From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751393AbdJBUIx (ORCPT ); Mon, 2 Oct 2017 16:08:53 -0400 Received: from gate.crashing.org ([63.228.1.57]:55938 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751059AbdJBUIw (ORCPT ); Mon, 2 Oct 2017 16:08:52 -0400 Date: Mon, 2 Oct 2017 15:08:05 -0500 From: Segher Boessenkool To: Kees Cook Cc: Benjamin Herrenschmidt , Michael Ellerman , David Laight , Linux-MM , LKML , Jinbum Park , Andrew Morton , "linuxppc-dev@lists.ozlabs.org" Subject: Re: [PATCH] mm: fix RODATA_TEST failure "rodata_test: test data was not read only" Message-ID: <20171002200805.GF8421@gate.crashing.org> References: <20170921093729.1080368AC1@po15668-vm-win7.idsi0.si.c-s.fr> <20170925073721.GM8421@gate.crashing.org> <063D6719AE5E284EB5DD2968C1650D6DD007F58B@AcuExch.aculab.com> <20170925194130.GV8421@gate.crashing.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.3i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 02, 2017 at 12:29:45PM -0700, Kees Cook wrote: > On Mon, Sep 25, 2017 at 12:41 PM, Segher Boessenkool > wrote: > > On Mon, Sep 25, 2017 at 04:01:55PM +0000, David Laight wrote: > >> From: Segher Boessenkool > >> > The compiler puts this item in .sdata, for 32-bit. There is no .srodata, > >> > so if it wants to use a small data section, it must use .sdata . > >> > > >> > Non-external, non-referenced symbols are not put in .sdata, that is the > >> > difference you see with the "static". > >> > > >> > I don't think there is a bug here. If you think there is, please open > >> > a GCC bug. > >> > >> The .sxxx sections are for 'small' data that can be accessed (typically) > >> using small offsets from a global register. > >> This means that all sections must be adjacent in the image. > >> So you can't really have readonly small data. > >> > >> My guess is that the linker script is putting .srodata in with .sdata. > > > > .srodata does not *exist* (in the ABI). > > So, I still think this is a bug. The variable is marked const: this is > not a _suggestion_. :) If the compiler produces output where the > variable is writable, that's a bug. C11 6.7.3/6: "If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined." And that is all that "const" means. The compiler is free to put this var in *no* data section, or to copy it to the stack before using it, or anything else it thinks is a good idea. If you think it would be a good idea for the compiler to change its behaviour here, please file a PR (or send a patch). Please bring arguments why we would want to change this. > I can't tell if this bug is related: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=9571 I don't think so: the only remaining bug there is that a copy of the constant is put in .rodata.cst8 (although there is a copy in .sdata2 already). Thanks, Segher