linux-kernel - Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals delivered to 64-bit programs

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Message-ID: <20150318182522.GA1832@redhat.com>
Date:	Wed, 18 Mar 2015 19:25:22 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	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 03/18, Cyrill Gorcunov 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.

But, say, __USER_CS can be changed in kernel, and nobody should notice this.

But in this case "restore on another machine" or "restore after kernel
upgrade" can fail.

So probably restore_gpregs() should only change the general-purpose regs,
as its name suggests.

Oleg.

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