Subject: Re: [bpf-next PATCH v3 4/6] bpf, sockmap: remove dropped data on errors in redirect case

From: Jakub Sitnicki <[email-masked]>
Date: Mon, 12 Oct 2020 14:04:19 +0200

On Fri, Oct 09, 2020 at 08:37 PM CEST, John Fastabend wrote:
> In the sk_skb redirect case we didn't handle the case where we overrun
> the sk_rmem_alloc entry on ingress redirect or sk_wmem_alloc on egress.
> Because we didn't have anything implemented we simply dropped the skb.
> This meant data could be dropped if socket memory accounting was in
> place.
>
> This fixes the above dropped data case by moving the memory checks
> later in the code where we actually do the send or recv. This pushes
> those checks into the workqueue and allows us to return an EAGAIN error
> which in turn allows us to try again later from the workqueue.
>
> Fixes: 51199405f9672 ("bpf: skb_verdict, support SK_PASS on RX BPF path")
> Signed-off-by: John Fastabend <[email-masked]>
> ---
>  net/core/skmsg.c |   28 ++++++++++++++--------------
>  1 file changed, 14 insertions(+), 14 deletions(-)
>

[...]

> @@ -709,30 +711,28 @@ static void sk_psock_skb_redirect(struct sk_buff *skb)
>  {
>  	struct sk_psock *psock_other;
>  	struct sock *sk_other;
> -	bool ingress;
>
>  	sk_other = tcp_skb_bpf_redirect_fetch(skb);
> +	/* This error is a buggy BPF program, it returned a redirect
> +	 * return code, but then didn't set a redirect interface.
> +	 */
>  	if (unlikely(!sk_other)) {
>  		kfree_skb(skb);
>  		return;
>  	}
>  	psock_other = sk_psock(sk_other);
> +	/* This error indicates the socket is being torn down or had another
> +	 * error that caused the pipe to break. We can't send a packet on
> +	 * a socket that is in this state so we drop the skb.
> +	 */
>  	if (!psock_other || sock_flag(sk_other, SOCK_DEAD) ||
>  	    !sk_psock_test_state(psock_other, SK_PSOCK_TX_ENABLED)) {
>  		kfree_skb(skb);
>  		return;
>  	}
>
> -	ingress = tcp_skb_bpf_ingress(skb);
> -	if ((!ingress && sock_writeable(sk_other)) ||
> -	    (ingress &&
> -	     atomic_read(&sk_other->sk_rmem_alloc) <=
> -	     sk_other->sk_rcvbuf)) {

I'm wondering why the check for going over socket's rcvbuf was removed?

I see that we now rely exclusively on
sk_psock_skb_ingress→sk_rmem_schedule for sk_rmem_alloc checks, which I
don't think applies the rcvbuf limit.

> -		skb_queue_tail(&psock_other->ingress_skb, skb);
> -		schedule_work(&psock_other->work);
> -	} else {
> -		kfree_skb(skb);
> -	}
> +	skb_queue_tail(&psock_other->ingress_skb, skb);
> +	schedule_work(&psock_other->work);
>  }
>
>  static void sk_psock_tls_verdict_apply(struct sk_buff *skb, int verdict)