Re: [PATCH] bpf, sockmap: Fix self-redirect copied_seq double-counting

[Jiayuan Chen <jiayuan.chen@linux.dev>]

on 9/1/26 7:38 PM, Jakub Sitnicki wrote:
> On Sat, Aug 29, 2026 at 10:00 AM +08, Geliang Tang wrote:
>> From: Geliang Tang <tanggeliang@kylinos.cn>
>>
>> When a BPF stream_verdict program redirects an skb back to the same
>> socket (self-redirect with BPF_F_INGRESS), sk_psock_verdict_apply()
>> calls tcp_eat_skb() which advances tcp_sk->copied_seq. However, the
>> skb is then delivered to the socket's psock ingress queue and later
>> read by tcp_bpf_recvmsg_parser(), which also advances copied_seq via

Hi Geliang,

tcp_eat_skb() will test 'skb_bpf_strparser(skb)' then skip the 
calculation of copied_seq.



>> the copied_from_self accounting path. This double-counting causes
>> copied_seq to advance by 2x the actual data length, triggering:
>>
>>    TCP recvmsg seq # bug 2: copied BF2E806, seq BF2E7FD, \
>> 			   rcvnxt BF2E806, fl 0
>>    WARNING: net/ipv4/tcp.c:2745 at tcp_recvmsg_locked+0x72b/0x2640
>>    Call Trace:
>>     tcp_recvmsg+0x10a/0x500
>>     sock_recvmsg+0x168/0x1d0
>>     __sys_recvfrom+0x19a/0x2a0
>>     __x64_sys_recvfrom+0xe4/0x1f0
>>     do_syscall_64+0xf7/0x530
>>     entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>
>>    cleanup rbuf bug: copied BF2E806 seq BF2E806 rcvnxt BF2E806
>>    WARNING: net/ipv4/tcp.c:1609 at tcp_cleanup_rbuf+0xf2/0x1c0
>>    Call Trace:
>>     tcp_recvmsg_locked+0x8d1/0x2640
>>     tcp_recvmsg+0x10a/0x500
>>     sock_recvmsg+0x168/0x1d0
>>     __sys_recvfrom+0x19a/0x2a0
>>     __x64_sys_recvfrom+0xe4/0x1f0
>>     do_syscall_64+0xf7/0x530
>>     entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>
>> Fix this by checking if the redirect destination is the same socket.
>> For self-redirect (dst == psock->sk), skip tcp_eat_skb() since the
>> copied_seq will be advanced when the data is actually read from the
>> ingress queue. For cross-socket redirects, tcp_eat_skb() is still
>> needed to account for data leaving the source socket.
>>
>> Fixes: e5c6de5fa025 ("bpf, sockmap: Incorrectly handling copied_seq")
>> Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
>> ---
>> Hi,
>>
>> I encountered this while adding MPTCP BPF sockmap support. The existing
>> TCP sockmap selftests don't cover self-redirect, but the MPTCP tests do,
>> exposing this latent issue.
>>
>> With this fix, both TCP and MPTCP tests pass, validating self-redirect
>> functionality.
>> ---
>>   net/core/skmsg.c | 8 ++++++--
>>   1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
>> index 2521b643fa05..5fa7b9639eef 100644
>> --- a/net/core/skmsg.c
>> +++ b/net/core/skmsg.c
>> @@ -1039,10 +1039,14 @@ static int sk_psock_verdict_apply(struct sk_psock *psock, struct sk_buff *skb,
>>   				goto out_free;
>>   		}
>>   		break;
>> -	case __SK_REDIRECT:
>> -		tcp_eat_skb(psock->sk, skb);
>> +	case __SK_REDIRECT: {
>> +		struct sock *dst = skb_bpf_redirect_fetch(skb);
>> +
>> +		if (dst != psock->sk)
>> +			tcp_eat_skb(psock->sk, skb);
>>   		err = sk_psock_skb_redirect(psock, skb);
>>   		break;
>> +	}
>>   	case __SK_DROP:
>>   	default:
>>   out_free:
> Isn't the source of problem on the read-side (tcp_bpf_recvmsg_parser)?


Right, I think I already fixed the parser side.


> We should be advancing copied_seq only for skbs that we received from
> the tcp stack. That's why we have the copied_from_self detection in
> tcp_bpf_recvmsg_parser.
>
> I think the problem is that we set msg->sk when we call
> sk_psock_skb_ingress_self from sk_psock_skb_ingress, so on SK_REDIRECT
> path, not the SK_PASS path.


I sucessfully use this selftest to reproduce the splat:

diff --git a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c 
b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
index 1fef6ec2ba7a..58a90f2e3602 100644
--- a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
+++ b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
@@ -1173,6 +1173,15 @@ static void test_sockmap_copied_seq(bool strp)
         if (!ASSERT_OK(err, "bpf_map_update_elem(p1)"))
                 goto end;

+       /* self redirect: data sent by c1 is redirected back to p1 itself */
+       sent = xsend(c1, buf, sizeof(buf), 0);
+       if (!ASSERT_EQ(sent, sizeof(buf), "xsend(c1), self"))
+               goto end;
+
+       recvd = recv_timeout(p1, rcv, sizeof(buf), MSG_DONTWAIT, 1);
+       if (!ASSERT_EQ(recvd, sent, "recv_timeout(p1), self"))
+               goto end;
+
         /* just trigger sockamp: data sent by c0 will be received by p1 */
         sent = xsend(c0, buf, sizeof(buf), 0);
         if (!ASSERT_EQ(sent, sizeof(buf), "xsend(c0), bpf"))
@@ -1364,6 +1373,8 @@ static void test_sockmap_no_verdict_fionread(void)

  void test_sockmap_basic(void)
  {
+       test_sockmap_copied_seq(false);
+       return;
         if (test__start_subtest("sockmap create_update_free"))
test_sockmap_create_update_free(BPF_MAP_TYPE_SOCKMAP);
         if (test__start_subtest("sockhash create_update_free"))




> REDIRECT-to-self should really be a PASS, see [1]. My suggestion - fixup
> the verdict:
>
>          if (verdict == __SK_REDIRECT && skb->sk == psock->sk)
>                  verdict = __SK_PASS;


Agree. It's simple and clear.


+        if (verdict == __SK_REDIRECT && skb_bpf_ingress(skb) &&

+            skb_bpf_redirect_fetch(skb) == psock->sk)
+                verdict = __SK_PASS;

         switch (verdict) {


> Then we can remove the sk_psock_skb_ingress_self call from
> sk_psock_skb_ingress, and kill take_ref param in
> sk_psock_skb_ingress_enqueue.
>
> John, Jiayuan, thoughts?
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2443ca66676d50a4eb3305c236bccd84a9828ce2