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

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Date:	Thu, 19 Jan 2012 02:45:01 +0100
From:	"Indan Zupancic" <indan@....nu>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	"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, January 18, 2012 22:26, Linus Torvalds wrote:
> On Wed, Jan 18, 2012 at 1:04 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> Fair enough.  Sigh.  I still think an actual pseudo-register would be
>> better.
>
> .. and that breaks existing binaries too, because the indexing is
> based on offsets into "struct pt_regs", and while we *could* change
> that - leave pt_regs untouched but add a new virtual register - it
> would be problematic.
>
> We could add a whole new ptrace() access command (eg
> PTRACE_GETSYSTEMREGSET), of course. But that's a lot of effort for
> very little gain.
>
> So on the whole, putting it in eflags seemed like the *much* simpler approach.

For security reasons it should be impossible for userspace to set those bits
themselves, otherwise the tracer can be easily fooled on an old kernel. That
seems to be the case for the higher bits of eflags, so eflags would work. And
the current code checks cs, also checking eflags would be very easy to add.

Greetings,

Indan


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