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=-5.8 required=3.0 tests=BAYES_00,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 A602DC48BC2 for ; Mon, 7 Jun 2021 09:22:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8F99560232 for ; Mon, 7 Jun 2021 09:22:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230262AbhFGJYf (ORCPT ); Mon, 7 Jun 2021 05:24:35 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41354 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230203AbhFGJYc (ORCPT ); Mon, 7 Jun 2021 05:24:32 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BAD5AC061789 for ; Mon, 7 Jun 2021 02:22:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=7UNlj+FWBEeACCbOEiLMCdZwBprY1+ow/vEJ/vwRuwk=; b=wUfPXZ6F9OhgnIF0VXeCQY4Nwr +DofzgeIpIg14E17XpuwZ38q0GgMQ49xqAT/HCoIOVB5qIsam7d3oCByMy8oDqCe2XmN+/Mpb23Qy 8de/EwGMHzN+HVdioolFui2rYPp+O1j67XVOZWoZo+if2LaERH7wgXaZQIJmhZJAeja0XOIw72zKM zbifJl5+IOj/+rwBTU4JXnqMs1kb77wFZkSuaj55Dz8truCJqz3WMG0c31m4A4yvBSlw1jUfpdpV1 Fr4ASg2Zw9Nd6XeoWxpuxySW70vLoa2QfuIOcXHGOa7fgD4B+NzETnKnHyqx2Y+6XaLkmmY1H8lHw EqC8OVgw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94 #2 (Red Hat Linux)) id 1lqBSW-00FZjV-Ce; Mon, 07 Jun 2021 09:22:17 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 468723001E3; Mon, 7 Jun 2021 11:22:11 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 2D0982CEABC0F; Mon, 7 Jun 2021 11:22:11 +0200 (CEST) Date: Mon, 7 Jun 2021 11:22:11 +0200 From: Peter Zijlstra To: =?utf-8?B?RsSBbmctcnXDrCBTw7JuZw==?= Cc: Nick Desaulniers , Josh Poimboeuf , lma@semihalf.com, Guenter Roeck , Juergen Gross , lb@semihalf.com, LKML , mbenes@suse.com, =?utf-8?B?UmFkb3PFgmF3?= Biernacki , upstream@semihalf.com, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , clang-built-linux , Nathan Chancellor , Sami Tolvanen Subject: Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls Message-ID: References: <20210604205018.2238778-1-ndesaulniers@google.com> <20210604235046.w3hazgcpsg4oefex@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jun 07, 2021 at 09:56:48AM +0200, Peter Zijlstra wrote: > On Sat, Jun 05, 2021 at 06:58:39PM -0700, Fāng-ruì Sòng wrote: > > On Sat, Jun 5, 2021 at 3:39 AM Peter Zijlstra wrote: > > > > I think you've absolutely nailed it; but would you have more information > > > or a code reference to what you're speaking about? My complete ELF > > > and libelf knowledge is very limited and as demonstrated here, I'm not > > > at all sure how all that extended index stuff is supposed to work. > > > > The section index field of an Elf{32,64}_Sym (st_shndx) is 16-bit, so > > it cannot represent a section index greater than 0xffff. > > ELF actually reserves values in 0xff00~0xff00 for other purposes, so > > st_shndx cannot represent a section whose index is greater or equal to > > 0xff00. > > Right, that's about as far as I got, but never could find details on how > the extension worked in detail, and I clearly muddled it :/ OK, so I'm all confused again... So a .symtab entry has: st_name -- strtab offset for the name string st_value -- where this symbol lives st_size -- size of symbol in bytes st_shndx -- section index to interpret the @st_value above st_info -- type+bind st_other -- visibility The thing is, we're adding UNDEF symbols, for the linker to resolve. UNDEF has: st_value := 0 st_size := 0 st_shndx := 0 st_info := GLOBAL + NOTYPE st_other := 0 Per that, sh_shndx isn't >= SHN_LORESERVE, and I figured we all good. Is the problem that .symtab_shndx is expected to contain the exact same number of entries as .symtab? And I'm adding to .symtab and not to .symtab_shndx, hence getting them out of sync? Let me try adding 0s to .symtab_shndx. See if that makes readelf happier.