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=-2.7 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_NEOMUTT autolearn=unavailable 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 05041C31E49 for ; Thu, 13 Jun 2019 16:10:21 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id D1FA420866 for ; Thu, 13 Jun 2019 16:10:20 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="q8ctyTll" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D1FA420866 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=mdWHlgxtoSGpxMbF6rp5x+dAZXYhdcZPGFpzTuePiIk=; b=q8ctyTllhuJKZ1 YRDWGi9RbR/J+XhkEqlc8dXkJvKL05CnJQXOP3/za+aUH11oUFdPzIBZA1kLhirnGvmLEhaFb1265 W2HR1KccdcKWque6JfvvFh/OIEl6SNyb3/ntQtbGu9lEhGHOIkRYzUETe0asWiLlUp0vYqOozEG7c rE/DaE9tLy2GbCdOUFXMM+xkMhn2N/8suwHZMwyVW+vqzjU7SQ/6V62Bg3uX9cMUWkdKVKIcdnpUn 4uiponHV22/02Q3qzoZ/W589nZNdFAUgDPlLO60IOCVFQrQy3GjIG1rrG9wEgPWAA30z3LiE95rbn mqJoOBNTSha3sMAFaCrQ==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1hbSIt-00081A-F1; Thu, 13 Jun 2019 16:10:19 +0000 Received: from mx1.redhat.com ([209.132.183.28]) by bombadil.infradead.org with esmtps (Exim 4.92 #3 (Red Hat Linux)) id 1hbSIp-0007zX-SC for linux-arm-kernel@lists.infradead.org; Thu, 13 Jun 2019 16:10:17 +0000 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 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-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) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190613_091015_951525_1CDAB9EF X-CRM114-Status: GOOD ( 36.63 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Julien Thierry , "peterz@infradead.org" , Catalin Marinas , Will Deacon , "linux-kernel@vger.kernel.org" , Kamalesh Babulal , "linux-arm-kernel@lists.infradead.org" Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel