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=-1.1 required=3.0 tests=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 B2170C433E0 for ; Thu, 25 Jun 2020 08:58:16 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 0E2D32072E for ; Thu, 25 Jun 2020 08:58:15 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="dhVJmH7i" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0E2D32072E Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-19164-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 5733 invoked by uid 550); 25 Jun 2020 08:58:08 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 5701 invoked from network); 25 Jun 2020 08:58:07 -0000 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; 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=oaHjYuJa/3zuClzIH+/g+fhPoe4mw4jvJXlUfLpBRwo=; b=dhVJmH7iCebSecH6ztLVh/a9AX q/FRl+8k3rSxsroM7W9JVbb4ruGWGY7b3ylToiN3VKMgxN8q00TRkiBpCp8haU8MShPGRK/s8jkgg w3YBszL7uKIGNLBTLhESs/x80y6d0palNb62gZGEGSMiegKRbb9GXJ9f1wHqqmzeO1jX7JiN9/HsX aTa1JqeZNh93t14b58sd/SSqgpOlSZ8pEHH5Qo6ELxJU7Vf2AGwoPRrjAW1izwzD6GTzOsKkAJ0LL Ve/bAcUlLidRSKJGMMkj+0+VwBHrbE7cgi6LsUKtOByycYPJrSQmUl57IXr4MTgW/JLH3ka/h2HLc xcYWPkgA==; Date: Thu, 25 Jun 2020 10:57:45 +0200 From: Peter Zijlstra To: Nick Desaulniers Cc: Sami Tolvanen , Masahiro Yamada , Will Deacon , Greg Kroah-Hartman , "Paul E. McKenney" , Kees Cook , clang-built-linux , Kernel Hardening , linux-arch , Linux ARM , Linux Kbuild mailing list , LKML , linux-pci@vger.kernel.org, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" Subject: Re: [PATCH 00/22] add support for Clang LTO Message-ID: <20200625085745.GD117543@hirez.programming.kicks-ass.net> References: <20200624203200.78870-1-samitolvanen@google.com> <20200624211540.GS4817@hirez.programming.kicks-ass.net> <20200625080313.GY4817@hirez.programming.kicks-ass.net> <20200625082433.GC117543@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200625082433.GC117543@hirez.programming.kicks-ass.net> On Thu, Jun 25, 2020 at 10:24:33AM +0200, Peter Zijlstra wrote: > On Thu, Jun 25, 2020 at 10:03:13AM +0200, Peter Zijlstra wrote: > > I'm sure Will will respond, but the basic issue is the trainwreck C11 > > made of dependent loads. > > > > Anyway, here's a link to the last time this came up: > > > > https://lore.kernel.org/linux-arm-kernel/20171116174830.GX3624@linux.vnet.ibm.com/ > > Another good read: > > https://lore.kernel.org/lkml/20150520005510.GA23559@linux.vnet.ibm.com/ > > and having (partially) re-read that, I now worry intensily about things > like latch_tree_find(), cyc2ns_read_begin, __ktime_get_fast_ns(). > > It looks like kernel/time/sched_clock.c uses raw_read_seqcount() which > deviates from the above patterns by, for some reason, using a primitive > that includes an extra smp_rmb(). > > And this is just the few things I could remember off the top of my head, > who knows what else is out there. As an example, let us consider __ktime_get_fast_ns(), the critical bit is: seq = raw_read_seqcount_latch(&tkf->seq); tkr = tkf->base + (seq & 0x01); now = tkr->base; And we hard rely on that being a dependent load, so: LOAD seq, (tkf->seq) LOAD tkr, tkf->base AND seq, 1 MUL seq, sizeof(tk_read_base) ADD tkr, seq LOAD now, (tkr->base) Such that we obtain 'now' as a direct dependency on 'seq'. This ensures the loads are ordered. A compiler can wreck this by translating it into something like: LOAD seq, (tkf->seq) LOAD tkr, tkf->base AND seq, 1 CMP seq, 0 JE 1f ADD tkr, sizeof(tk_read_base) 1: LOAD now, (tkr->base) Because now the machine can speculate and load now before seq, breaking the ordering.