[PATCH bpf v3 0/2] bpf, sockmap: Fix FIONREAD for sockets without a verdict program

[PATCH bpf v3 0/2] bpf, sockmap: Fix FIONREAD for sockets without a verdict program

[Mattia Meleleo via B4 Relay]

Sockets added to a sockmap/sockhash with no stream/skb verdict program
attached answer FIONREAD with 0 even when unread data is pending in
sk_receive_queue. Fix tcp_bpf_ioctl() to account for the receive queue
in that case, and add a selftest.

Changes in v3:
- Remove unused sk_psock_msg_inq()
- Link to v2: https://patch.msgid.link/20260708-fionread-no-verdict-v2-0-29dd293621c7@coralogix.com

Changes in v2:
- Split the fix and the selftest into separate patches
- Use READ_ONCE() to read the verdict program pointers
- Link to v1: https://patch.msgid.link/20260707-fionread-no-verdict-v1-1-ce94a72357ec@coralogix.com

Signed-off-by: Mattia Meleleo <mattia.meleleo@coralogix.com>
---
Mattia Meleleo (2):
      bpf, sockmap: Account for receive queue in FIONREAD without a verdict program
      selftests/bpf: Test FIONREAD on a sockmap socket without a verdict program

 include/linux/skmsg.h                              | 14 --------
 net/ipv4/tcp_bpf.c                                 | 17 +++++++++-
 .../selftests/bpf/prog_tests/sockmap_basic.c       | 39 ++++++++++++++++++++++
 3 files changed, 55 insertions(+), 15 deletions(-)
---
base-commit: d2c9a99135da931377240942d44f3dea104cedb8
change-id: 20260707-fionread-no-verdict-a4f8697ac9f9

Best regards,
--  
Mattia Meleleo <mattia.meleleo@coralogix.com>

[PATCH bpf v3 1/2] bpf, sockmap: Account for receive queue in FIONREAD without a verdict program

[Mattia Meleleo via B4 Relay]

From: Mattia Meleleo <mattia.meleleo@coralogix.com>

tcp_bpf_ioctl() answers SIOCINQ from psock->msg_tot_len, which only
counts bytes in ingress_msg. Without a stream/skb verdict program
nothing is diverted there: data stays in sk_receive_queue, so FIONREAD
returns 0 even though read() returns data.

Add tcp_inq() to the reported value when the psock has no verdict
program. The two queues are disjoint, so bytes redirected into
ingress_msg from other sockets stay correctly accounted through
msg_tot_len.

Remove unused sk_psock_msg_inq().

Fixes: 929e30f93125 ("bpf, sockmap: Fix FIONREAD for sockmap")
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Mattia Meleleo <mattia.meleleo@coralogix.com>
---
 include/linux/skmsg.h | 14 --------------
 net/ipv4/tcp_bpf.c    | 17 ++++++++++++++++-
 2 files changed, 16 insertions(+), 15 deletions(-)

diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
index a8553401b..d5e35f247 100644
--- a/include/linux/skmsg.h
+++ b/include/linux/skmsg.h
@@ -551,20 +551,6 @@ static inline void psock_progs_drop(struct sk_psock_progs *progs)
 	psock_set_prog(&progs->skb_verdict, NULL);
 }
 
-/* for tcp only, sk is locked */
-static inline ssize_t sk_psock_msg_inq(struct sock *sk)
-{
-	struct sk_psock *psock;
-	ssize_t inq = 0;
-
-	psock = sk_psock_get(sk);
-	if (likely(psock)) {
-		inq = sk_psock_get_msg_len_nolock(psock);
-		sk_psock_put(sk, psock);
-	}
-	return inq;
-}
-
 /* for udp only, sk is not locked */
 static inline ssize_t sk_msg_first_len(struct sock *sk)
 {
diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
index cc0bd73f3..8e905b50d 100644
--- a/net/ipv4/tcp_bpf.c
+++ b/net/ipv4/tcp_bpf.c
@@ -334,6 +334,7 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk,
 
 static int tcp_bpf_ioctl(struct sock *sk, int cmd, int *karg)
 {
+	struct sk_psock *psock;
 	bool slow;
 
