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 10:35:12 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Pavel Emelyanov <xemul@...allels.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrey Wagin <avagin@...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 Wed, Mar 18, 2015 at 03:03:27PM -0700, Andy Lutomirski wrote:
> On Wed, Mar 18, 2015 at 2:34 PM, Pavel Emelyanov <xemul@...allels.com> wrote:
> > 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.
> 
> It's certainly parseable by things like gdb.  But it's also supposed
> to be extensible.  hpa, any thoughts here?
> 
> >
> >> 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.
> 
> Yeah, that's unfortunate.
> 
> I don't have a great idea for how to work around this, unfortunately.
> Ideally we'd increment some kind of version counter or use an
> extension mechanism rather than shoving ss into a field that used to
> be padding.

fwiw currently we're passing zero in this __pad0 (replying to your
previous email, so we can workaround in the kernel assuming zero
as a special case, not that good but better than nothing).

> 
> --Andy
> 
> >
> >> (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
> >
> 
> 
> 
> -- 
> Andy Lutomirski
> AMA Capital Management, LLC
> 

	Cyrill
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