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

[Jakub Sitnicki <jakub@cloudflare.com>]

On Tue, Dec 24, 2024 at 03:16 PM +08, Jiayuan Chen wrote:
> On Mon, Dec 23, 2024 at 11:57:58PM +0800, Jakub Sitnicki wrote:
>> 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;
>
> This modification definitely reduces code duplication and makes it more
> elegant compared to my previous idea. Also If we want to avoid modifying
> the strp code and not introduce new ops, perhaps we could revert to the
> simplest solution:
> '''
> void sk_psock_start_strp(struct sock *sk, struct sk_psock *psock)
> {
>     ...
>     sk->sk_data_ready = sk_psock_strp_data_ready;
>     /* Replacement */
>     psock->saved_read_sock = sk->sk_socket->ops->read_sock;
>     sk->sk_socket->ops->read_sock = tcp_read_sock_noack;
> }
> '''
> If acceptable, I can incorporate this approach in the next patch version.

SGTM