[PATCH v5 0/2] bpf, sockmap: handle spurious tcp_msg_wait_data() wakeup

[PATCH v5 0/2] bpf, sockmap: handle spurious tcp_msg_wait_data() wakeup

[Nnamdi Onyeyiri]

Spurious wakeups in tcp_msg_wait_data() isn'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 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
  selftest/bpf: added selftest sockmap_basic/sockmap recvfrom eagain

 net/ipv4/tcp_bpf.c                            |  69 +++++++++--
 .../selftests/bpf/prog_tests/sockmap_basic.c  | 115 ++++++++++++++++++
 2 files changed, 175 insertions(+), 9 deletions(-)

-- 
2.52.0

[PATCH v5 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 | 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;
+		}
+
+		if (sk->sk_state == TCP_CLOSE) {
+			ret = -ENOTCONN;
+			goto unlock;
+		}
+
+		if (!timeo) {
+			ret = -EAGAIN;
+			goto unlock;
+		}
+
+		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;
-- 
2.52.0

[PATCH v5 2/2] selftest/bpf: added selftest sockmap_basic/sockmap recvfrom eagain

[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  | 115 ++++++++++++++++++
 1 file changed, 115 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..852a12b17e6b 100644
--- a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
+++ b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
@@ -1373,6 +1373,117 @@ 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);
+
+	return result;
+}
+
+static void test_sockmap_recvfrom_eagain(bool with_verdict)
+{
+	struct test_sockmap_pass_prog *skel = NULL;
+	struct bpf_program *prog = NULL;
+	size_t buflen = 1024 * 1024 * 25;
+	char *buf = NULL;
+	int map, err;
+
+	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_msg);
+
+	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 < 200; ++i) {
+		ssize_t sent;
+		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) {
+			sent = xsend(c, buf + (buflen - rem), rem, 0);
+			if (sent == -1)
+				goto end_attempt;
+			rem -= sent;
+		}
+
+		err = recvfrom(c, ignored, sizeof(ignored), 0, NULL, NULL);
+		if (err < 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);
+			ASSERT_NULL(retval, "retval");
+		}
+
+		if (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 +1562,8 @@ 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 v5 2/2] selftest/bpf: added selftest sockmap_basic/sockmap recvfrom eagain

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 1000 bytes --]

> selftest/bpf: added selftest sockmap_basic/sockmap recvfrom eagain
>
> 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>

this isn't a bug, but could the subject use the imperative mood, for
example "selftests/bpf: add sockmap recvfrom EAGAIN selftest", to match
the changelog convention and the sibling commit in the series?  The
current "added selftest sockmap_basic/sockmap recvfrom eagain" also
repeats "selftest".


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/29595053560