From: Arnd Bergmann <arnd@arndb.de>
To: "Russell King - ARM Linux" <linux@arm.linux.org.uk>
Subject: Re: [PATCH v2 00/10] uaccess: better might_sleep/might_fault behavior
Date: Wed, 22 May 2013 16:04:48 +0200
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Cc: "Michael S. Tsirkin" <mst@redhat.com>, linux-m32r-ja@ml.linux-m32r.org,
        kvm@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
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        Koichi Yasutake <yasutake.koichi@jp.panasonic.com>,
        linuxppc-dev@lists.ozlabs.org
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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. 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
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