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 0A6E7EB64DA for ; Thu, 6 Jul 2023 00:47:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232259AbjGFArf (ORCPT ); Wed, 5 Jul 2023 20:47:35 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35844 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232483AbjGFArf (ORCPT ); Wed, 5 Jul 2023 20:47:35 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 31AA8171D; Wed, 5 Jul 2023 17:47:30 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id C4491617D1; Thu, 6 Jul 2023 00:47:29 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 767ECC433C7; Thu, 6 Jul 2023 00:47:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1688604449; bh=lQCNaknqsctPMwh5xkWdlQlbt6s32SgZ8UYnURcKZCI=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=hZcKKxeaPoG3PuRs06XSBFbd4rJPydqxtqdf4lYwf1diJLXcuY6dQaQCpGsT15gV/ 5seM9FMnXK5j7ufmpvUR69zsDEZOBWP42gPwolTlZoYxhZpbjUPOQZ+G6VmyHTbJn6 ixetbr9rKUM7b1vn06K/UEqBlACKm/vnkscGhgcCe9J/zMPr0PluFHMLielLMFKpMk 8Avwn8m0L905PX4grCVms6QD88A1BfOT2sU80hYlTHKWDPkNecs3LsMT92fqVxFDSu P3Mtn1GMRoPlyyjmgyrhDkbWTTLGHZZP3x4/PFzBbZhFJNuKY5VI0aXGl522bZ5hJg 9vM9a+hovUZuw== Date: Thu, 6 Jul 2023 09:47:23 +0900 From: Masami Hiramatsu (Google) To: Peter Zijlstra Cc: Petr Pavlu , tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, samitolvanen@google.com, x86@kernel.org, linux-trace-kernel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] x86/retpoline,kprobes: Avoid treating rethunk as an indirect jump Message-Id: <20230706094723.6934105e03f652923796bf7e@kernel.org> In-Reply-To: <20230705145017.GC4253@hirez.programming.kicks-ass.net> References: <20230705081547.25130-1-petr.pavlu@suse.com> <20230705081547.25130-3-petr.pavlu@suse.com> <20230705085857.GG462772@hirez.programming.kicks-ass.net> <20230705232038.3a6d03e18f7bafb14cdfed42@kernel.org> <20230705145017.GC4253@hirez.programming.kicks-ass.net> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-trace-kernel@vger.kernel.org On Wed, 5 Jul 2023 16:50:17 +0200 Peter Zijlstra wrote: > On Wed, Jul 05, 2023 at 11:20:38PM +0900, Masami Hiramatsu wrote: > > On Wed, 5 Jul 2023 10:58:57 +0200 > > Peter Zijlstra wrote: > > > > > On Wed, Jul 05, 2023 at 10:15:47AM +0200, Petr Pavlu wrote: > > > > Functions can_optimize() and insn_is_indirect_jump() consider jumps to > > > > the range [__indirect_thunk_start, __indirect_thunk_end] as indirect > > > > jumps and prevent use of optprobes in functions containing them. > > > > > > Why ?!? I mean, doing an opt-probe of an indirect jump/call instruction > > > itself doesn't really make sense and I can see why you'd want to not do > > > that. But why disallow an opt-probe if there's one in the function as a > > > whole, but not the probe target? > > > > Here we need to clarify the reason why functions which have indirect jumps > > are not allowed to use opt-probe. Since optprobe can replace multiple > > instructions with a jump, if any jmp (is used for jump inside same function) > > jumps to the second and subsequent instructions replaced by optprobe's jump, > > that target instruction can not be optimized. > > > > The problem of indirect jump (which jumps to the same function) is that > > we don't know which addresses will be the target of the indirect jump. > > So, for safety, I disallow optprobe for such function. In that case, normal > > kprobe is used because it replaces only one instruction. > > Ah, you're worried about jump-tables; you don't want to optimize across > a jump-table target because then things go *boom*. > > There's two things: > > - when X86_KERNEL_IBT=y any indirect jump target should be an ENDBR > instruction, so jump-table targets can be easily detected. > > - when RETPOLINE=y || X86_KERNEL_IBT=y we have jump-tables disabled, > search for -fno-jump-table in arch/x86/Makefile. > > At some point in the future we should be able to allow jump-tables for > RETPOLINE=n && IBT=y builds (provided the compilers behave), but we > currently don't bother to find out. > > Therefore, when either CONFIG option is found, you can assume that any > indirect jump will be to another function. OK, I confirmed that '-fno-jump-tables' is set when X86_KERNEL_IBT=y || RETPOLINE=y so we can skip this indirect jump check. That makes things simpler. > > > If I understand correctly, all indirect jump will be replaced with JMP_NOSPEC. > > If you read the insn_jump_into_range, I onlu jecks the jump code, not call. > > So the functions only have indirect call still allow optprobe. > > With the introduction of kCFI JMP_NOSPEC is no longer an equivalent to a > C indirect jump. If I understand correctly, kCFI is enabled by CFI_CLANG, and clang is not using jump-tables by default, so we can focus on gcc. In that case current check still work, correct? Thank you, -- Masami Hiramatsu (Google)