Subject: Re: [PATCH 2/2] af_unix: If we don't care about credentials coallesce all messages

From: dingtianhong <[email-masked]>
Date: Thu, 4 Apr 2013 15:56:37 +0800

On 2013/4/4 10:14, Eric W. Biederman wrote:
> 
> It was reported that the following LSB test case failed
> https://lsbbugs.linuxfoundation.org/attachment.cgi?id=2144 because we
> were not coallescing unix stream messages when the application was
> expecting us to.
> 
> The problem was that the first send was before the socket was accepted
> and thus sock->sk_socket was NULL in maybe_add_creds, and the second
> send after the socket was accepted had a non-NULL value for sk->socket
> and thus we could tell the credentials were not needed so we did not
> bother.
> 
> The unnecessary credentials on the first message cause
> unix_stream_recvmsg to start verifying that all messages had the same
> credentials before coallescing and then the coallescing failed because
> the second message had no credentials.
> 
> Ignoring credentials when we don't care in unix_stream_recvmsg fixes a
> long standing pessimization which would fail to coallesce messages when
> reading from a unix stream socket if the senders were different even if
> we did not care about their credentials.
> 
> I have tested this and verified that the in the LSB test case mentioned
> above that the messages do coallesce now, while the were failing to
> coallesce without this change.
> 
> Reported-by: Karel Srot <[email-masked]>
> Reported-by: Ding Tianhong <[email-masked]>
> Signed-off-by: "Eric W. Biederman" <[email-masked]>
> ---
>  net/unix/af_unix.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index f153a8d..2db702d 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -1993,7 +1993,7 @@ again:
>  			if ((UNIXCB(skb).pid  != siocb->scm->pid) ||
>  			    (UNIXCB(skb).cred != siocb->scm->cred))
>  				break;
> -		} else {
> +		} else if (test_bit(SOCK_PASSCRED, &sock->flags)) {
>  			/* Copy credentials */
>  			scm_set_cred(siocb->scm, UNIXCB(skb).pid, UNIXCB(skb).cred);
>  			check_creds = 1;
> 

As your opinion, I think the way is better:

		if (test_bit(SOCK_PASSCRED, &sock->flags)) {
                        if (check_creds) {
                                /* Never glue messages from different writers */
                                if ((UNIXCB(skb).pid  != siocb->scm->pid) ||
                                (UNIXCB(skb).cred != siocb->scm->cred))
                                        break;
                        } else {
                                /* Copy credentials */
                                scm_set_cred(siocb->scm, UNIXCB(skb).pid, UNIXCB(skb).cred);
                                check_creds = 1;
                        }
                }

Ding