[PATCH v7 0/2] bpf, sockmap: handle spurious tcp_msg_wait_data() wakeup

[PATCH v7 0/2] bpf, sockmap: handle spurious tcp_msg_wait_data() wakeup

[Nnamdi Onyeyiri]

Spurious wakeups in tcp_msg_wait_data() aren't being handled by
tcp_bpf_recvmsg() and tcp_bpf_recvmsg_parser(), leading to unexpected
EAGAIN errors returned by recvfrom()/recv().  Adding handling for the
wakeup and a selftest.

This issue was first discovered in an application that adds sockets to
a sockmap as a way to view the received data.  No redirects or any
other operations are performed.

Sashiko has noted a few other pre-existing issues in the same area
(https://patch.msgid.link/20260714205118.17DB11F000E9@smtp.kernel.org)
that can lead to EAGAIN in tcp_bpf_recvmsg() and
tcp_bpf_recvmsg_parser().  A local run of Sashiko also identified a
potential issue in tcp_bpf_recvmsg() handling zero-data FIN packets when
the MSG_PEEK flag is set.  To prevent this patchset from growing too
large, I intend to submit follow up patches to address these once this
one has been accepted.

Changes in v7:
- In tcp_bpf_recvmsg check for data in the TCP_CLOSE, !timeo and
  signal_pending branches.
- Refactored changes to tcp_bpf_recvmsg to reduce duplication.
- Increase the number of iterations in the selftests.
- Documented selftests numeric constants.
- Link to v6: https://patch.msgid.link/20260720171535.67867-1-nnamdio@gmail.com

Changes in v6:
- Closing the file descriptor in the selftest worker thread.
- Ensuring the selftest loop breaks early for an error in the worker
  thread.
- Added comments to selftest regarding focus on EAGAIN error.
- Updated selftest commit message to imperative mood.
- Link to v5: https://patch.msgid.link/20260717155348.54975-1-nnamdio@gmail.com

Changes in v5:
- Move selftest from net into bpf/prog_tests/sockmap_basic.c
- Link to v4: https://patch.msgid.link/20260715213538.37229-1-nnamdio@gmail.com

Changes in v4:
- Fix potential data loss in tcp_bpf_recvmsg() when a FIN or RST has
  been received.
- Check the return code of pthread_create() in the selftest.
- Fix race caused by using EXPECT macros in a thread in selftest.
- Link to v3: https://patch.msgid.link/20260714203927.32289-1-nnamdio@gmail.com

Changes in v3:
- Added the sockmap_recvfrom selftest.
- Link to v2: https://patch.msgid.link/alFRK66z45eDNZA7@localhost.localdomain

Changes in v2:
- In tcp_bpf_recvmsg, handle signals and the socket closing in the loop.
- Fix spurious wakeups when SO_RCVTIMEO has been set on the socket.
- Link to v1: https://patch.msgid.link/ak_rR-Skd8Mvn4mH@localhost.localdomain

Signed-off-by: Nnamdi Onyeyiri <nnamdio@gmail.com>
---
Nnamdi Onyeyiri (2):
  bpf, sockmap: handle spurious tcp_msg_wait_data() wakeup
  selftests/bpf: add sockmap recvfrom EAGAIN selftest

 net/ipv4/tcp_bpf.c                            |  72 ++++++++--
 .../selftests/bpf/prog_tests/sockmap_basic.c  | 136 ++++++++++++++++++
 2 files changed, 199 insertions(+), 9 deletions(-)

-- 
2.52.0

[PATCH v7 1/2] bpf, sockmap: handle spurious tcp_msg_wait_data() wakeup

[Nnamdi Onyeyiri]

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 | 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;
+		}
+
+		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,
-- 
2.52.0

[PATCH v7 2/2] selftests/bpf: add sockmap recvfrom EAGAIN selftest

[Nnamdi Onyeyiri]

These selftests exercise the tcp_bpf_recvmsg() and tcp_bpf_recvmsg_parser()
functions, to ensure that they are properly handling spurious wakeups in
tcp_msg_wait_data().

The expected behaviour is that recvfrom() does not return an EAGAIN
error.  If the spurious wakeups are incorrectly handled, this assertion
will fail.

Signed-off-by: Nnamdi Onyeyiri <nnamdio@gmail.com>
---
 .../selftests/bpf/prog_tests/sockmap_basic.c  | 136 ++++++++++++++++++
 1 file changed, 136 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
index cb3229711f93..afda4d90e573 100644
--- a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
+++ b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
@@ -1373,6 +1373,138 @@ static void test_sockmap_multi_channels(int sotype)
 	test_sockmap_pass_prog__destroy(skel);
 }
 
