Re: [PATCH bpf v3 1/2] bpf: fix wrong copied_seq calculation

[Jakub Sitnicki <jakub@cloudflare.com>]

On Mon, Dec 23, 2024 at 09:57 PM +01, Jakub Sitnicki wrote:
> On Thu, Dec 19, 2024 at 05:30 PM +08, Jiayuan Chen wrote:
>> Currently, not all modules using strparser have issues with
>> copied_seq miscalculation. The issue exists mainly with
>> bpf::sockmap + strparser because bpf::sockmap implements a
>> proprietary read interface for user-land: tcp_bpf_recvmsg_parser().
>>
>> Both this and strp_recv->tcp_read_sock update copied_seq, leading
>> to errors.
>>
>> This is why I rewrote the tcp_read_sock() interface specifically for
>> bpf::sockmap.
>
> All right. Looks like reusing read_skb is not going to pan out.
>
> But I think we should not give up just yet. It's easy to add new code.
>
> We can try to break up and parametrize tcp_read_sock - if other
> maintainers are not against it. Does something like this work for you?
>
>   https://github.com/jsitnicki/linux/commits/review/stp-copied_seq/idea-2/

Actually it reads better if we just add early bailout to tcp_read_sock:

  https://github.com/jsitnicki/linux/commits/review/stp-copied_seq/idea-2.1/

---8<---
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 6a07d98017f7..6564ea3b6cd4 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1565,12 +1565,13 @@ EXPORT_SYMBOL(tcp_recv_skb);
  *	  or for 'peeking' the socket using this routine
  *	  (although both would be easy to implement).
  */
-int tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
-		  sk_read_actor_t recv_actor)
+static inline int __tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
+				  sk_read_actor_t recv_actor, bool noack,
+				  u32 *copied_seq)
 {
 	struct sk_buff *skb;
 	struct tcp_sock *tp = tcp_sk(sk);
-	u32 seq = tp->copied_seq;
+	u32 seq = *copied_seq;
 	u32 offset;
 	int copied = 0;
 
@@ -1624,9 +1625,12 @@ int tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
 		tcp_eat_recv_skb(sk, skb);
 		if (!desc->count)
 			break;
-		WRITE_ONCE(tp->copied_seq, seq);
+		WRITE_ONCE(*copied_seq, seq);
 	}
-	WRITE_ONCE(tp->copied_seq, seq);
+	WRITE_ONCE(*copied_seq, seq);
+
+	if (noack)
+		goto out;
 
 	tcp_rcv_space_adjust(sk);
 
@@ -1635,10 +1639,25 @@ int tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
 		tcp_recv_skb(sk, seq, &offset);
 		tcp_cleanup_rbuf(sk, copied);
 	}
+out:
 	return copied;
 }
+
+int tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
+		  sk_read_actor_t recv_actor)
+{
+	return __tcp_read_sock(sk, desc, recv_actor, false,
+			       &tcp_sk(sk)->copied_seq);
+}
 EXPORT_SYMBOL(tcp_read_sock);
 
+int tcp_read_sock_noack(struct sock *sk, read_descriptor_t *desc,
+			sk_read_actor_t recv_actor, u32 *copied_seq)
+{
+	return __tcp_read_sock(sk, desc, recv_actor, true, copied_seq);
+}
+EXPORT_SYMBOL(tcp_read_sock_noack);
+
 int tcp_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
 {
 	struct sk_buff *skb;