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 11:20:08 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	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 11:17 AM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Wed, Mar 18, 2015 at 11:06:00AM -0700, Andy Lutomirski wrote:
>> > --- 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.
>
> Yes we are. And why it can't work reliably? As to CPU -- we're
> testing that cpu features saved in image should match ones
> provided by the kernel runtime, ie on the machine where we're
> restoring.
>
>> 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.
>
> Basically the initial registers values are fetched with ptrace
> when program is been dumped, then on restore we copy them back
> into sigcontext. That said we don't mangle fs/gs anyhow simply
> restore the values back from dump.

Wouldn't it be a little safer to have the kernel make you a sane
sigcontext by raising a signal and catching it rather than writing a
new one from scratch?  Grr, maybe the kernel should version its
sigcontext structures.

Getting fs from ptrace and restoring it to sigcontext is useless --
ptrace handles fs correctly, but that sigcontext field is just
padding.

Is CRIU at least reliably putting zero in the ss field?  If so, we
could add a special case to translate zero to __USER_DS on restore.
(And then I'll update my test case, and we'll have to document it,
etc.)

--Andy
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