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

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Date:	Thu, 19 Jan 2012 16:05:47 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	Andrew Lutomirski <luto@....edu>, 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 3:54 PM, Jamie Lokier <jamie@...reable.org> wrote:
>
> Mine was: strace currently checks the CS value and may have a bug on
> existing/older kernels if Xen is involved when using the *normal*
> syscall entry point (not int $0x80).  Can we patch strace to solve
> that on those kernels in a generic way, or does the fix need to
> hard-code knowledge of Xen's CS values (and any similar PV hypervisors
> if there are any).
>
> No amount of patching newer kernels will fix that, but it would be
> nice if newer kernels made it unambiguous.

Ok.  So yeah, I think the heuristics for strace could possibly be
improved when running under Xen, I agree. See my suggestion for taking
other register contents into account (%rsp in particular - the code
segment tends to be mapped low in 64-bit mode, but the stack is almost
always high unless you are doing something really odd).

So heuristics improvements could be a good idea. Very few real
programs will use "int 0x80" in long mode, since it's slow and limited
to 32 bit.

               Linus
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