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

[Soheil Hassas Yeganeh <soheil@google.com>]

On Wed, Nov 2, 2016 at 5:41 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> From: Eric Dumazet <edumazet@google.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@google.com>
> Cc: Willem de Bruijn <willemb@google.com>
> Cc: Soheil Hassas Yeganeh <soheil@google.com>
> Cc: Yuchung Cheng <ycheng@google.com>
> Cc: Neal Cardwell <ncardwell@google.com>

Tested-by: Soheil Hassas Yeganeh <soheil@google.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!