linux-kernel - Re: 32bit binaries on x86_64/Xen segfaults in syscall-vdso

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Message-ID: <20090904174605.GA8396@wavehammer.waldi.eu.org>
Date:	Fri, 4 Sep 2009 19:46:05 +0200
From:	Bastian Blank <waldi@...ian.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	544145@...s.debian.org, Keir Fraser <keir.fraser@...citrix.com>
Subject: Re: 32bit binaries on x86_64/Xen segfaults in syscall-vdso

On Fri, Sep 04, 2009 at 09:07:39AM -0700, Jeremy Fitzhardinge wrote:
> On 09/03/09 15:36, Bastian Blank wrote:
> > This function looks weird. It tries to restores the user code segment.
> > But the documentation from AMD explicitely stat that the CS and SS are
> > restored from the STAR register.
> 
> And STAR is always set with:
>     wrmsrl(MSR_STAR,  ((u64)__USER32_CS)<<48  | ((u64)__KERNEL_CS)<<32);

No. This is the normal kernel setup. But the Xen setup (the relevant
one) looks different:

| #define FLAT_RING3_CS32 0xe023
| wrmsr(MSR_STAR, 0, (FLAT_RING3_CS32<<16) | __HYPERVISOR_CS);

But this does not match my observation either.

And even the native Linux kernel uses "iret" to jump out of a compat
(32bit) syscall. No, I don't want to understand this, but it looks
highly broken.

Bastian

-- 
Captain's Log, star date 21:34.5...
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