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Date: Fri, 14 Aug 2015 10:22:05 +0300
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>
Cc: Pavel Emelyanov <xemul@parallels.com>, Stas Sergeev <stsp@list.ru>,
        Linux kernel <linux-kernel@vger.kernel.org>
Subject: Re: [regression] x86/signal/64: Fix SS handling for signals
 delivered to 64-bit programs breaks dosemu
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On Thu, Aug 13, 2015 at 01:09:47PM -0700, Linus Torvalds wrote:
> On Thu, Aug 13, 2015 at 1:08 PM, Cyrill Gorcunov <gorcunov@gmail.com> wrote:
> >
> > If only I'm not missin something obvious this should not hurt us.
> > But I gonna build test kernel and check to be sure tomorrow, ok?

Managed to test it. And criu works fine with the revert as expected.
Actually it's because of commit c6f2062935c8 Oleg made us a patch:

 | commit 07dcf0dbb6ff97c255bc6b06569255a9479bccdd
 | Author: Oleg Nesterov <oleg@redhat.com>
 | Date:   Thu Mar 19 19:14:00 2015 +0300
 |
 |     restore/x86: restore_gpregs() needs to initialize ->ss as well
 |
 |     Before the recent "x86_64,signal: Fix SS handling for signals delivered
 |     to 64-bit programs" kernel patch, sigreturn paths forgot to restore ->ss
 |     after return from the signal handler.
 |
 |     Now that the kernel was fixed, restore_gpregs() has to initialize ->ss
 |     too, it is no longer ignored.
 | ...
 |
 | +++ b/arch/x86/include/asm/restorer.h
 | @@ -53,7 +53,7 @@ struct rt_sigcontext {
 |         unsigned short                  cs;
 |         unsigned short                  gs;
 |         unsigned short                  fs;
 | -       unsigned short                  __pad0;
 | +       unsigned short                  ss;
 |         unsigned long                   err;
 |         unsigned long                   trapno;
 |         unsigned long                   oldmask;

IOW we've been not setting up __pad0 which became ss
inside the kernel (in result we've been passing 0 here,
which caused the problem).

fwiw, we declare that new criu versions may require new
kernels to work but never promised that new criu gonna
be compatible with old kernels. That said if something
get changed inside sigcontext structure in future
we may update criu code as well.