 	if (cmd != SIOCINQ)
@@ -344,7 +345,21 @@ static int tcp_bpf_ioctl(struct sock *sk, int cmd, int *karg)
 		return -EINVAL;
 
 	slow = lock_sock_fast(sk);
-	*karg = sk_psock_msg_inq(sk);
+	psock = sk_psock_get(sk);
+	if (unlikely(!psock)) {
+		unlock_sock_fast(sk, slow);
+		return tcp_ioctl(sk, cmd, karg);
+	}
+	*karg = sk_psock_get_msg_len_nolock(psock);
+	/* Without a verdict program, ingress data is never diverted to
+	 * ingress_msg: it stays in sk_receive_queue and is read through
+	 * the fallback to tcp_recvmsg(), so account for it like
+	 * tcp_ioctl() does.
+	 */
+	if (!READ_ONCE(psock->progs.stream_verdict) &&
+	    !READ_ONCE(psock->progs.skb_verdict))
+		*karg += tcp_inq(sk);
+	sk_psock_put(sk, psock);
 	unlock_sock_fast(sk, slow);
 
 	return 0;

-- 
Git-155)

Re: [PATCH bpf v3 1/2] bpf, sockmap: Account for receive queue in FIONREAD without a verdict program

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider:

New issues:
- [High] FIONREAD (SIOCINQ) double-counts available bytes when a verdict program is detached, exposes negative values, and under-reports bytes when a verdict program is active.

Pre-existing issues:
- [High] udp_bpf_ioctl() suffers from the exact same bug that this patch fixes for tcp_bpf_ioctl(). Without a verdict program, data stays in sk_receive_queue and UDP FIONREAD returns 0.
- [High] Data race and sequence number corruption in psock->ingress_bytes during SK_REDIRECT.
--

--- Patch [1]: [PATCH bpf v3 1/2] bpf, sockmap: Account for receive queue in FIONREAD without a verdict program ---
commit fc144ea49f61a51adcd404a526d68065c61d0452
Author: Mattia Meleleo <mattia.meleleo@coralogix.com>

bpf, sockmap: Account for receive queue in FIONREAD without a verdict program

This commit modifies tcp_bpf_ioctl() to add the output of tcp_inq() to the
reported FIONREAD value when no stream or skb verdict program is attached.
This correctly accounts for data resting in sk_receive_queue that has not
been diverted to the ingress_msg list.

> diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
> index cc0bd73f36b6d..8e905b50deadb 100644
> --- a/net/ipv4/tcp_bpf.c
> +++ b/net/ipv4/tcp_bpf.c
[ ... ]
> @@ -344,7 +345,21 @@ static int tcp_bpf_ioctl(struct sock *sk, int cmd, int *karg)
>  		return -EINVAL;
>  
>  	slow = lock_sock_fast(sk);
> -	*karg = sk_psock_msg_inq(sk);
> +	psock = sk_psock_get(sk);
> +	if (unlikely(!psock)) {
> +		unlock_sock_fast(sk, slow);
> +		return tcp_ioctl(sk, cmd, karg);
> +	}
> +	*karg = sk_psock_get_msg_len_nolock(psock);
> +	/* Without a verdict program, ingress data is never diverted to
> +	 * ingress_msg: it stays in sk_receive_queue and is read through
> +	 * the fallback to tcp_recvmsg(), so account for it like
> +	 * tcp_ioctl() does.
> +	 */
> +	if (!READ_ONCE(psock->progs.stream_verdict) &&
> +	    !READ_ONCE(psock->progs.skb_verdict))
> +		*karg += tcp_inq(sk);

[Severity: High]
Does this conditional logic double-count available bytes or under-report them
in some situations?

If a stream verdict program is attached, data is queued (which delays
tp->copied_seq), and then the program is detached. The unread bytes sit in
both msg_tot_len (ingress_msg) and are also reflected in tcp_inq(sk) because
copied_seq was held back. Adding them together here would double-count the
available data.

Conversely, when a verdict program is active, tcp_inq(sk) is ignored. Could
this fail to account for unparsed data in sk_receive_queue, leading to an
under-reported value?

