netdev - Re: [PATCH bpf] bpf: tcp: recv() should return 0 when the peer socket is closed

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Message-ID: <20200612121526.4810a073@toad>
Date:   Fri, 12 Jun 2020 12:15:26 +0200
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     Sabrina Dubroca <sd@...asysnail.net>
Cc:     netdev@...r.kernel.org, John Fastabend <john.fastabend@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>, bpf@...r.kernel.org
Subject: Re: [PATCH bpf] bpf: tcp: recv() should return 0 when the peer
 socket is closed

On Wed, 10 Jun 2020 12:19:43 +0200
Sabrina Dubroca <sd@...asysnail.net> wrote:

> If the peer is closed, we will never get more data, so
> tcp_bpf_wait_data will get stuck forever. In case we passed
> MSG_DONTWAIT to recv(), we get EAGAIN but we should actually get
> 0.
> 
> From man 2 recv:
> 
>     RETURN VALUE
> 
>     When a stream socket peer has performed an orderly shutdown, the
>     return value will be 0 (the traditional "end-of-file" return).
> 
> This patch makes tcp_bpf_wait_data always return 1 when the peer
> socket has been shutdown. Either we have data available, and it would
> have returned 1 anyway, or there isn't, in which case we'll call
> tcp_recvmsg which does the right thing in this situation.
> 
> Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
> Signed-off-by: Sabrina Dubroca <sd@...asysnail.net>
> ---
>  net/ipv4/tcp_bpf.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
> index 2b915aafda42..7aa68f4aae6c 100644
> --- a/net/ipv4/tcp_bpf.c
> +++ b/net/ipv4/tcp_bpf.c
> @@ -245,6 +245,9 @@ static int tcp_bpf_wait_data(struct sock *sk, struct sk_psock *psock,
>  	DEFINE_WAIT_FUNC(wait, woken_wake_function);
>  	int ret = 0;
>  
> +	if (sk->sk_shutdown & RCV_SHUTDOWN)
> +		return 1;
> +
>  	if (!timeo)
>  		return ret;
>  

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