netdev - Re: [PATCH bpf v6 06/12] bpf: sockmap, wake up polling after data copy

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Date:   Thu, 13 Apr 2023 15:00:02 +0200
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     daniel@...earbox.net, lmb@...valent.com, edumazet@...gle.com,
        bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...nel.org,
        andrii@...nel.org, will@...valent.com
Subject: Re: [PATCH bpf v6 06/12] bpf: sockmap, wake up polling after data copy

On Fri, Apr 07, 2023 at 10:16 AM -07, John Fastabend wrote:
> When TCP stack has data ready to read sk_data_ready() is called. Sockmap
> overwrites this with its own handler to call into BPF verdict program.
> But, the original TCP socket had sock_def_readable that would additionally
> wake up any user space waiters with sk_wake_async().
>
> Sockmap saved the callback when the socket was created so call the saved
> data ready callback and then we can wake up any epoll() logic waiting
> on the read.
>
> Note we call on 'copied >= 0' to account for returning 0 when a FIN is
> received because we need to wake up user for this as well so they
> can do the recvmsg() -> 0 and detect the shutdown.
>
> Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()")
> Signed-off-by: John Fastabend <john.fastabend@...il.com>
> ---

One observation. On the happy path, we will be hitting the recently
introduced sk_data_ready tracepoint [1] twice. However, we have the
caller IP there, so we can differentiate.

[1] 40e0b0908142 ("net/sock: Introduce trace_sk_data_ready()")

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