERRQUEUE)) > return inet_recv_error(sk, msg, len); > @@ -371,14 +375,52 @@ static int tcp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, > return tcp_recvmsg(sk, msg, len, flags); > } > lock_sock(sk); > + > + timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT); > + > msg_bytes_ready: > copied = sk_msg_recvmsg(sk, psock, msg, len, flags); > if (!copied) { > - long timeo; > int data; > > - timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT); > - data = tcp_msg_wait_data(sk, psock, timeo); > + if (sock_flag(sk, SOCK_DONE)) { > + ret = 0; > + goto check_queues; > + } > + > + if (sk->sk_err) { > + if (!sk_psock_queue_empty(psock)) > + goto msg_bytes_ready; > + if (!skb_queue_empty(&sk->sk_receive_queue)) { > + release_sock(sk); > + sk_psock_put(sk, psock); > + return tcp_recvmsg(sk, msg, len, flags); > + } > + ret = sock_error(sk); > + goto unlock; > + } Isn't that branch the same as: if (sk->sk_err) { ret = sock_error(sk); goto check_queues; } ? > + > + if (sk->sk_shutdown & RCV_SHUTDOWN) { > + ret = 0; > + goto check_queues; > + } > + > + if (sk->sk_state == TCP_CLOSE) { > + ret = -ENOTCONN; > + goto check_queues; > + } > + > + if (!timeo) { > + ret = -EAGAIN; > + goto check_queues; > + } > + > + if (signal_pending(current)) { > + ret = sock_intr_errno(timeo); > + goto check_queues; > + } > + > + data = tcp_msg_wait_data(sk, psock, &timeo); > if (data < 0) { > ret = data; > goto unlock; > @@ -390,6 +432,8 @@ 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 > 0) > + goto msg_bytes_ready; > copied = -EAGAIN; > } > ret = copied; > @@ -398,6 +442,16 @@ static int tcp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, > release_sock(sk); > sk_psock_put(sk, psock); > return ret; > + > +check_queues: > + if (!sk_psock_queue_empty(psock)) > + goto msg_bytes_ready; > + if (!skb_queue_empty(&sk->sk_receive_queue)) { > + release_sock(sk); > + sk_psock_put(sk, psock); > + return tcp_recvmsg(sk, msg, len, flags); > + } > + goto unlock; > } > > static int tcp_bpf_send_verdict(struct sock *sk, struct sk_psock *psock,