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=-0.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no 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 9A2A3C83008 for ; Tue, 28 Apr 2020 14:15:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 740E82072A for ; Tue, 28 Apr 2020 14:15:11 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="EeqclIcM" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728044AbgD1OPK (ORCPT ); Tue, 28 Apr 2020 10:15:10 -0400 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:56143 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727899AbgD1OPJ (ORCPT ); Tue, 28 Apr 2020 10:15:09 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1588083307; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=NFbXtHTyHEyaaSu7Ifnn6u756qhZlAY5OEZBR4sIRJw=; b=EeqclIcMQMolv/Py2Q7cG8qb9Dv6b9J50LVt+33G6raSuuJg/ZE/kzYZhFfjlya2PFBuTo 2f3JD5OmBVVvUX/cdaUofhb8OrDp5BzCIGtVZeNiRWFLwAz6CSzOlb195Pntu9W+vMXziD K6cXBx6PmGJ+fU0EqJt6wKOcfcZ1feA= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-237-_TfdQzz5NlG6mmdrW38tEg-1; Tue, 28 Apr 2020 10:15:03 -0400 X-MC-Unique: _TfdQzz5NlG6mmdrW38tEg-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id BB15B83DE6C; Tue, 28 Apr 2020 14:15:01 +0000 (UTC) Received: from treble (ovpn-112-209.rdu2.redhat.com [10.10.112.209]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 56A0E611A9; Tue, 28 Apr 2020 14:14:59 +0000 (UTC) Date: Tue, 28 Apr 2020 09:14:57 -0500 From: Josh Poimboeuf To: Peter Zijlstra Cc: Jann Horn , Andy Lutomirski , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , the arch/x86 maintainers , kernel list , alexandre.chartre@oracle.com Subject: Re: x86 entry perf unwinding failure (missing IRET_REGS annotation on stack switch?) Message-ID: <20200428141457.4il3bgjjhifs47zc@treble> References: <20200302151829.brlkedossh7qs47s@treble> <20200302155239.7ww7jfeu4yeevpkb@treble> <20200428070450.w5l5ey54dtmqy5ph@treble> <20200428124627.GC13558@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200428124627.GC13558@hirez.programming.kicks-ass.net> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 28, 2020 at 02:46:27PM +0200, Peter Zijlstra wrote: > > I'm thinking something like this should fix it. Peter, does this look > > ok? > > Unfortunate. But also, I fear, insufficient. Specifically consider > things like: > > ALTERNATIVE "jmp 1f", > "alt... > "..." > "...insn", X86_FEAT_foo > 1: > > This results in something like: > > > .text .altinstr_replacement > e8 xx ... > 90 > 90 > ... > 90 > > Where all our normal single byte nops (0x90) are unreachable with > undefined CFI, but the alternative might have CFI, which is never > propagated. > > We ran into this with the validate_alternative stuff from Alexandre. I don't get what you're saying. We decided not to allow CFI changes in alternatives. And how does this relate to my patch? > > @@ -773,12 +772,26 @@ static int handle_group_alt(struct objtool_file *file, > > struct instruction *last_orig_insn, *last_new_insn, *insn, *fake_jump = NULL; > > unsigned long dest_off; > > > > + /* > > + * For uaccess checking, propagate the STAC/CLAC from the alternative > > + * to the original insn to avoid paths where we see the STAC but then > > + * take the NOP instead of CLAC (and vice versa). > > + */ > > + if (!orig_insn->ignore_alts && orig_insn->type == INSN_NOP && > > + *new_insn && > > + ((*new_insn)->type == INSN_STAC || > > + (*new_insn)->type == INSN_CLAC)) > > + orig_insn->type = (*new_insn)->type; > > Also, this generates a mis-match between actual instruction text and > type. We now have a single byte instruction (0x90) with the type of a 3 > byte (SLAC/CLAC). Which currently isn't a problem, but I'm looking at > adding infrastructure for having objtool rewrite instructions. But it doesn't actually change the original instruction bytes, it just changes the decoding. Is that really going to be a problem? We do that in other places as well, and it helps simplify code flow. Also might I ask why you're going to be rewriting instructions? That sounds scary. > So rather than hacking around this issue, should we not make > create_orc() smarter? Maybe, though I don't see how that logic belongs in create_orc(). It might be tricky distinguishing between normal undefined and "undefined because of a skip_orig". Right now create_orc() is blissfully ignorant. -- Josh