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

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Message-Id: <20130405.004737.1210876879365869656.davem@davemloft.net>
Date:	Fri, 05 Apr 2013 00:47:37 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	ebiederm@...ssion.com
Cc:	svenjoac@....de, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	dingtianhong@...wei.com, edumazet@...gle.com, luto@...capital.net,
	ksrot@...hat.com, netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH 2/2] af_unix: If we don't care about credentials
 coallesce all messages

From: ebiederm@...ssion.com (Eric W. Biederman)
Date: Wed, 03 Apr 2013 19:14:47 -0700

> 
> 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 <ksrot@...hat.com>
> Reported-by: Ding Tianhong <dingtianhong@...wei.com>
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>

Applied.
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