netdev - Re: [PATCH bpf v10 08/14] bpf: sockmap, incorrectly handling copied

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Date: Tue, 23 May 2023 11:09:45 +0200
From: Jakub Sitnicki <jakub@...udflare.com>
To: John Fastabend <john.fastabend@...il.com>
Cc: daniel@...earbox.net, bpf@...r.kernel.org, netdev@...r.kernel.org,
 edumazet@...gle.com, ast@...nel.org, andrii@...nel.org, will@...valent.com
Subject: Re: [PATCH bpf v10 08/14] bpf: sockmap, incorrectly handling
 copied_seq

On Mon, May 22, 2023 at 07:56 PM -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 <john.fastabend@...il.com>
> ---

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