Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756252Ab3EVOpg (ORCPT ); Wed, 22 May 2013 10:45:36 -0400 Received: from mx1.redhat.com ([209.132.183.28]:8891 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756113Ab3EVOpe (ORCPT ); Wed, 22 May 2013 10:45:34 -0400 Date: Wed, 22 May 2013 17:44:06 +0300 From: "Michael S. Tsirkin" To: Arnd Bergmann Cc: Russell King - ARM Linux , 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 Subject: Re: [PATCH v2 00/10] uaccess: better might_sleep/might_fault behavior Message-ID: <20130522144406.GB21886@redhat.com> References: <201305221125.36284.arnd@arndb.de> <20130522134124.GD18614@n2100.arm.linux.org.uk> <201305221604.49185.arnd@arndb.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <201305221604.49185.arnd@arndb.de> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3270 Lines: 77 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/