linux-kernel - 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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Date:	Wed, 18 Jan 2012 15:06:12 +0100
From:	Martin Mares <mj@....cz>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Jamie Lokier <jamie@...reable.org>,
	Andi Kleen <andi@...stfloor.org>,
	Indan Zupancic <indan@....nu>,
	Andrew Lutomirski <luto@....edu>,
	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,
	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!? [was: Re:
	[RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF]

Hello!

> > That would be incompatible.
> 
> No it wouldn't.
> 
> We'd only do it for the case that everybody gets wrong: int80 from a
> 64-bit context.

Not everybody. There are programs which try hard to distinguish between
int80 and syscall. One such example is a sandbox for programming contests
I wrote several years ago. It analyses the instruction before EIP and as
it does not allow threads nor executing writeable memory, it should be
correct.

The change you propose would break it. It is not a huge deal, I can fix it
in a minute, but I suspect there are other such pieces of code in the wild.

However, having TS_COMPAT available through ptrace would be great and I do not
see any other nice way how to export it to userspace, so maybe breaking the
ABI in this case is acceptable.

				Have a nice fortnight
-- 
Martin `MJ' Mares                          <mj@....cz>   http://mj.ucw.cz/
Faculty of Math and Physics, Charles University, Prague, Czech Rep., Earth
Anything is good and useful if it's made of chocolate.
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