netdev - Re: [PATCH net] tcp: fix return value for partial writes

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Date:   Wed, 2 Nov 2016 17:47:23 -0400
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net] tcp: fix return value for partial writes

On Wed, Nov 2, 2016 at 5:41 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> After my commit, tcp_sendmsg() might restart its loop after
> processing socket backlog.
>
> If sk_err is set, we blindly return an error, even though we
> copied data to user space before.
>
> We should instead return number of bytes that could be copied,
> otherwise user space might resend data and corrupt the stream.
>
> This might happen if another thread is using recvmsg(MSG_ERRQUEUE)
> to process timestamps.
>
> Issue was diagnosed by Soheil and Willem, big kudos to them !
>
> Fixes: d41a69f1d390f ("tcp: make tcp_sendmsg() aware of socket backlog")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Willem de Bruijn <willemb@...gle.com>
> Cc: Soheil Hassas Yeganeh <soheil@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>

Tested-by: Soheil Hassas Yeganeh <soheil@...gle.com>

> ---
>  net/ipv4/tcp.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 3251fe71f39f..19e1468bf8ea 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1164,7 +1164,7 @@ int tcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
>
>         err = -EPIPE;
>         if (sk->sk_err || (sk->sk_shutdown & SEND_SHUTDOWN))
> -               goto out_err;
> +               goto do_error;
>
>         sg = !!(sk->sk_route_caps & NETIF_F_SG);
>
>
>

Nice fix. Thanks, Eric!