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

["Kumar Kartikeya Dwivedi" <memxor@gmail.com>]

On Tue Jul 7, 2026 at 7:19 PM CEST, Emil Tsalapatis wrote:
> On Tue Jul 7, 2026 at 12:15 PM EDT, Mattia Meleleo via B4 Relay wrote:
>> 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.
>>
>> Add a selftest covering FIONREAD without a verdict program.
>>
>> Fixes: 929e30f93125 ("bpf, sockmap: Fix FIONREAD for sockmap")
>> Signed-off-by: Mattia Meleleo <mattia.meleleo@coralogix.com>
>
> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
>
> I don't think the Sashiko READ_ONCE() recommendation is that important,
> we're just checking for the pointers' existence and I don't see how the
> reads can be moved/optimized out/merged in a way that breaks this code.
>

It would be necessary if the xchg() can happen even when the lock is held, at
the very least, to suppress potential KCSAN warnings, I think. Even for the
theoretical load tearing that causes false positive, it would be benign due to
wrong accounting.

>> ---
>>  net/ipv4/tcp_bpf.c                                 | 16 ++++++++-
>>  .../selftests/bpf/prog_tests/sockmap_basic.c       | 39 ++++++++++++++++++++++
>>  2 files changed, 54 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
>> index cc0bd73f3..a001b1fff 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 +345,20 @@ 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 (!psock->progs.stream_verdict && !psock->progs.skb_verdict)
>> +		*karg += tcp_inq(sk);
>> +	sk_psock_put(sk, psock);
>>  	unlock_sock_fast(sk, slow);
>>
>>  	return 0;
>> 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"))
>>
>> ---
>> base-commit: d2c9a99135da931377240942d44f3dea104cedb8
>> change-id: 20260707-fionread-no-verdict-a4f8697ac9f9
>>
>> Best regards,
>> --
>> Mattia Meleleo <mattia.meleleo@coralogix.com>