linux-kernel - Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals delivered to 64-bit programs

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Date:	Thu, 19 Mar 2015 00:34:58 +0300
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>
CC:	Andrey Wagin <avagin@...il.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Andy Lutomirski <luto@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Al Viro <viro@...iv.linux.org.uk>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals delivered
 to 64-bit programs

On 03/19/2015 12:26 AM, Andy Lutomirski wrote:
> On Wed, Mar 18, 2015 at 1:02 PM, Oleg Nesterov <oleg@...hat.com> wrote:
>> On 03/18, Andrey Wagin wrote:
>>>
>>> This patch fixes the problem. Oleg, could you send this path in the
>>> criu maillist?
>>
>> Sure, will do.
> 
> We still haven't answered one question: what's the kernel's position
> on ABI stability wrt CRIU?  We clearly shouldn't make changes that
> break the principle of CRIU, but CRIU encodes so many tricky
> assumptions about the inner workings of the kernel that it's really
> tough to avoid breaking old CRIU versions.

Well, we try hard to use only documented kernel API-s. Isn't the sigframe
considered to be some sort of "stable API"? I mean -- it's visible by the
userspace, nobody prevents glibc or gdb from messing with this stuff just
by reading it from memory.

If it's "parse-able" e.g. like VDSO is, but we don't do it in CRIU -- then
it's definitely a CRIU BUG to be fixed.

> So... do we introduce somewhat nasty code into the kernel to keep old
> CRIU versions working, or do we require that users who want to restore
> onto new kernels use new CRIU?

It's OK (I think) to require newer versions of CRIU, it's easy to update
one unlike the kernel ;)

But if "old" version of CRIU just crash the restored processes on "new"
kernels and there's no way to detect this properly -- that's the problem.

> (It seems clear to me that CRIU should apply the patch regardless of
> what the kernel does.  It will enable CRIU to work on the same class
> of programs that are fixed by the kernel change that started this
> thread.)
> 
> --Andy
> .
> 

Thanks,
Pavel

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