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=-3.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 1B882C10F03 for ; Tue, 23 Apr 2019 21:09:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E63B3218D2 for ; Tue, 23 Apr 2019 21:09:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727718AbfDWVJw (ORCPT ); Tue, 23 Apr 2019 17:09:52 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35460 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726029AbfDWVJw (ORCPT ); Tue, 23 Apr 2019 17:09:52 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id A2B2D81245; Tue, 23 Apr 2019 21:09:51 +0000 (UTC) Received: from treble (ovpn-123-99.rdu2.redhat.com [10.10.123.99]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 850CF1001DD2; Tue, 23 Apr 2019 21:09:49 +0000 (UTC) Date: Tue, 23 Apr 2019 16:09:47 -0500 From: Josh Poimboeuf To: Raphael Gault Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, peterz@infradead.org, catalin.marinas@arm.com, will.deacon@arm.com, julien.thierry@arm.com Subject: Re: [PATCH 0/6] objtool: Add support for Arm64 Message-ID: <20190423210947.b2qomzj3qb4pzfr5@treble> References: <20190409135243.12424-1-raphael.gault@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190409135243.12424-1-raphael.gault@arm.com> User-Agent: NeoMutt/20180716 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Tue, 23 Apr 2019 21:09:51 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 09, 2019 at 02:52:37PM +0100, Raphael Gault wrote: > Hi, > > As of now, objtool only supports the x86_64 architecture but the > groundwork has already been done in order to add support for other > architecture without too much effort. > > This series of patches adds support for the arm64 architecture > based on the Armv8.5 Architecture Reference Manual. > > * Patch 1 adapts the existing code to be able to add support for other > architecture. > * Patch 2 provide implementation of the required function for the arm64 > architecture. > * Patch 3 adapts the checking of the stack state for the arm64 > architecture. > * Patch 4 & 5 fix some warning objtool raised in some particular > functions of ~/arch/arm64/kernel/sleep.S. Patch 4 add a macro to > signal that some function should be ignored by objtool. > * Patch 6 enables stack validation for arm64. > > Theses patches should provide support for the main cases and behaviour. > However a few corner cases are not yet handled by objtool: > > * In the `~/arch/arm64/crypto/` directory, I noticed that some plain > data are sometimes stored in the `.text` section causing objtool to mistake > this for instructions and trying (and failing) to interprete them. If someone > could explain to me why we store data directly in the .text section I would > appreciate it. I haven't looked, but it should probably be moved to .rodata. We had cases like that for x86. > * In the support for arm32 architecture such as in `~/arch/arm64/kernel/kuser32.S` > some A32 instructions are used but such instructions are not understood by > objtool causing a warning. > > I also have a few unclear points I would like to bring to your > attention: > > * For x86_64, when looking for a symbol relocation with explicit > addend, objtool systematically adds a +4 offset to the addend. > I don't understand why even if I have a feeling it is related > to the type of relacation. This is because of how relative call/jump addresses are implemented on x86. It calculates the call/jump destination by adding the encoded offset to the *ending* address of the instruction, rather than to the address of the encoded offset itself. For example: 119ca0: e8 00 00 00 00 callq 119ca5 <__ia32_sys_sched_rr_get_interval+0x5> 119ca1: R_X86_64_PC32 __fentry__-0x4 This instruction is a call to the __fentry__ function. The rela addend is the address of the destination function (__fentry__) minus 4. After applying the relocation, it resolves to: ffffffff81002010: e8 eb f7 9f 00 callq ffffffff81a01800 <__fentry__> The destination address is "0x9ff7eb", which is indeed __fentry__ - 4. x86 expects it to be that way, because the x86 CPU adds the offset to the *end* of the instruction: ffffffff81002015 (last digit is 5, not 0). 0xffffffff81002015 + 0x9ff7eb = 0xfffffff81a01800, which is indeed the address of __fentry__. And there's always a 4-byte gap between the relocation target and the end of the instruction, so the rela addend always has the -4. So when reading the relocation we have to add the 4 bytes back to get the actual destination address. > * I currently don't have a clear understanding about how switch-tables > are generated on arm64 and how to retrieve them (based on relocations). This is indeed a bit tricky on x86. > Please provide me with any feedback and comments as well on the content > than the style of these patches. Overall it looks like a great start. I added some per-patch comments. Has the cross-compile path been tested? Specifically, compiling for a arm64 target on an x86 host? In other words, objtool would be an x86 binary which reads arm64 objects. I imagine that will be a semi-common use case. Objtool already supports cross-compiling for an x86-64 target on an x86-32 host (and also a powerpc host IIRC), so it should be do-able in theory, and it might "just work". For the next version, please base it on the -tip tree, as that's where all the latest objtool changes are. Peter refactored some code which has some minor conflicts with yours. -- Josh