Subject: Re: [PATCH bpf v7 08/13] bpf: sockmap, incorrectly handling copied_seq

From: Jakub Sitnicki <[email-masked]>
Date: Fri, 05 May 2023 14:14:12 +0200

On Tue, May 02, 2023 at 08:51 AM -07, John Fastabend wrote:
> The read_skb() logic is incrementing the tcp->copied_seq which is used for
> among other things calculating how many outstanding bytes can be read by
> the application. This results in application errors, if the application
> does an ioctl(FIONREAD) we return zero because this is calculated from
> the copied_seq value.
>
> To fix this we move tcp->copied_seq accounting into the recv handler so
> that we update these when the recvmsg() hook is called and data is in
> fact copied into user buffers. This gives an accurate FIONREAD value
> as expected and improves ACK handling. Before we were calling the
> tcp_rcv_space_adjust() which would update 'number of bytes copied to
> user in last RTT' which is wrong for programs returning SK_PASS. The
> bytes are only copied to the user when recvmsg is handled.
>
> Doing the fix for recvmsg is straightforward, but fixing redirect and
> SK_DROP pkts is a bit tricker. Build a tcp_psock_eat() helper and then
> call this from skmsg handlers. This fixes another issue where a broken
> socket with a BPF program doing a resubmit could hang the receiver. This
> happened because although read_skb() consumed the skb through sock_drop()
> it did not update the copied_seq. Now if a single reccv socket is
> redirecting to many sockets (for example for lb) the receiver sk will be
> hung even though we might expect it to continue. The hang comes from
> not updating the copied_seq numbers and memory pressure resulting from
> that.
>
> We have a slight layer problem of calling tcp_eat_skb even if its not
> a TCP socket. To fix we could refactor and create per type receiver
> handlers. I decided this is more work than we want in the fix and we
> already have some small tweaks depending on caller that use the
> helper skb_bpf_strparser(). So we extend that a bit and always set
> the strparser bit when it is in use and then we can gate the
> seq_copied updates on this.
>
> Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()")
> Signed-off-by: John Fastabend <[email-masked]>
> ---
>  include/net/tcp.h  | 10 ++++++++++
>  net/core/skmsg.c   |  7 +++++--
>  net/ipv4/tcp.c     | 10 +---------
>  net/ipv4/tcp_bpf.c | 28 +++++++++++++++++++++++++++-
>  4 files changed, 43 insertions(+), 12 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index db9f828e9d1e..76bf0a11bdc7 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -1467,6 +1467,8 @@ static inline void tcp_adjust_rcv_ssthresh(struct sock *sk)
>  }
>  
>  void tcp_cleanup_rbuf(struct sock *sk, int copied);
> +void __tcp_cleanup_rbuf(struct sock *sk, int copied);
> +
>  
>  /* We provision sk_rcvbuf around 200% of sk_rcvlowat.
>   * If 87.5 % (7/8) of the space has been consumed, we want to override
> @@ -2323,6 +2325,14 @@ int tcp_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool restore);
>  void tcp_bpf_clone(const struct sock *sk, struct sock *newsk);
>  #endif /* CONFIG_BPF_SYSCALL */
>  
> +#ifdef CONFIG_INET
> +void tcp_eat_skb(struct sock *sk, struct sk_buff *skb);
> +#else
> +static inline void tcp_eat_skb(struct sock *sk, struct sk_buff *skb)
> +{
> +}
> +#endif
> +
>  int tcp_bpf_sendmsg_redir(struct sock *sk, bool ingress,
>  			  struct sk_msg *msg, u32 bytes, int flags);
>  #endif /* CONFIG_NET_SOCK_MSG */
> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> index 3c0663f5cc3e..18c4f4015559 100644
> --- a/net/core/skmsg.c
> +++ b/net/core/skmsg.c
> @@ -1017,11 +1017,14 @@ static int sk_psock_verdict_apply(struct sk_psock *psock, struct sk_buff *skb,
>  		}
>  		break;
>  	case __SK_REDIRECT:
> +		tcp_eat_skb(psock->sk, skb);
>  		err = sk_psock_skb_redirect(psock, skb);
>  		break;
>  	case __SK_DROP:
>  	default:
>  out_free:
> +		tcp_eat_skb(psock->sk, skb);
> +		skb_bpf_redirect_clear(skb);
>  		sock_drop(psock->sk, skb);
>  	}
>  

I have a feeling you wanted to factor out the common
skb_bpf_redirect_clear() into out_free: block, but maybe forgot to
update the jump sites?

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