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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5D5B0C38A2D for ; Mon, 24 Oct 2022 11:19:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=TpiuXIuHUfRU9v0hVTxKHKLxCzK0dwcRxvSVw1N29rU=; b=JUJs4ME8QZHC0O D4UqIpUjPaYD7yjMnmv9eoPJ8ARPOjChXqCWHudEO7hKJR3Hx0R1IaFhns410oRjxVzYgFQN9IoWh WQIiVac7rTOcPf/ltTe8nZIzcjlHL2WdgMYEIvmxLHSBs7VrRzng9pglxSRshiQLg7zQ4z9py1zT3 7iIYNFGH6z0UfKJ5KpsuWJY11km/RCTPIGNYFf9pKiE3xEIJNa62V71x/WGwRxU+sgweuL3zQ1CwH ak71VQzImLzEt/51WdSfYUgv+m91lflGede8FMNqrAnJzScoaVcxGQa8PPuj855PHyKuwA+yBPqLK BNYsjCInqlnzNeSJ7V1g==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1omvTY-000x6E-Q9; Mon, 24 Oct 2022 11:18:36 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1omvTV-000x4Y-C0 for linux-arm-kernel@lists.infradead.org; Mon, 24 Oct 2022 11:18:35 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id D8E23D6E; Mon, 24 Oct 2022 04:18:36 -0700 (PDT) Received: from FVFF77S0Q05N (unknown [10.57.7.186]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D162B3F7B4; Mon, 24 Oct 2022 04:18:28 -0700 (PDT) Date: Mon, 24 Oct 2022 12:18:26 +0100 From: Mark Rutland To: Fangrui Song Cc: Sami Tolvanen , llvm@lists.linux.dev, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Joao Moreira , Josh Poimboeuf , Kees Cook , Nathan Chancellor , Nick Desaulniers , Peter Zijlstra Subject: Re: kCFI && patchable-function-entry=M,N Message-ID: References: MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20221024_041833_529956_716D7BF3 X-CRM114-Status: GOOD ( 39.55 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Fri, Oct 21, 2022 at 09:14:41PM -0700, Fangrui Song wrote: > On Fri, Oct 21, 2022 at 10:39 AM Sami Tolvanen wrote: > > > > On Fri, Oct 21, 2022 at 8:56 AM Mark Rutland wrote: > > > > > > Hi, > > > > > > For arm64, I'd like to use -fatchable-function-entry=M,N (where N > 0), for our > > > ftrace implementation, which instruments *some* but not all functions. > > > Unfortuntately, this doesn't play nicely with -fsanitize=kcfi, as instrumented > > > and non-instrumented functions don't agree on where the type hash should live > > > relative to the function entry point, making them incompatible with one another. > > > > Yes, the current implementation assumes that if prefix nops are used, > > all functions have the same number of them. > > > > > Is there any mechanism today that we could use to solve this, or could we > > > extend clang to have some options to control this behaviour? > > > > I don't think there's a mechanism to work around the issue right now, > > but we could just change where the hash is emitted on arm64. > > > > > It would also be helpful to have a symbol before both the hash and pre-function > > > NOPs so that we can filter those out of probes patching (I see that x86 does > > > this with the __cfi_function symbol). > > > > Adding a symbol before the hash isn't a problem, but if we move the > > hash and want the symbol to be placed before the prefix nops as well, > > we might need a flag to control this. Fangrui, what do you think? > > > > Sami > > Since the kcfi code expects the hash to appear in a specific location > so that an instrumented indirect jump can reliably obtain the hash. > For a translation unit `-fpatchable-function-entry=N,M` may be > specified or not, and we want both to work. Therefore, I agree that a > consistent hash location will help. This argument favors placing M > nops before the hash. The downside is a restriction on how the M nops > can be used. Previously if M>0, the runtime code needs to check > whether a BTI exists to locate the N-M after-function-entry NOPs. If > the hash appears after the M nops, the runtime code needs to > additionally knows whether the hash exists. My question is how to > reliably detect this. That's a fair point. For detecting BTI we can scan the binary for BTI/NOP at M instructions into the patch-site, but a similar approach won't be reliable for the type hash since the type hash itself could have the same bit pattern as an instruction. > If there is motivation using M>0, I'd like to know the concrete code > sequence for `-fpatchable-function-entry=N,M` and how the runtime code > detects the NOPs with optional hash and optional BTI. For the BTI case alone, I have code at: https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=arm64/ftrace/per-callsite-ops&id=272a580fd5b7acc31747505d71530cee7cc2837d ... the gist being that it checks the instruction M insns after the start of the patch site. For the type hash, I think there are a few options, e.g. * Restricting the type hash to a set of values that can be identified (e.g. encoding those as a permanently-undefined UDF with a 16-bit immediate). * Adding options to record additional metadata along with the pointer to the patch-site in the __patchable_function_entries section. * Adding an option to record patch-site variants to sub-sections of the __patchable_function_entries section, so that at link time these can be grouped separately, e.g. * __patchable_function_entries.??? // no BTI, no type hash * __patchable_function_entries.bti // has BTI * __patchable_function_entries.bti_cfi // has BTI and type hash * __patchable_function_entries.cfi // has type hash Do any of those approaches sound plausible to you? Thanks, Mark. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel