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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 C1CEFC43331 for ; Wed, 1 Apr 2020 13:48:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9FA0D206F5 for ; Wed, 1 Apr 2020 13:48:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732767AbgDANsX (ORCPT ); Wed, 1 Apr 2020 09:48:23 -0400 Received: from mx2.suse.de ([195.135.220.15]:45556 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732289AbgDANsX (ORCPT ); Wed, 1 Apr 2020 09:48:23 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 5B314AE4D; Wed, 1 Apr 2020 13:48:21 +0000 (UTC) Date: Wed, 1 Apr 2020 15:48:20 +0200 (CEST) From: Miroslav Benes To: Julien Thierry cc: linux-kernel@vger.kernel.org, jpoimboe@redhat.com, peterz@infradead.org, raphael.gault@arm.com Subject: Re: [PATCH v2 04/10] objtool: check: Ignore empty alternative groups In-Reply-To: <1f53ee68-3bcc-7de9-beb9-df812b2e3613@redhat.com> Message-ID: References: <20200327152847.15294-1-jthierry@redhat.com> <20200327152847.15294-5-jthierry@redhat.com> <1f53ee68-3bcc-7de9-beb9-df812b2e3613@redhat.com> User-Agent: Alpine 2.21 (LSU 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 1 Apr 2020, Julien Thierry wrote: > > > On 4/1/20 1:53 PM, Miroslav Benes wrote: > > On Fri, 27 Mar 2020, Julien Thierry wrote: > > > >> Atlernative section can contain entries for alternatives with no > >> instructions. Objtool will currently crash when handling such an entry. > >> > >> Just skip that entry, but still give a warning to discourage useless > >> entries. > >> > >> Signed-off-by: Julien Thierry > >> --- > >> tools/objtool/check.c | 6 ++++++ > >> 1 file changed, 6 insertions(+) > >> > >> diff --git a/tools/objtool/check.c b/tools/objtool/check.c > >> index 74353b2c39ce..5c03460f1f07 100644 > >> --- a/tools/objtool/check.c > >> +++ b/tools/objtool/check.c > >> @@ -904,6 +904,12 @@ static int add_special_section_alts(struct > >> objtool_file *file) > >> } > >> > >> if (special_alt->group) { > >> + if (!special_alt->orig_len) { > >> + WARN_FUNC("empty alternative entry", > >> + orig_insn->sec, orig_insn->offset); > >> + continue; > >> + } > >> + > >> ret = handle_group_alt(file, special_alt, orig_insn, > >> &new_insn); > >> if (ret) > > > > Probably the first time I am looking at alternatives handling in objtool, > > so I must be missing something, but is this even possible now? I mean > > get_alt_entry() in special.c sets alt->orig_len when alt->group is true > > (which means .alternatives section) to something which cannot be zero. > > > > What I see is: > > if (alt->group) { > alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset + > entry->orig_len); > alt->new_len = *(unsigned char *)(sec->data->d_buf + offset + > entry->new_len); > } Now that you copy-pasted the code here, I see that I completely missed there is dereference (for obvious reasons) right before the type cast, so all is fine. My mistake, I need more tea. > And as far as I can tell, "alt->orig_len" can be 0 if the entry in the > .altinstructions section of the .o file has the length set to 0. Yes > I don't know how the alternative section generation works on x86, but on arm64 > it's just a computed assembly offset which can be 0. > > > Is this a preparatory patch for arm64, where this could happen? If yes, it > > would be better to mention it in the changelog. > > > > It used to happen on arm64, but the fix [1] was picked. > > I can add that link to the commit if necessary. No, I think the check makes sense on its own. Thanks Miroslav