> +	sk_psock_put(sk, psock);
>  	unlock_sock_fast(sk, slow);
>  
>  	return 0;

[Severity: High]
This isn't a bug introduced by this patch, but udp_bpf_ioctl() in
net/ipv4/udp_bpf.c appears to suffer from the exact same bug that this patch
fixes for tcp_bpf_ioctl().

net/ipv4/udp_bpf.c:udp_bpf_ioctl() {
    ...
    *karg = sk_msg_first_len(sk);
    ...
}

Without a verdict program, data stays in sk_receive_queue, and UDP FIONREAD
constantly returns 0. Could a similar fallback to the native receive queue be
applied there to prevent user-space applications from hanging or dropping
packets when relying on ioctl(FIONREAD)?

[Severity: High]
This is a pre-existing issue, but is there a data race and sequence number
corruption in psock->ingress_bytes during SK_REDIRECT?

In sk_psock_skb_ingress_enqueue(), psock->ingress_bytes is incremented without
locks:

net/core/skmsg.c:sk_psock_skb_ingress_enqueue() {
    ...
    psock->ingress_bytes += len;
    ...
}

Concurrently, tcp_bpf_strp_read_sock() locklessly resets psock->ingress_bytes
to 0 and uses it to adjust tp->copied_seq:

net/ipv4/tcp_bpf.c:tcp_bpf_strp_read_sock() {
    ...
    psock->ingress_bytes = 0;
    ...
    tp->copied_seq = psock->copied_seq - psock->ingress_bytes;
    ...
}

If another socket redirects data to this socket, the target socket's
psock->ingress_bytes is incremented by the workqueue or softirq. Does the race
with tcp_bpf_strp_read_sock() cause redirected bytes to be incorrectly
subtracted from the target socket's tp->copied_seq, artificially holding back
the ACK sequence and corrupting the TCP state?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260708-fionread-no-verdict-v3-0-b4ee31b3af53@coralogix.com?part=1

[PATCH bpf v3 2/2] selftests/bpf: Test FIONREAD on a sockmap socket without a verdict program

[Mattia Meleleo via B4 Relay]

From: Mattia Meleleo <mattia.meleleo@coralogix.com>

Add a test validating that FIONREAD on a TCP socket in a sockmap
without a verdict program reports data pending in sk_receive_queue.

Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Mattia Meleleo <mattia.meleleo@coralogix.com>
---
 .../selftests/bpf/prog_tests/sockmap_basic.c       | 39 ++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
index cb3229711..f0f368201 100644
--- a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
+++ b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
@@ -1373,6 +1373,43 @@ static void test_sockmap_multi_channels(int sotype)
 	test_sockmap_pass_prog__destroy(skel);
 }
 
+/* A socket in a sockmap without a verdict program keeps its ingress data
+ * in sk_receive_queue: FIONREAD must account for it.
+ */
+static void test_sockmap_no_verdict_fionread(void)
+{
+	int err, map, zero = 0, sent, avail;
+	int c0 = -1, c1 = -1, p0 = -1, p1 = -1;
+	struct test_sockmap_pass_prog *skel;
+	char buf[256] = "0123456789";
+
+	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);
+
+	err = create_socket_pairs(AF_INET, SOCK_STREAM, &c0, &c1, &p0, &p1);
+	if (!ASSERT_OK(err, "create_socket_pairs()"))
+		goto out;
+
+	err = bpf_map_update_elem(map, &zero, &c1, BPF_NOEXIST);
+	if (!ASSERT_OK(err, "bpf_map_update_elem(c1)"))
+		goto out_close;
+
+	sent = xsend(p1, &buf, sizeof(buf), 0);
+	ASSERT_EQ(sent, sizeof(buf), "xsend(p1)");
+	avail = wait_for_fionread(c1, sizeof(buf), IO_TIMEOUT_SEC);
+	ASSERT_EQ(avail, sizeof(buf), "ioctl(FIONREAD)");
+
+out_close:
+	close(c0);
+	close(p0);
+	close(c1);
+	close(p1);
+out:
+	test_sockmap_pass_prog__destroy(skel);
+}
+
 void test_sockmap_basic(void)
 {
 	if (test__start_subtest("sockmap create_update_free"))
@@ -1415,6 +1452,8 @@ void test_sockmap_basic(void)
 		test_sockmap_skb_verdict_shutdown();
 	if (test__start_subtest("sockmap skb_verdict fionread"))
 		test_sockmap_skb_verdict_fionread(true);
+	if (test__start_subtest("sockmap no_verdict fionread"))
+		test_sockmap_no_verdict_fionread();
 	if (test__start_subtest("sockmap skb_verdict fionread on drop"))
 		test_sockmap_skb_verdict_fionread(false);
 	if (test__start_subtest("sockmap skb_verdict change tail"))

