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[86.153.201.25]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47f63e499c2sm31525849f8f.4.2026.07.20.15.53.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Jul 2026 15:53:25 -0700 (PDT) Date: Mon, 20 Jul 2026 23:53:23 +0100 From: Nnamdi Onyeyiri To: Emil Tsalapatis Cc: bpf@vger.kernel.org, davem@davemloft.net, edumazet@google.com, horms@kernel.org, jakub@cloudflare.com, 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 Subject: Re: [PATCH v6 1/2] bpf, sockmap: handle spurious tcp_msg_wait_data() wakeup Message-ID: References: <20260720171535.67867-1-nnamdio@gmail.com> <20260720171535.67867-2-nnamdio@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Jul 20, 2026 at 05:16:08PM -0400, Emil Tsalapatis wrote: > On Mon Jul 20, 2026 at 1:15 PM EDT, 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 | 69 ++++++++++++++++++++++++++++++++++++++++------ > > 1 file changed, 60 insertions(+), 9 deletions(-) > > > > diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c > > index cc0bd73f36b6..aa5c5d741599 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 +375,59 @@ 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 unlock; > > + } > > + > > + 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; > > + } > > + > > + if (sk->sk_shutdown & RCV_SHUTDOWN) { > > + 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 = 0; > > + goto unlock; > > These two error handling routines above look identical. Can you refactor > them? > Will do. My understanding is the same logic is needed to address the issue Sashiko raised with the SOCK_DONE check as well. > > + } > > + > > + if (sk->sk_state == TCP_CLOSE) { > > + ret = -ENOTCONN; > > + goto unlock; > > + } > > + > > + if (!timeo) { > > + ret = -EAGAIN; > > + goto unlock; > > + } > > + > > Since this handling (which Sashiko flags by the way, correctly AFAICT) > are taken from tcp_bpf_recvmsg, there is obvious overlap between the two > functions. Please factor those out so that they share the logic between > them. > > pw-bot: cr > Sashiko highlighted the "if (!timeo)" and signal_pending early returns when MSG_DONTWAIT is set, but I think I'm missing part of the picture. By the time we reach these branches, haven't we already checked for data in sk_receive_queue (line 372, after the patch is applied to 7.2-rc2) [copied below for ease of viewing]: if (!skb_queue_empty(&sk->sk_receive_queue) && sk_psock_queue_empty(psock)) { sk_psock_put(sk, psock); return tcp_recvmsg(sk, msg, len, flags); } and in the psock (line 382) [again copied below for viewing]: copied = sk_msg_recvmsg(sk, psock, msg, len, flags); > > + if (signal_pending(current)) { > > + ret = sock_intr_errno(timeo); > > + goto unlock; > > + } > > + > > + data = tcp_msg_wait_data(sk, psock, &timeo); > > if (data < 0) { > > ret = data; > > goto unlock; > > @@ -390,6 +439,8 @@ 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; >