linux-kernel - Re: [PATCH v2 00/10] uaccess: better might_sleep/might

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Date:	Wed, 22 May 2013 14:41:24 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	linux-m32r-ja@...linux-m32r.org, kvm@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	David Howells <dhowells@...hat.com>, linux-mm@...ck.org,
	Paul Mackerras <paulus@...ba.org>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	linux-am33-list@...hat.com,
	Hirokazu Takata <takata@...ux-m32r.org>, x86@...nel.org,
	Ingo Molnar <mingo@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	microblaze-uclinux@...e.uq.edu.au,
	Chris Metcalf <cmetcalf@...era.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-arm-kernel@...ts.infradead.org,
	Michal Simek <monstr@...str.eu>, linux-m32r@...linux-m32r.org,
	linux-kernel@...r.kernel.org,
	Koichi Yasutake <yasutake.koichi@...panasonic.com>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 00/10] uaccess: better might_sleep/might_fault
	behavior

On Wed, May 22, 2013 at 11:25:36AM +0200, Arnd Bergmann wrote:
> Given the most commonly used functions and a couple of architectures
> I'm familiar with, these are the ones that currently call might_fault()
> 
> 			x86-32	x86-64	arm	arm64	powerpc	s390	generic
> copy_to_user		-	x	-	-	-	x	x
> copy_from_user		-	x	-	-	-	x	x
> put_user		x	x	x	x	x	x	x
> get_user		x	x	x	x	x	x	x
> __copy_to_user		x	x	-	-	x	-	-
> __copy_from_user	x	x	-	-	x	-	-
> __put_user		-	-	x	-	x	-	-
> __get_user		-	-	x	-	x	-	-
> 
> WTF?

I think your table is rather screwed - especially on ARM.  Tell me -
how can __copy_to_user() use might_fault() but copy_to_user() not when
copy_to_user() is implemented using __copy_to_user() ?  Same for
copy_from_user() but the reverse argument - there's nothing special
in our copy_from_user() which would make it do might_fault() when
__copy_from_user() wouldn't.

The correct position for ARM is: our (__)?(pu|ge)t_user all use
might_fault(), but (__)?copy_(to|from)_user do not.  Neither does
(__)?clear_user.  We might want to fix those to use might_fault().
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