linux-kernel - Re: Compat 32-bit syscall entry from 64-bit task!?

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Date:	Thu, 19 Jan 2012 11:37:50 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Jamie Lokier <jamie@...reable.org>, Indan Zupancic <indan@....nu>,
	Andi Kleen <andi@...stfloor.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	keescook@...omium.org, john.johansen@...onical.com,
	serge.hallyn@...onical.com, coreyb@...ux.vnet.ibm.com,
	pmoore@...hat.com, eparis@...hat.com, djm@...drot.org,
	segoon@...nwall.com, rostedt@...dmis.org, jmorris@...ei.org,
	scarybeasts@...il.com, avi@...hat.com, penberg@...helsinki.fi,
	viro@...iv.linux.org.uk, mingo@...e.hu, akpm@...ux-foundation.org,
	khilman@...com, borislav.petkov@....com, amwang@...hat.com,
	ak@...ux.intel.com, eric.dumazet@...il.com, gregkh@...e.de,
	dhowells@...hat.com, daniel.lezcano@...e.fr,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com,
	Roland McGrath <mcgrathr@...omium.org>
Subject: Re: Compat 32-bit syscall entry from 64-bit task!?

On Thu, Jan 19, 2012 at 11:30 AM, Andrew Lutomirski <luto@....edu> wrote:
>
> Unless you're writing a debugger and you want to disassemble the code
> that's being executed (i.e. normal code, not a system call).  I wonder
> how gdb guesses whether the cpu is in long mode.

Yes, if you need to disassemble user space you would need to figure
out the mode.

I would suggest looking at 'rip/rsp' first, though, and just say that
if it's >32-bit, it's flat mode. Only if both rsp and rip fit in 32
bits should you even bother start guessing.

Because technically I suspect you really do need to look it up in the
segment descriptors, and I don't think we have that kind of interface
(nor do I think we really want to expose one).

                          Linus
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