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

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Date:	Fri, 10 Feb 2012 23:42:01 +0100
From:	"Indan Zupancic" <indan@....nu>
To:	"H. Peter Anvin" <hpa@...or.com>
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 Fri, February 10, 2012 16:53, H. Peter Anvin wrote:
> On 02/09/2012 11:42 PM, Indan Zupancic wrote:>
>> Patch implementing this below. It uses bit 3 for task mode and bit 5
>> for syscall mode. Those bits are only valid if VIF is set. It increases
>> the kernel size by around 50 bytes, 6 for a 32-bit kernel.
>>
>> Any objections?
>
> #include <stdnak.h>

Could you please elaborate? Is it just the stealing of eflags bits that
irks you or are there technical problems too?

I understand some people would prefer a new regset, but that would force
everyone to use PTRACE_GETREGSET instead of whatever they are using now.
The problem with that is that not all archs support PTRACE_GETREGSET, so
the user space ptrace code needs to use different ptrace calls depending
on the architecture for no good reason. If PEEK_USER works then it's less
of a problem, then it's one extra ptrace call compared to the eflag way
if PTRACE_GETREGS is used. If this new info is exposed with a special
regset instead of being appended to normal regs then one extra ptrace
call per system call event needs to be done. You can as well add special
x86 ptrace requests then.

Or is the main advantage of using a regset that it shows up in coredumps?
That would merit the extra effort at least.

Stealing eflags bits may be ugly like hell, but it's very simple for both
the kernel and user space to implement.

I think everyone agrees that this kind of info needs to be exposed somehow.
In the end I don't care how it is done, as long as the info is easily
available.

Greetings,

Indan

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