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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7049ECCA479 for ; Mon, 4 Jul 2022 11:44:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233240AbiGDLoS (ORCPT ); Mon, 4 Jul 2022 07:44:18 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55430 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232529AbiGDLoQ (ORCPT ); Mon, 4 Jul 2022 07:44:16 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 75BE4F584 for ; Mon, 4 Jul 2022 04:44:13 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; 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; bh=XiB+m1tbZAMLJR0lpeEa1msl+Okwne9w6qdT+sTSXdI=; b=DL5O9fjnqjIlwKjHOj4LvSCbSv xg8Lk9q384dEivTYB93x6oCtZQM4qWhzjen7a88iTwvDjLzTKHiXHm1vcPKemPWv6UiFQGKdnlOnj gN5S1hAXoYRPwhZkBcrNz9wu+QhAvm4VbhqRB7mfSoiQfSRj+0ftuwUv8nzIRv46TWcLKwqmgJy4Q aNAGRWcxIZYAayFoSvey9CQxEvs53pIrOCtTzbaiXIlGVziUUK+3uVUFSwUSfFi+TaQpWABHtiyr1 +7ucvluEhfmBqShzjEfz8UIXW3c5TlbVuCvdZuIPsFTcpTkd6DGkitpxqGMrAGFliw4dW6VfMRGm1 eZ+LRikw==; Received: from dhcp-077-249-017-003.chello.nl ([77.249.17.3] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1o8KU1-00H70w-Uh; Mon, 04 Jul 2022 11:43:18 +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 (4096 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id DE2D830003A; Mon, 4 Jul 2022 13:43:15 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id C4F632028F029; Mon, 4 Jul 2022 13:43:15 +0200 (CEST) Date: Mon, 4 Jul 2022 13:43:15 +0200 From: Peter Zijlstra To: "Naveen N. Rao" Cc: Christophe Leroy , "linuxppc-dev@lists.ozlabs.org" , Sathvika Vasireddy , Sathvika Vasireddy , "aik@ozlabs.ru" , "benh@kernel.crashing.org" , Chen Zhongjin , "jpoimboe@redhat.com" , Linux ARM , "linux-kernel@vger.kernel.org" , Marc Zyngier , "mbenes@suse.cz" , "mingo@redhat.com" , "mpe@ellerman.id.au" , "paulus@samba.org" , "rostedt@goodmis.org" Subject: Re: [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON() Message-ID: References: <20220624183238.388144-1-sv@linux.ibm.com> <20220624183238.388144-12-sv@linux.ibm.com> <70b6d08d-aced-7f4e-b958-a3c7ae1a9319@csgroup.eu> <92eae2ef-f9b6-019a-5a8e-728cdd9bbbc0@linux.vnet.ibm.com> <1656572413.pbaqjnrrcl.naveen@linux.ibm.com> <1656583960.0nqsj977sr.naveen@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1656583960.0nqsj977sr.naveen@linux.ibm.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 30, 2022 at 04:07:47PM +0530, Naveen N. Rao wrote: > Objtool classifies 'ud2' as INSN_BUG, and 'int3' as INSN_TRAP. In x86 BUG(), Yes, ud2 is the traditional 'kill' instruction and a number of emulators treat it as such, however it also being the shortest encoding (2 bytes) for #UD Linux has opted to (ab)use it to implement WARN/BUG. As such interpretation of 'ud2' needs to assume control flow stops (compiler will also emit ud2 in a number of cases with that intent). However, if it's used as WARN we then need to annotate the thing to not be terminal. > there is no need for an annotation since objtool assumes that 'ud2' > terminates control flow. But, for __WARN_FLAGS(), since 'ud2' is used, an > explicit annotate_reachable() is needed. That's _reachable_, to indicate > that the control flow can continue with the next instruction. > > On powerpc, we should (eventually) classify all trap variants as INSN_TRAP. Careful.. INSN_TRAP is mostly used for purposes of speculation stop and padding. That is, INSN_TRAP does indeed not affect control flow, but the way objtool treats it might not be quite what you want. Specifically, straight-line-speculation checks want INT3 after indirect control transfers (indirect jump and return -- indirect call is 'difficult'); these locations are architecturally not executed and as such placing a random trap instruction there is 'harmless'. Of course, were the branch predictor to go wobbly and attempt to execute it, the fact that it's a trap will stop speculation dead. Additionally, int3, being a single byte instruction, is also used to fill dead code space, any #BP trap on it will not have a descriptor and mostly cause the kernel to go splat. Per the last usage, validate_reachable_instructions() will ignore it. I'm not sure you want to always ignore all your (unreachable) trap instructions.