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Date: Wed, 18 Mar 2015 21:17:18 +0300
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Oleg Nesterov <oleg@redhat.com>, Andrey Wagin <avagin@gmail.com>,
        Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Andi Kleen <andi@firstfloor.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Al Viro <viro@zeniv.linux.org.uk>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Borislav Petkov <bp@alien8.de>, Pavel Emelyanov <xemul@openvz.org>
Subject: Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals
 delivered to 64-bit programs
Message-ID: <20150318181718.GG2255@moon>
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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.