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

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Date:   Fri, 12 Jun 2020 15:52:55 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Jakub Sitnicki <jakub@...udflare.com>
Cc:     Sabrina Dubroca <sd@...asysnail.net>,
        Network Development <netdev@...r.kernel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf] bpf: tcp: recv() should return 0 when the peer socket
 is closed

Alexei Starovoitov wrote:
> On Fri, Jun 12, 2020 at 3:18 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
> >
> > 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>
> 
> Applied. Thanks

Thanks for the patch. LGTM, I guess we should also break the
copy loop in __tcp_bpf_recvmsg if RCV_SHUTDOWN is set it
looks like TCP stack does. I'll send a follow up thanks!