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

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Date:	Wed, 22 May 2013 17:44:06 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	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 04:04:48PM +0200, Arnd Bergmann wrote:
> On Wednesday 22 May 2013, Russell King - ARM Linux wrote:
> > 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.
> 
> I think something went wrong with formatting of the tabstobs in
> the table. I've tried to correct it above to the same version I
> see on the mailing list.
> 
> > 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().
> 
> Yes, that sounds like a good idea, especially since they are all
> implemented out-of-line.
> 
> For __get_user()/__put_user(), I would probably do the reverse and make
> them not call might_fault() though, like we do on most other architectures:
> 
> Look at the object code produced for setup_sigframe for instance, it calls
> might_fault() around 25 times where one should really be enough.

Well it depends on what config options you set.
But with VOLUNTARY you are right.
Also, look at memcpy_fromiovec and weep.

> Using
> __put_user() instead of put_user() is normally an indication that the
> author of that function has made performance considerations and move the
> (trivial) access_ok() call out, but now we add a more expensive
> call instead.
> 
> 	Arnd

I think exactly the same rules should apply to __XXX_user and
__copy_XXX_user - otherwise it's really confusing.

Maybe a preempt point in might_fault should go away?
Basically

#define might_fault() __might_sleep(__FILE__, __LINE__, 0)

Possibly adding the in_atomic() etc checks that Peter suggested.

Ingo, what do you think? And what testing would be appropriate
for such a change?


Thanks,

-- 
MST
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