linux-kernel - Re: [PATCH] tty: Fix SIGTTOU not sent with tcflush()

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Date:	Thu, 26 Sep 2013 21:10:34 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	codonell <codonell@...hat.com>, Eduard Benes <ebenes@...hat.com>,
	Karel Srot <ksrot@...hat.com>,
	Matt Newsome <mnewsome@...hat.com>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] tty: Fix SIGTTOU not sent with tcflush()

On 09/26, Oleg Nesterov wrote:
>
> Thanks Peter.
>
> Well. I'am afraid my testing was wrong, because Karel reports
> it fixes the problem...
>
> But. I applied this patch to my 3.11 tree (last commit is bff157b3a)
> which also has the additional patch (03e12617 "tty: disassociate_ctty()
> sends the extra SIGCONT"), and
>
> 	TET_CONFIG=CFG TET_ROOT=.  ./T.tcflush 4
>
> still fails... T.tcflush was compiled on another (Karel's) machine,
> perhaps there is something in libc, I do not know.
>
> So let me ask just in case, I assume the fix below doesn't depend on
> other changes I could miss?
>
> I'll retest after git-pull and report...

Still fails under the Linus's tree + this patch.

However!!!

Tested-by: Oleg Nesterov <oleg@...hat.com>




It turns out, T.tcflush doesn't expect it can start as a process group
leader. In this case setsid() fails, then tty_open() doesn't set
signal->tty, and thus this patch makes no difference: tty_check_change()
fails because tty doesn't match signal->tty.

And indeed, this test passes if you run it under strace, or simply do

	$ TET_CONFIG=CFG TET_ROOT=. perl -e "system './T.tcflush 4'"

And damn, the fact it doesn't fail under strace doesn't allow you to
see that setsid() fails ;)

This is probably explains why Karel reported success, perhaps he
didn't run this test individually.

Thanks again Peter. Perhaps my analysis was wrong (and I do not see
setsid() in the sources), I do not really care. but perhaps tcflush.c
should be updated.

Oleg.

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