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

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Date:	Sat, 21 Jan 2012 00:49:24 +0100
From:	"Indan Zupancic" <indan@....nu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Roland McGrath" <mcgrathr@...gle.com>,
	"Denys Vlasenko" <vda.linux@...glemail.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Andi Kleen" <andi@...stfloor.org>,
	"Jamie Lokier" <jamie@...reable.org>,
	"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,
	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
Subject: Re: Compat 32-bit syscall entry from 64-bit task!?

On Fri, January 20, 2012 23:41, H. Peter Anvin wrote:
> On 01/20/2012 02:40 PM, Roland McGrath wrote:
>> If you change the size of a regset, then the new full size will be the size
>> of the core file notes.  Existing userland tools will not be expecting
>> this, they expect a known exact size.  If you need to add new stuff, it
>> really is easier all around to add a new regset flavor.  When adding a new
>> one, you can make it variable-sized from the start so as to be extensible
>> in the future.  We did this for NT_X86_XSTATE, for example.
>>
>
> Yes, that definitely seems cleaner.

I would prefer Linus' way of just stuffing it into cs. Jamie also wanted
a bit telling in what mode the userspace is running. That's 3 bits in total,
with one bit telling whether the other bits are valid or not. Anything else?
Maybe a bit telling whether it is syscall entry or exit?

As all this is very x86_64 specific and cs is already used to figure out
the mode, it seems overkill to add a new regset just for this.

It's a lot easier for existing code to add an extra cs check than to use
different register sets and different ptrace commands. Considering that
PTRACE_GETREGSET is undocumented it's likely that existing code isn't
using it much.

Greetings,

Indan


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