linux-kernel - Re: [PATCH v2 0/4] x86: sigcontext fixes, again

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Message-ID: <56309B10.50706@list.ru>
Date:	Wed, 28 Oct 2015 12:53:20 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Toshi Kani <toshi.kani@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Toshi Kani <toshi.kani@...com>, Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Stas Sergeev <stsp@...rs.sourceforge.net>
Subject: Re: [PATCH v2 0/4] x86: sigcontext fixes, again

28.10.2015 03:04, Toshi Kani пишет:
> On Wed, 2015-10-28 at 07:37 +0900, Linus Torvalds wrote:
>> On Tue, Oct 27, 2015 at 11:05 PM, Stas Sergeev <stsp@...t.ru> wrote:
>>>
>>> I can't easily post an Oops: under X it doesn't even appear -
>>> machine freezes immediately, and under non-KMS console it is
>>> possible to get one, but difficult to screen-shot (using bare
>>> metal, not VM). Also the Oops was seemingly unrelated.
>>> And if you run "dosemu -s" under non-KMS console, you'll also
>>> reproduce this one:
>>> https://bugzilla.kernel.org/show_bug.cgi?id=97321
>>
>> Hmm. Andrew Morton responded to that initially, but then nothing
>> happened, and now it's been another six months. Andrew?
>>
>> The arch/x86/mm/pat.c error handling does seem to be suspect. This is
>> all code several years old, so none of this is new, and I think Suresh
>> is gone.  Adding a few other people with recent sign-offs to that
>> file, in the hope that somebody feels like they own it..
> 
> In the case of PFNMAP, the range should always be mapped.  So, I wonder why
> follow_phys() failed with the !pte_present() check.
> 
> Stas, do you have a test program that can reproduce 97321?
Get dosemu2 from here:
https://github.com/stsp/dosemu2/releases
or from git, or get dosemu1.
Then boot your kernel with "nomodeset=1" to get a text console.
Run

dosemu -s

and you'll get the bug.
You don't even need to mess with FreeDOS or whatever, because
the problem happens on a very start. And since it is WARN_OCNCE,
you'll need to reboot to get it again.
You will probably need to adjust /etc/sudoers because -s means
that dosemu (a wrapper script for dosemu.bin) will try sudo so
that dosemu.bin can grab /dev/mem as root.

If it is too complicated, let me know and I'll try to code up
a reduced test-case (but can't promise to get to that quickly).

Thanks to everyone (esp Linus of course :-) who got involved in
finally supporting dosemu again - it was rather ill for the last
years.
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