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

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Date:	Thu, 19 Jan 2012 11:30:41 -0800
From:	Andrew Lutomirski <luto@....edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jamie Lokier <jamie@...reable.org>, 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 11:21 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Jan 19, 2012 at 8:01 AM, Jamie Lokier <jamie@...reable.org> wrote:
>> Andrew Lutomirski wrote:
>>> It's reasonable, obvious, and even more wrong than it appears.  On
>>> Xen, there's an extra 64-bit GDT entry, and it gets used by default.
>>> (I got bitten by this in some iteration of the vsyscall emulation
>>> patches -- see user_64bit_mode for the correct and
>>> unusable-from-user-mode way to do this.)
>>
>> Here it is:
>>
>>        static inline bool user_64bit_mode(struct pt_regs *regs)
>
> This is pointless, even if it worked, which it clearly doesn't on Xen
> (or other random situations).
>
> Why would you care?
>
> The issue is *not* whether somebody is running in 32-bit mode or 64-bit mode.
>
> The problem is the system call itself, and that can be 32-bit or
> 64-bit independently of the execution mode. So knowing the user-mode
> mode is simply not relevant.

Unless you're writing a debugger and you want to disassemble the code
that's being executed (i.e. normal code, not a system call).  I wonder
how gdb guesses whether the cpu is in long mode.

--Andy
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