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:	Wed, 18 Mar 2015 11:06:00 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	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>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	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 10:48 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 03/18, Andrey Wagin wrote:
>>
>> This commit breaks CRIU. I don't have any details yet. I'm going to
>> investigate this issue and provide more details tomorrow.
>>
>> [root@...gin-fc19-cr criu]# setsid sleep 1000 &
>> [1] 1225
>> [root@...gin-fc19-cr criu]# ps -C sleep
>>   PID TTY          TIME CMD
>>  1226 ?        00:00:00 sleep
>> [root@...gin-fc19-cr criu]# ./criu dump -t 1226 -D dump --shell-job
>> [root@...gin-fc19-cr criu]# ./criu restore -D dump --shell-job
>> Error (parasite-syscall.c:923): Task is in unexpected state: b7f (SIGSEGV)
>
> This is funny. Because currenty I am looking into criu sources for quite
> different reason (and I HATE this reason ;)
>
> Shot in a dark afer a quick grep: restore_gpregs() should initialize ->ss?
>
> perhaps something like below... obviously uncompiled/untested.
>
> And my grep can't find the definition of UserX86RegsEntry in crtools...
> Perhaps the change below needs CPREG1(ss, anothername).
>
> Seriously, where is UserX86RegsEntry?
>
> Oleg.
>
>
> --- a/arch/x86/crtools.c
> +++ b/arch/x86/crtools.c
> @@ -475,6 +475,7 @@ int restore_gpregs(struct rt_sigframe *f, UserX86RegsEntry *r)
>         CPREG2(rip, ip);
>         CPREG2(eflags, flags);
>         CPREG1(cs);
> +       CPREG1(ss);
>         CPREG1(gs);
>         CPREG1(fs);

Huh?  Is CRIU actually trying to build an entire sigcontext from
scratch here?  I don't see how this can reliably work across kernel
versions or CPU versions.

Also, what's up with CPREG1(gs) and CPREG1(fs)?  I assume that's
redundant, because that hasn't worked for many years, but CRIU works,
so there must be correct code somewhere to restore those regs.

--Andy
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