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Date: Wed, 18 Jan 2012 02:04:53 +0000
From: Jamie Lokier <jamie@shareable.org>
To: Andi Kleen <andi@firstfloor.org>
Cc: Indan Zupancic <indan@nul.nu>, Andrew Lutomirski <luto@mit.edu>,
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Subject: Re: Compat 32-bit syscall entry from 64-bit task!? [was: Re:
 [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF]
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Andi Kleen wrote:
> > Our ptrace jailer is checking cs to figure out if a task is a compat task
> > or not, if the kernel can change that behind our back it means our jailer
> 
> Every user program change it behind your back.
..
> Look up far jumps in any x86 manual.

I'm pretty sure this isn't about changing cs or far jumps

I think Indan means code is running with 64-bit cs, but the kernel
treats int $0x80 as a 32-bit syscall and sysenter as a 64-bit syscall,
and there's no way for the ptracer to know which syscall the kernel
will perform, even by looking at all registers.  It looks like a hole
in ptrace which could be fixed.

-- Jamie