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

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Date:	Thu, 09 Feb 2012 17:15:28 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Indan Zupancic <indan@....nu>
CC:	"H.J. Lu" <hjl.tools@...il.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,
	Roland McGrath <mcgrathr@...omium.org>
Subject: Re: Compat 32-bit syscall entry from 64-bit task!?

On 02/09/2012 05:09 PM, Indan Zupancic wrote:
> On Thu, February 9, 2012 17:00, H.J. Lu wrote:
>> GDB uses CS value to tell ia32 process from x86-64 process.
> 
> Are there any cases when this doesn't work? Someone said Xen can
> have different CS values, but looking at the source it seems it's
> using the same ones, at least with a Linux hypervisor. So perhaps
> it was KVM. Looking at the header it seems paravirtualisation uses
> different cs values. On the upside, it seems we can just use that
> user_64bit_mode() to know whether it is 32 or 64 bit mode, so
> adding a bit telling the process mode is easier than I thought.
> 
> Currently there is a need to tell if the 32 or 64-bit syscall
> path is being taken, which is independent of the process mode.
> 

There are definitely cases where the current reliance on magic CS values
doesn't work; never mind the fact that it's just broken.

>> At minimum, we need a bit in CS for GDB.  But any changes
>> will break old GDB.
> 
> Would adding bits to the upper 32-bit of rflags break GDB?

It doesn't work for i386, never mind that this is reserved hardware
state and we don't have an OK at this time to redeclare them available.

> Do you also need a way to know whether the kernel was entered via
> int 0x80, SYSCALL32/64 or SYSENTER?

gdb, probably not.  That came from another user (pin, I think, but I'm
not sure.)

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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