netdev - Re: [V4.4-rc6 Regression] af_unix: Revert 'lock_interruptible' in stream receive code

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Message-ID: <87io20upk9.fsf@doppelsaurus.mobileactivedefense.com>
Date:	Sun, 07 Feb 2016 22:24:22 +0000
From:	Rainer Weikusat <rweikusat@...ileactivedefense.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Rainer Weikusat <rweikusat@...ileactivedefense.com>,
	davem@...emloft.net, hannes@...essinduktion.org,
	edumazet@...gle.com, dhowells@...hat.com, ying.xue@...driver.com,
	"netdev\@vger.kernel.org" <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"stable\@vger.kernel.org" <stable@...r.kernel.org>,
	Joseph Salisbury <joseph.salisbury@...onical.com>
Subject: Re: [V4.4-rc6 Regression] af_unix: Revert 'lock_interruptible' in stream receive code

Rainer Weikusat <rw@...pelsaurus.mobileactivedefense.com> writes:

[...]

> The start uses that to record an error which might need to be
> reported, the return statement uses it to indicate that an error has
> occurred. Hence, some kind of in-between translation must occur.  The
> mutex_lock_interruptible happened to do that but that was never it's
> intended purpose.

Additional information: The 'trick' of using recvmsg w/o a receive
buffer in order to retrieve control messages in fact wouldn't have
worked with the unix_stream_recvmsg prior to introduction of the
interruptible lock as that (judging from the git source) would have
triggered all the same issues,

	- -EOPNOTSUP if a msg was available

        - -EAGAIN if the code had to wait

        - not receiving the creds if the -EAGAIN hadn't happened because
          of the continue (that's the other patch)

IOW, that's a feature inadvertendly added by an otherwise useless code
change (mea culpa).