From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752701AbdBUVnf (ORCPT ); Tue, 21 Feb 2017 16:43:35 -0500 Received: from merlin.infradead.org ([205.233.59.134]:58182 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751271AbdBUVn0 (ORCPT ); Tue, 21 Feb 2017 16:43:26 -0500 Date: Tue, 21 Feb 2017 22:43:26 +0100 From: Peter Zijlstra To: Josh Poimboeuf Cc: Ingo Molnar , linux-kernel@vger.kernel.org Subject: Re: [PATCH] objtool: improve detection of BUG() and other dead ends Message-ID: <20170221214326.GL6515@twins.programming.kicks-ass.net> References: <41a6d33971462ebd944a1c60ad4bf5be86c17b77.1487712920.git.jpoimboe@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <41a6d33971462ebd944a1c60ad4bf5be86c17b77.1487712920.git.jpoimboe@redhat.com> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 21, 2017 at 03:35:32PM -0600, Josh Poimboeuf wrote: > The BUG() macro's use of __builtin_unreachable() via the unreachable() > macro tells gcc that the instruction is a dead end, and that it's safe > to assume the current code path will not execute past the previous > instruction. > > On x86, the BUG() macro is implemented with the 'ud2' instruction. When > objtool's branch analysis sees that instruction, it knows the current > code path has come to a dead end. > > Peter Zijlstra has been working on a patch to change the WARN macros to > use 'ud2'. That patch will break objtool's assumption that 'ud2' is > always a dead end. > > Generally it's best for objtool to avoid making those kinds of > assumptions anyway. The more ignorant it is of kernel code internals, > the better. > > So create a more generic way for objtool to detect dead ends by adding > an annotation to the unreachable() macro. The annotation stores a > pointer to the end of the unreachable code path in an '__unreachable' > section. Objtool can read that section to find the dead ends. > Tested-by: Peter Zijlstra (Intel)