+static void *test_sockmap_recvfrom_eagain_thread(void *arg)
+{
+	int fd = *(int *)arg;
+	char buf[1024];
+	void *result = NULL;
+
+	while (true) {
+		ssize_t len = recvfrom(fd, buf, sizeof(buf), 0, NULL, NULL);
+
+		if (len == -1) {
+			if (errno == EINTR)
+				continue;
+			result = (void *)1;
+			break;
+		}
+
+		if (!len || buf[len - 1] == 'e')
+			break;
+	}
+
+	send(fd, "test", 4, MSG_NOSIGNAL);
+
+	close(fd);
+
+	return result;
+}
+
+static void test_sockmap_recvfrom_eagain(bool with_verdict)
+{
+	struct test_sockmap_pass_prog *skel = NULL;
+	struct bpf_program *prog = NULL;
+	char *buf = NULL;
+	int map, err;
+
+	/*
+	 * the size of the buffer to send.  completion of the sending can trigger a spurious wake
+	 * up.  larger values make the issue more likely, but make successful test runs longer.
+	 */
+	const size_t buflen = 1024 * 1024 * 25;
+
+	/*
+	 * maximum number of attempts to reproduce EAGAIN on spurious wake up.  more attempts
+	 * increases the chances of triggering the issue, and the test ends as soon as we do.
+	 * however, successful runs would take longer to complete.
+	 */
+	const int max_attempts = 300;
+
+	skel = test_sockmap_pass_prog__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		return;
+
+	map = bpf_map__fd(skel->maps.sock_map_rx);
+
+	if (with_verdict) {
+		prog = skel->progs.prog_skb_verdict;
+		err = bpf_prog_attach(bpf_program__fd(prog), map, BPF_SK_SKB_STREAM_VERDICT, 0);
+		if (!ASSERT_OK(err, "bpf_prog_attach verdict"))
+			goto cleanup;
+	}
+
+	buf = malloc(buflen);
+	if (!ASSERT_OK_PTR(buf, "malloc buf"))
+		goto cleanup;
+	memset(buf, 0, buflen);
+	buf[buflen - 1] = 'e';
+
+	for (int i = 0; i < max_attempts; ++i) {
+		ssize_t len;
+		char ignored[128];
+		pthread_t thread;
+		bool thread_created = false;
+		size_t rem = buflen;
+		int c = -1, p = -1, zero = 0;
+		bool success = false;
+
+		err = create_pair(AF_INET, SOCK_STREAM, &c, &p);
+		if (!ASSERT_OK(err, "create_pair"))
+			goto end_attempt;
+
+		err = pthread_create(&thread, NULL, &test_sockmap_recvfrom_eagain_thread, &p);
+		if (!ASSERT_OK(err, "pthread_create"))
+			goto end_attempt;
+		thread_created = true;
+
+		err = bpf_map_update_elem(map, &zero, &c, BPF_ANY);
+		if (!ASSERT_OK(err, "bpf_map_update_elem"))
+			goto end_attempt;
+
+		while (rem) {
+			len = xsend(c, buf + (buflen - rem), rem, 0);
+			if (len == -1)
+				goto end_attempt;
+			rem -= len;
+		}
+
+		/* we cannot use recv_timeout(), otherwise EAGAIN would be an expected errno. */
+		len = recvfrom(c, ignored, sizeof(ignored), 0, NULL, NULL);
+
+		/*
+		 * we are checking for the invalid return of EAGAIN, any other return is considered
+		 * successful for the purposes of this test.
+		 */
+		if (len < 0 && !ASSERT_NEQ(errno, EAGAIN, "recvfrom eagain"))
+			goto end_attempt;
+
+		success = true;
+
+end_attempt:
+		if (c >= 0)
+			close(c);
+
+		if (thread_created) {
+			void *retval = NULL;
+
+			pthread_join(thread, &retval);
+			if (!ASSERT_NULL(retval, "retval"))
+				success = false;
+		}
+
+		if (!thread_created && p >= 0)
+			close(p);
+		if (!success)
+			break;
+	}
+
+cleanup:
+	if (buf)
+		free(buf);
+
+	test_sockmap_pass_prog__destroy(skel);
+}
+
 void test_sockmap_basic(void)
 {
 	if (test__start_subtest("sockmap create_update_free"))
@@ -1451,4 +1583,8 @@ void test_sockmap_basic(void)
 		test_sockmap_multi_channels(SOCK_STREAM);
 	if (test__start_subtest("sockmap udp multi channels"))
 		test_sockmap_multi_channels(SOCK_DGRAM);
+	if (test__start_subtest("sockmap recvfrom eagain"))
+		test_sockmap_recvfrom_eagain(false);
+	if (test__start_subtest("sockmap recvfrom eagain with verdict"))
+		test_sockmap_recvfrom_eagain(true);
 }
-- 
2.52.0

Re: [PATCH v7 1/2] bpf, sockmap: handle spurious tcp_msg_wait_data() wakeup

[Jakub Sitnicki]

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 <nnamdio@gmail.com>
> ---
>  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,

Re: [PATCH v7 1/2] bpf, sockmap: handle spurious tcp_msg_wait_data() wakeup

[Nnamdi Onyeyiri]

On Wed, Jul 22, 2026 at 01:01:09PM +0200, Jakub Sitnicki wrote:
> 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 <nnamdio@gmail.com>
> > ---
> >  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;
>         }
> 
> ?
> 

sock_error clears the error in sk->sk_err.  using check_queues means
tcp_recvmsg would not see it.  is that ok?

> > +
> > +		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,