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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_MUTT 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 75768C48BD3 for ; Wed, 26 Jun 2019 16:31:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4A8B92183F for ; Wed, 26 Jun 2019 16:31:27 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="fIMR7D57" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726478AbfFZQb0 (ORCPT ); Wed, 26 Jun 2019 12:31:26 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:37220 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726006AbfFZQbZ (ORCPT ); Wed, 26 Jun 2019 12:31:25 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=K6GM20Sjlcpa53T5ol2yNh7EWFpyKggKBswitXSEOg4=; b=fIMR7D57/qq7CZ0w47lXiEAtp HW7ySd5Ansu8NUsr6aQNqXdPSCWfyAPOpMDHPZYWku9omzeBDHETeqZanh1mvwOF5nbQlIwd+QNJ4 v717E9r6Olyo2bKQkbmiwGkqYucSLFgZo6v9T0QMa9Unw/216EXNdAsG1hG9FkSYW3AJJZkWS2eHR ribaReaj6lI+jwMQDGlmNzc2PIo5R2Sr+emQKjvs/w+D1RrTGHG263RmYYYyYWTDE247hu/8PhOgo 85VYpbp+rAGmxs0BaPG9NPwr6jqdzpiqQJjpdbXncoelsMnTzCuMMhHGQMALn7V4K7REU9I7l32tw PWAqQ839Q==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1hgAp1-0004Qp-BK; Wed, 26 Jun 2019 16:30:59 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 67A96209CEDB5; Wed, 26 Jun 2019 18:30:57 +0200 (CEST) Date: Wed, 26 Jun 2019 18:30:57 +0200 From: Peter Zijlstra To: Thomas Gleixner Cc: Nathan Chancellor , Kees Cook , Miguel Ojeda , "Gustavo A. R. Silva" , Joe Perches , Ingo Molnar , Arnaldo Carvalho de Melo , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Borislav Petkov , "H. Peter Anvin" , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , Kan Liang , linux-kernel , Shawn Landden , clang-built-linux@googlegroups.com, Josh Poimboeuf Subject: Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs Message-ID: <20190626163057.GN3419@hirez.programming.kicks-ass.net> References: <20190624203737.GL3436@hirez.programming.kicks-ass.net> <3dc75cd4-9a8d-f454-b5fb-64c3e6d1f416@embeddedor.com> <20190625071846.GN3436@hirez.programming.kicks-ass.net> <201906251009.BCB7438@keescook> <20190625180525.GA119831@archlinux-epyc> <20190625202746.GA83499@archlinux-epyc> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 25, 2019 at 11:47:06PM +0200, Thomas Gleixner wrote: > > On Tue, Jun 25, 2019 at 09:53:09PM +0200, Thomas Gleixner wrote: > > > but it also makes objtool unhappy: > > > arch/x86/kernel/cpu/mtrr/generic.o: warning: objtool: get_fixed_ranges()+0x9b: unreachable instruction > I just checked two of them in the disassembly. In both cases it's jump > label related. Here is one: > > asm volatile("1: rdmsr\n" > 410: b9 59 02 00 00 mov $0x259,%ecx > 415: 0f 32 rdmsr > 417: 49 89 c6 mov %rax,%r14 > 41a: 48 89 d3 mov %rdx,%rbx > return EAX_EDX_VAL(val, low, high); > 41d: 48 c1 e3 20 shl $0x20,%rbx > 421: 48 09 c3 or %rax,%rbx > 424: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) > 429: eb 0f jmp 43a > do_trace_read_msr(msr, val, 0); > 42b: bf 59 02 00 00 mov $0x259,%edi <------- "unreachable" > 430: 48 89 de mov %rbx,%rsi > 433: 31 d2 xor %edx,%edx > 435: e8 00 00 00 00 callq 43a > 43a: 44 89 35 00 00 00 00 mov %r14d,0x0(%rip) # 441 Thomas provided the actual .o file, and from that we find that the .rela__jump_table entries look like: 000000000010 000100000002 R_X86_64_PC32 0000000000000000 .text + 3e9 000000000014 000100000002 R_X86_64_PC32 0000000000000000 .text + 3f0 000000000018 006100000018 R_X86_64_PC64 0000000000000000 __tracepoint_read_msr + 8 000000000020 000100000002 R_X86_64_PC32 0000000000000000 .text + 424 000000000024 000100000002 R_X86_64_PC32 0000000000000000 .text + 3f0 000000000028 006100000018 R_X86_64_PC64 0000000000000000 __tracepoint_read_msr + 8 >From this we find that the jump target that goes with the NOP at +424 is +3f0, not +42b as one would expect. And as Josh noted, it is also 'weird' that this +3f0 is the very same as the target for the previous entry. When we compare the code at both sites, we find: 3f0: bf 58 02 00 00 mov $0x258,%edi 3f5: 48 89 de mov %rbx,%rsi 3f8: 31 d2 xor %edx,%edx 3fa: e8 00 00 00 00 callq 3ff 3fb: R_X86_64_PC32 do_trace_read_msr-0x4 vs 42b: bf 59 02 00 00 mov $0x259,%edi 430: 48 89 de mov %rbx,%rsi 433: 31 d2 xor %edx,%edx 435: e8 00 00 00 00 callq 43a 436: R_X86_64_PC32 do_trace_read_msr-0x4 Which is not in fact the same code. So for some reason the .rela__jump_table are buggy on this clang build.