-- 
Git-155)

Re: [PATCH bpf v3 2/2] selftests/bpf: Test FIONREAD on a sockmap socket without a verdict program

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider:
- [Medium] The macro `IO_TIMEOUT_SEC` (30) is passed as the `timeout_ms` argument to `wait_for_fionread()`, resulting in a very short 30 millisecond timeout instead of 30 seconds.
- [Low] Multi-line comment does not follow the strict BPF subsystem comment style.
- [Low] Unnecessary creation and cleanup of an unused socket pair (`c0` and `p0`).
--

--- Patch [2]: [PATCH bpf v3 2/2] selftests/bpf: Test FIONREAD on a sockmap socket without a verdict program ---
commit ef6ce74d985414c924fd029034cc51c133f6ad26
Author: Mattia Meleleo <mattia.meleleo@coralogix.com>

selftests/bpf: Test FIONREAD on a sockmap socket without a verdict program

Add a test validating that FIONREAD on a TCP socket in a sockmap
without a verdict program reports data pending in sk_receive_queue.

> diff --git a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> index cb3229711f93a..f0f368201ddad 100644
> --- a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> +++ b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> @@ -1373,6 +1373,43 @@ static void test_sockmap_multi_channels(int sotype)
>  	test_sockmap_pass_prog__destroy(skel);
>  }
>  
> +/* A socket in a sockmap without a verdict program keeps its ingress data
> + * in sk_receive_queue: FIONREAD must account for it.
> + */

[Severity: Low]
This isn't a bug, but does this multi-line comment follow the strict BPF
subsystem style guidelines? It appears the opening /* is expected to be
on its own line.

> +static void test_sockmap_no_verdict_fionread(void)
> +{
> +	int err, map, zero = 0, sent, avail;
> +	int c0 = -1, c1 = -1, p0 = -1, p1 = -1;
> +	struct test_sockmap_pass_prog *skel;
> +	char buf[256] = "0123456789";
> +
> +	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);
> +
> +	err = create_socket_pairs(AF_INET, SOCK_STREAM, &c0, &c1, &p0, &p1);

[Severity: Low]
Is there a reason to allocate four socket file descriptors here?

It looks like the c0 and p0 sockets are created but never used in the test.
Could create_pair() be used instead to avoid unnecessarily creating and
closing the unused socket pair?

> +	if (!ASSERT_OK(err, "create_socket_pairs()"))
> +		goto out;
> +
> +	err = bpf_map_update_elem(map, &zero, &c1, BPF_NOEXIST);
> +	if (!ASSERT_OK(err, "bpf_map_update_elem(c1)"))
> +		goto out_close;
> +
> +	sent = xsend(p1, &buf, sizeof(buf), 0);
> +	ASSERT_EQ(sent, sizeof(buf), "xsend(p1)");
> +	avail = wait_for_fionread(c1, sizeof(buf), IO_TIMEOUT_SEC);

[Severity: Medium]
Will this result in a flaky test on slow or heavily loaded CI systems?

Looking at the wait_for_fionread() implementation, the third parameter is
timeout_ms, which dictates the number of 1 millisecond sleep iterations:

	while (elapsed < timeout_ms) {
		if (ioctl(fd, FIONREAD, &avail) < 0)
			return -errno;
		if (avail >= expected)
			return avail;
		usleep(1000);
		elapsed++;
	}

By passing IO_TIMEOUT_SEC (which is typically 30), it seems the test will only
wait 30 milliseconds instead of 30 seconds for the packet to traverse the local
network stack.

> +	ASSERT_EQ(avail, sizeof(buf), "ioctl(FIONREAD)");
> +
> +out_close:
> +	close(c0);
> +	close(p0);
> +	close(c1);
> +	close(p1);
> +out:
> +	test_sockmap_pass_prog__destroy(skel);
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260708-fionread-no-verdict-v3-0-b4ee31b3af53@coralogix.com?part=2