Re: [PATCH v6 1/2] bpf, sockmap: handle spurious tcp_msg_wait_data() wakeup

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Mon Jul 20, 2026 at 6:53 PM EDT, Nnamdi Onyeyiri wrote:
> 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 <nnamdio@gmail.com>
>> > ---
>> >  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 I'm understanding your question correctly, and AFAICT:
sk_msg_recvmsg only drains the psock out of sk_msg data. If any data is
still in the backing struct sock but has _not_ been drained into the psock
it gets missed under the new code.

>
>> > +		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;
>>