From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_NEOMUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3D462C31E45 for ; Thu, 13 Jun 2019 16:10:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1BEFD20866 for ; Thu, 13 Jun 2019 16:10:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2391433AbfFMQKL (ORCPT ); Thu, 13 Jun 2019 12:10:11 -0400 Received: from mx1.redhat.com ([209.132.183.28]:24684 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2391415AbfFMQKF (ORCPT ); Thu, 13 Jun 2019 12:10:05 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 6D35681DEB; Thu, 13 Jun 2019 16:09:52 +0000 (UTC) Received: from treble (ovpn-121-232.rdu2.redhat.com [10.10.121.232]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 28DD919700; Thu, 13 Jun 2019 16:09:49 +0000 (UTC) Date: Thu, 13 Jun 2019 11:09:47 -0500 From: Josh Poimboeuf To: Raphael Gault Cc: "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "peterz@infradead.org" , Catalin Marinas , Will Deacon , Julien Thierry , Kamalesh Babulal Subject: Re: [RFC V2 00/16] objtool: Add support for Arm64 Message-ID: <20190613160947.vlysyfmwuo3xshsc@treble> References: <20190516103655.5509-1-raphael.gault@arm.com> <20190516142917.nuhh6dmfiufxqzls@treble> <26692833-0e5b-cfe0-0ffd-c2c2f0815935@arm.com> <20190528222415.x63qw55ujm33dozb@treble> <09745535-2782-fa11-ed65-3119b9455e79@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <09745535-2782-fa11-ed65-3119b9455e79@arm.com> User-Agent: NeoMutt/20180716 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Thu, 13 Jun 2019 16:10:04 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 13, 2019 at 04:55:31PM +0100, Raphael Gault wrote: > Hi Josh, > > On 5/28/19 11:24 PM, Josh Poimboeuf wrote: > > On Tue, May 21, 2019 at 12:50:57PM +0000, Raphael Gault wrote: > > > Hi Josh, > > > > > > Thanks for offering your help and sorry for the late answer. > > > > > > My understanding is that a table of offsets is built by GCC, those > > > offsets being scaled by 4 before adding them to the base label. > > > I believe the offsets are stored in the .rodata section. To find the > > > size of that table, it is needed to find a comparison, which can be > > > optimized out apprently. In that case the end of the array can be found > > > by locating labels pointing to data behind it (which is not 100% safe). > > > > > > On 5/16/19 3:29 PM, Josh Poimboeuf wrote: > > > > On Thu, May 16, 2019 at 11:36:39AM +0100, Raphael Gault wrote: > > > > > Noteworthy points: > > > > > * I still haven't figured out how to detect switch-tables on arm64. I > > > > > have a better understanding of them but still haven't implemented checks > > > > > as it doesn't look trivial at all. > > > > > > > > Switch tables were tricky to get right on x86. If you share an example > > > > (or even just a .o file) I can take a look. Hopefully they're somewhat > > > > similar to x86 switch tables. Otherwise we may want to consider a > > > > different approach (for example maybe a GCC plugin could help annotate > > > > them). > > > > > > > > > > The case which made me realize the issue is the one of > > > arch/arm64/kernel/module.o:apply_relocate_add: > > > > > > ``` > > > What seems to happen in the case of module.o is: > > > 334: 90000015 adrp x21, 0 > > > which retrieves the location of an offset in the rodata section, and a > > > bit later we do some extra computation with it in order to compute the > > > jump destination: > > > 3e0: 78625aa0 ldrh w0, [x21, w2, uxtw #1] > > > 3e4: 10000061 adr x1, 3f0 > > > 3e8: 8b20a820 add x0, x1, w0, sxth #2 > > > 3ec: d61f0000 br x0 > > > ``` > > > > > > Please keep in mind that the actual offsets might vary. > > > > > > I'm happy to provide more details about what I have identified if you > > > want me to. > > > > I get the feeling this is going to be trickier than x86 switch tables > > (which have already been tricky enough). > > > > On x86, there's a .rela.rodata section which applies relocations to > > .rodata. The presence of those relocations makes it relatively easy to > > differentiate switch tables from other read-only data. For example, we > > can tell that a switch table ends when either a) there's not a text > > relocation or b) another switch table begins. > > > > But with arm64 I don't see a deterministic way to do that, because the > > table offsets are hard-coded in .rodata, with no relocations. > > > > From talking with Kamalesh I got the impression that we might have a > > similar issue for powerpc. > > > > So I'm beginning to think we'll need compiler help. Like a GCC plugin > > that annotates at least the following switch table metadata: > > > > - Branch instruction address > > - Switch table address > > - Switch table entry size > > - Switch table size > > > > The GCC plugin could write all the above metadata into a special section > > which gets discarded at link time. I can look at implementing it, > > though I'll be traveling for two out of the next three weeks so it may > > be a while before I can get to it. > > > > I am completely new to GCC plugins but I had a look and I think I found a > possible solution to retrieve at least part of this information using the > RTL representation in GCC. I can't say it will work for sure but I would be > happy to discuss it with you if you want. > Although there are still some area I need to investigate related to > interacting with the RTL representation and storing info into the ELF > I'd be interested in giving it a try, if you are okay with that. Sounds promising. I've been stretched thin lately with other work, and I'll be out again next week, so please go ahead :-) -- Josh