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

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Date:	Thu, 9 Feb 2012 08:00:53 -0800
From:	"H.J. Lu" <hjl.tools@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Indan Zupancic <indan@....nu>,
	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,
	Roland McGrath <mcgrathr@...omium.org>
Subject: Re: Compat 32-bit syscall entry from 64-bit task!?

On Wed, Feb 8, 2012 at 8:29 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 02/08/2012 08:20 PM, Indan Zupancic wrote:
>>
>> CS is already available to user space, but any other value than 0x23 or 0x33
>> will confuse user space, as that is all they know about. Apparently Xen uses
>> different values, but if those are static then user space can check for them
>> separately. But if the values change dynamically then some other way may be
>> needed.
>>
>> But does it make much sense to pass the CPU mode of user space if that mode
>> can be changed at any moment? I don't think it really does. Can you give an
>> example of how that info can be used by a ptracer?
>>
>
> Uh... you could make THAT argument about ANY register state!
>
> I believe H.J. can fill you in about the usage.
>

GDB uses CS value to tell ia32 process from x86-64 process.
At minimum, we need a bit in CS for GDB.  But any changes
will break old GDB.

H.J.
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