From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756166AbZAEPe2 (ORCPT ); Mon, 5 Jan 2009 10:34:28 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754352AbZAEPeK (ORCPT ); Mon, 5 Jan 2009 10:34:10 -0500 Received: from ns2.suse.de ([195.135.220.15]:50998 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754077AbZAEPeJ (ORCPT ); Mon, 5 Jan 2009 10:34:09 -0500 Date: Mon, 5 Jan 2009 16:34:06 +0100 From: Nick Piggin To: Cyrill Gorcunov Cc: Christoph Lameter , Andrew Morton , Rik van Riel , LKML , Jiri Slaby Subject: Re: [PATCH] mm: __nr_to_section - make it safe against overflow Message-ID: <20090105153406.GB32675@wotan.suse.de> References: <20090105094034.GA7645@localhost> <20090105152848.GG7645@localhost> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090105152848.GG7645@localhost> User-Agent: Mutt/1.5.9i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 05, 2009 at 06:28:48PM +0300, Cyrill Gorcunov wrote: > [Christoph Lameter - Mon, Jan 05, 2009 at 09:10:57AM -0600] > | On Mon, 5 Jan 2009, Cyrill Gorcunov wrote: > | > | > /* > | > * This is, logically, a pointer to an array of struct > | > @@ -980,9 +986,12 @@ extern struct mem_section mem_section[NR > | > > | > static inline struct mem_section *__nr_to_section(unsigned long nr) > | > { > | > - if (!mem_section[SECTION_NR_TO_ROOT(nr)]) > | > + unsigned long idx = SECTION_NR_TO_ROOT(nr); > | > + WARN_ON_ONCE(idx >= NR_SECTION_ROOTS); > | > + > | > + if (idx >=NR_SECTION_ROOTS || !mem_section[idx]) > | > return NULL; > | > - return &mem_section[SECTION_NR_TO_ROOT(nr)][nr & SECTION_ROOT_MASK]; > | > + return &mem_section[idx][nr & SECTION_ROOT_MASK]; > | > } > | > extern int __section_nr(struct mem_section* ms); > | > extern unsigned long usemap_size(void); > | > | Not that you are adding code to numerous hot code path. Plus this is a > | frequently used inline. Code size is going to increase if you do this. > > yes, I know, that is why I've changed WARN_ON_ONCE to plain WARN_ON. Still costs. Putting it under a config option would be nice. > | I would think that the code does not have the tests because of performance > | and code size concerns. Can we just say that a sane nr must be passed to > | __nr_section? > | > > If you mean did I test this patch for speed regresson then to be fair -- > no, I didn't. BUT we have a number of macros wich are self protective > like present_section which is used havily too. On the other hand -- > bad argument passed to __nr_to_section will be (and it is now) really > harmfull -- since it would allow to reference a memory outside the > valid bounds. The second -- SECTION_ROOT_MASK wich is fragile, any > attempt to modify mem_section structure will silently lead to insane > referencing, that is why it deserve a comment on top of structure. > > Don't know Christoph, if it really that important to not spend a few > cycles here in a sake of safety -- we could easily drop this patch. The problem with testing every little slowdown for a speed regression is that they are just going to be in the noise. But we *know* it will go slower. The problem is that they add up. We just have to be sensible about it. Has there ever been a problem here before? Has it been a problem during development? (in which case putting it in a .config option might make sense).