From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx176.postini.com [74.125.245.176]) by kanga.kvack.org (Postfix) with SMTP id B88056B00C1 for ; Wed, 22 May 2013 10:07:48 -0400 (EDT) From: Arnd Bergmann Subject: Re: [PATCH v2 00/10] uaccess: better might_sleep/might_fault behavior Date: Wed, 22 May 2013 16:04:48 +0200 References: <201305221125.36284.arnd@arndb.de> <20130522134124.GD18614@n2100.arm.linux.org.uk> In-Reply-To: <20130522134124.GD18614@n2100.arm.linux.org.uk> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201305221604.49185.arnd@arndb.de> Sender: owner-linux-mm@kvack.org List-ID: To: Russell King - ARM Linux Cc: "Michael S. Tsirkin" , linux-m32r-ja@ml.linux-m32r.org, kvm@vger.kernel.org, Peter Zijlstra , Catalin Marinas , Will Deacon , David Howells , linux-mm@kvack.org, Paul Mackerras , "H. Peter Anvin" , linux-arch@vger.kernel.org, linux-am33-list@redhat.com, Hirokazu Takata , x86@kernel.org, Ingo Molnar , Benjamin Herrenschmidt , microblaze-uclinux@itee.uq.edu.au, Chris Metcalf , Thomas Gleixner , linux-arm-kernel@lists.infradead.org, Michal Simek , linux-m32r@ml.linux-m32r.org, linux-kernel@vger.kernel.org, Koichi Yasutake , linuxppc-dev@lists.ozlabs.org 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org