linux-kernel - Re: [PATCH] signal: Unexport sigsuspend()

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Date:	Fri, 20 Nov 2015 00:09:54 +0100
From:	Richard Weinberger <richard@....at>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, vegard.nossum@...cle.com,
	oleg@...hat.com, amanieu@...il.com, dave@...olabs.net,
	qiaowei.ren@...el.com, luto@...capital.net, palmer@...belt.com,
	vdavydov@...allels.com
Subject: Re: [PATCH] signal: Unexport sigsuspend()

Am 18.11.2015 um 21:47 schrieb Richard Weinberger:
> Am 18.11.2015 um 21:44 schrieb Andrew Morton:
>> On Mon, 16 Nov 2015 19:18:21 +0100 Richard Weinberger <richard@....at> wrote:
>>
>>> sigsuspend() is nowhere used except in signal.c itself,
>>> so we can mark it static do not pollute the global namespace.
>>>
>>> But this patch is more than a boring cleanup patch,
>>> it fixes a real issue on UserModeLinux.
>>> UML has a special console driver to display ttys using xterm,
>>> or other terminal emulators, on the host side.
>>> Vegard reported that sometimes UML is unable to spawn a xterm
>>> and he's facing the following warning:
>>> WARNING: CPU: 0 PID: 908 at include/linux/thread_info.h:128 sigsuspend+0xab/0xc0()
>>> It turned out that this warning makes absolutely no sense as
>>> the UML xterm code calls sigsuspend() on the host side, at least it tries.
>>> But as the kernel itself offers a sigsuspend() symbol the linker choose
>>> this one instead of the glibc wrapper. Interestingly this code used to
>>> work since ever but always blocked signals on the wrong side.
>>> Some recent kernel change made the WARN_ON() trigger and uncovered the bug.
>>>
>>
>> So we don't know what caused this or when it started happening.  hrm. 
>> I guess I'll stick a cc:stable in there as it's likely to affect 4.3
>> and perhaps earlier, OK?
> 
> I fear it has been this way for ever.

Did some research. It is not that bad.
It broke in v3.5 because of this commit:

commit 68f3f16d9ad0f1e28ab3fd0001ab5798c41f15a3
Author: Al Viro <viro@...iv.linux.org.uk>
Date:   Mon May 21 21:42:32 2012 -0400

    new helper: sigsuspend()

    guts of saved_sigmask-based sigsuspend/rt_sigsuspend.  Takes
    kernel sigset_t *.

    Open-coded instances replaced with calling it

Thanks,
//richard
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