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

From: Bastian Blank
Date: Fri Sep 04 2009 - 13:46:22 EST

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.


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