Re: [PATCH bpf v9 2/5] bpf: fix wrong copied_seq calculation

Jakub Sitnicki
2025-01-26T14:10:51Z

On Wed, Jan 22, 2025 at 06:09 PM +08, Jiayuan Chen wrote:
> 'sk->copied_seq' was updated in the tcp_eat_skb() function when the action
> of a BPF program was SK_REDIRECT. For other actions, like SK_PASS, the
> update logic for 'sk->copied_seq' was moved to tcp_bpf_recvmsg_parser()
> to ensure the accuracy of the 'fionread' feature.
>
> It works for a single stream_verdict scenario, as it also modified
> sk_data_ready->sk_psock_verdict_data_ready->tcp_read_skb
> to remove updating 'sk->copied_seq'.
>
> However, for programs where both stream_parser and stream_verdict are
> active (strparser purpose), tcp_read_sock() was used instead of
> tcp_read_skb() (sk_data_ready->strp_data_ready->tcp_read_sock).
> tcp_read_sock() now still updates 'sk->copied_seq', leading to duplicate
> updates.
>
> In summary, for strparser + SK_PASS, copied_seq is redundantly calculated
> in both tcp_read_sock() and tcp_bpf_recvmsg_parser().
>
> The issue causes incorrect copied_seq calculations, which prevent
> correct data reads from the recv() interface in user-land.
>
> We do not want to add new proto_ops to implement a new version of
> tcp_read_sock, as this would introduce code complexity [1].
>
> We could have added noack and copied_seq to desc, and then called
> ops->read_sock. However, unfortunately, other modules didn’t fully
> initialize desc to zero. So, for now, we are directly calling
> tcp_read_sock_noack() in tcp_bpf.c.
>
> [1]: https://lore.kernel.org/bpf/20241218053408.437295-1-mrpre@163.com
> Fixes: e5c6de5fa025 ("bpf, sockmap: Incorrectly handling copied_seq")
> Suggested-by: Jakub Sitnicki <jakub@cloudflare.com>
> Signed-off-by: Jiayuan Chen <mrpre@163.com>
> ---

I'm happy with how this turned out, but let's run it by Eric.

Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>