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=-19.4 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=ham 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 F3B80C47254 for ; Tue, 5 May 2020 22:02:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D08622075E for ; Tue, 5 May 2020 22:02:31 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="n4aYqZKu" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729387AbgEEWCa (ORCPT ); Tue, 5 May 2020 18:02:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37648 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727089AbgEEWC1 (ORCPT ); Tue, 5 May 2020 18:02:27 -0400 Received: from mail-pg1-x544.google.com (mail-pg1-x544.google.com [IPv6:2607:f8b0:4864:20::544]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 48A02C061A0F for ; Tue, 5 May 2020 15:02:27 -0700 (PDT) Received: by mail-pg1-x544.google.com with SMTP id 207so151029pgc.6 for ; Tue, 05 May 2020 15:02:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=meutyvpqi9+qTZ06UEsokNwwGPTMphxT3NcJSXe1Pt4=; b=n4aYqZKuQDutp5OyuzgrDrbOfw46qnS5p3WJa9OrEVIOe7YEDc6L8MVm0rNqy/CM0v EPXZs5B3vT9m/tCOyzVjW/ooXqk0Um9CTxmTKrdzOvr1UOvH2Qk+/a+xz1RKzAlgxxn/ +pdlW8483sPcBESg7tKJJzIWd/1En/kGux6bn2UKHxYtc/9s18CqMNz47UQ6MYEfRyON uM9jikZBuhRPBp+wtcIzN7M6atPXrcF1VQeK5vlUAZCtcNvMhPHTK3gJNWy4Je8tr7UI 10hnifAwGP79iZJF9wqmIjs6NXBm00T1eiSZGgBgbmWGwd3IxBq9B+PC6QeOHFPTBGiF B18Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=meutyvpqi9+qTZ06UEsokNwwGPTMphxT3NcJSXe1Pt4=; b=JJK9XJfHNhikRIYXlNRCyAlYwrzsK7ANmt3VgLL3NvuWAt/f7IBV/u6Qp3E5XpoiRs yR2jx4KqRnCYO/yWg0exExU021BTa5aoQzrA9OyPo/awhBynYB0t7sgtom4LEbSDQHDC qjoGbmN6vZZakfoMGVsX0Z8QcVV655IGxTUh8t4F6WFwnvtX/mLs7ZOabvlaopW60Dq3 Wod/ix2xhqzg/OeLbvV7BSBAUCJrir60zsrk4WyOHdGYElR4riNgOBF0X1r/byWbyiwb kqygQMRPfmaOekhfB/liirb3xF0eHz4YqqQhZ4a+MA9eJrz+5VA3ioFftRjjuMPd/rfk LXpQ== X-Gm-Message-State: AGi0PuYRVCFvHe5i1urOY6SMbV+dzyj9edrcV+H7n0Ze9SiP4fUNU6AI 2iyma+jOi6ZadT+3Bz+XwX6YfdiYcdi21042hGqXqQ== X-Google-Smtp-Source: APiQypII92fzip+jhi9hD7251MPhAj00NvB2R8OUBsp6nQByudV49fC3OMI4LGDz7JOTRwkDao417VvP6RNTwjyMCU0= X-Received: by 2002:a65:5ac4:: with SMTP id d4mr4267862pgt.381.1588716146345; Tue, 05 May 2020 15:02:26 -0700 (PDT) MIME-Version: 1.0 References: <20200505215503.691205-1-Jason@zx2c4.com> In-Reply-To: <20200505215503.691205-1-Jason@zx2c4.com> From: Nick Desaulniers Date: Tue, 5 May 2020 15:02:16 -0700 Message-ID: Subject: Re: [PATCH] Kbuild: disable FORTIFY_SOURCE on clang-10 To: "Jason A. Donenfeld" Cc: "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" , LKML , clang-built-linux , Arnd Bergmann , Kees Cook , George Burgess Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 5, 2020 at 2:55 PM Jason A. Donenfeld wrote: > > clang-10 has a broken optimization stage that doesn't enable the > compiler to prove at compile time that certain memcpys are within > bounds, and thus the outline memcpy is always called, resulting in > horrific performance, and in some cases, excessive stack frame growth. > Here's a simple reproducer: > > typedef unsigned long size_t; > void *c(void *dest, const void *src, size_t n) __asm__("memcpy"); > extern inline __attribute__((gnu_inline)) void *memcpy(void *dest, const void *src, size_t n) { return c(dest, src, n); } > void blah(char *a) > { > unsigned long long b[10], c[10]; > int i; > > memcpy(b, a, sizeof(b)); > for (i = 0; i < 10; ++i) > c[i] = b[i] ^ b[9 - i]; > for (i = 0; i < 10; ++i) > b[i] = c[i] ^ a[i]; > memcpy(a, b, sizeof(b)); > } > > Compile this with clang-9 and clang-10 and observe: > > zx2c4@thinkpad /tmp/curve25519-hacl64-stack-frame-size-test $ clang-10 -Wframe-larger-than=0 -O3 -c b.c -o c10.o > b.c:5:6: warning: stack frame size of 104 bytes in function 'blah' [-Wframe-larger-than=] > void blah(char *a) > ^ > 1 warning generated. > zx2c4@thinkpad /tmp/curve25519-hacl64-stack-frame-size-test $ clang-9 -Wframe-larger-than=0 -O3 -c b.c -o c9.o > > Looking at the disassembly of c10.o and c9.o, one can see that c9.o is > properly optimized in the obvious way one would expect, while c10.o has > blown up and includes extern calls to memcpy. > > This is present on the most trivial bits of code. Thus, for clang-10, we > just set __NO_FORTIFY globally, so that this issue won't be incurred. > > Cc: Arnd Bergmann > Cc: LKML > Cc: clang-built-linux > Cc: Kees Cook > Cc: George Burgess > Cc: Nick Desaulniers > Link: https://bugs.llvm.org/show_bug.cgi?id=45802 > Signed-off-by: Jason A. Donenfeld I'm going to request this not be merged until careful comment from George and Kees. My hands are quite full at the moment with other regressions. I suspect these threads may be relevant, though I haven't had time to read through them myself. https://github.com/ClangBuiltLinux/linux/issues/979 https://github.com/ClangBuiltLinux/linux/pull/980 > --- > Makefile | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/Makefile b/Makefile > index 49b2709ff44e..f022f077591d 100644 > --- a/Makefile > +++ b/Makefile > @@ -768,6 +768,13 @@ KBUILD_CFLAGS += -Wno-gnu > # source of a reference will be _MergedGlobals and not on of the whitelisted names. > # See modpost pattern 2 > KBUILD_CFLAGS += -mno-global-merge > + > +# clang-10 has a broken optimization stage that causes memcpy to always be > +# outline, resulting in excessive stack frame growth and poor performance. > +ifeq ($(shell test $(CONFIG_CLANG_VERSION) -ge 100000 && test $(CONFIG_CLANG_VERSION) -lt 110000; echo $$?),0) > +KBUILD_CFLAGS += -D__NO_FORTIFY > +endif > + > else > > # These warnings generated too much noise in a regular build. > -- > 2.26.2 > -- Thanks, ~Nick Desaulniers