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=-22.4 required=3.0 tests=BAYES_00,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=unavailable 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 89D2CC43465 for ; Mon, 21 Sep 2020 15:53:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2912222574 for ; Mon, 21 Sep 2020 15:53:53 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="wS3DJbGh" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727684AbgIUPxw (ORCPT ); Mon, 21 Sep 2020 11:53:52 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54516 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727030AbgIUPxv (ORCPT ); Mon, 21 Sep 2020 11:53:51 -0400 Received: from mail-pj1-x1043.google.com (mail-pj1-x1043.google.com [IPv6:2607:f8b0:4864:20::1043]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1EB0FC0613D1 for ; Mon, 21 Sep 2020 08:53:51 -0700 (PDT) Received: by mail-pj1-x1043.google.com with SMTP id gf14so4365pjb.5 for ; Mon, 21 Sep 2020 08:53:51 -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=Fn5z5p0KWkT8F7kdqXLtUSD00/eNhULjEnaQyt7HO3c=; b=wS3DJbGh/toEWGHMWfIYAyfl62a2xPnkTm7cNhPym3mzQxK28qL+kZCrSZggBSROXa NIIsRsLPprRNfrNxq/O7SgGIAQv+11RPoBUmYfVay0H+wQrvpSFqZGCCwczoE0PI3NwM BajK6bGuvdWSOxCULPkkElyJinqDQbfrg3BX4qxe0nSoLtxoqCUZ4DYY8jlgM9XNnsxV m952IZLwDFx/YDBae5MI6BosCFQlIVPnADU67SRlbPRPa3XZPP99q/tE/mqTrWmaFUGB Cr7PUOGoUwpYEL9jPXyQWHhnShzEybRi2A7whxiV331Jx4G8BVdc/1kDmDMbH8AHeYn9 IaBA== 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=Fn5z5p0KWkT8F7kdqXLtUSD00/eNhULjEnaQyt7HO3c=; b=mY9AToWa1bfRCnPpqK0Cewb0qz87LO5Mw+rKpa8cVwuS9RScl3SSwXo6FAhieXWlA8 tg1CHvmDYxd8OdLEZJKlD04D1Z10KvRBfkI4+KSpaSVc54JrPQKK2PExWMd/GzmVHgAV 2t44Q5uewgXDefG26YMuRygW5eIPA6yflQEV/kbKZqzSxuHl0uAG2OkX/0zkaGdDymEy TFgxSVdVuSPvUxFda00fo124/YHoaaP/RJ3YHpp8Yaa6r73I7ghKbpkQ5/tfC50xfu3b nq2OsB0RuQJ1IGQL3y+zN0cP8UxYtKHkko9IVfzHi2GxWzc6G0sV3VMO49ucsecTrw+3 CJ4Q== X-Gm-Message-State: AOAM530i/gLAEwzQHUkbhgShP9FeCIvIIVXCn0oczodPGwE582d6lkw0 vALMnmNGtvQjwF91DNWELIt/6mJ/yJlOCvs5BmJF8A== X-Google-Smtp-Source: ABdhPJxEx7C1NCFiroHZ1htfv1qkh42nY8oGzcnOH06wOhFq2SsYUT8w4mdvW5J7C4BtXsEWH+72M+TnlFjXmM5sSRs= X-Received: by 2002:a17:90b:889:: with SMTP id bj9mr28426pjb.101.1600703630255; Mon, 21 Sep 2020 08:53:50 -0700 (PDT) MIME-Version: 1.0 References: <20200918201436.2932360-1-samitolvanen@google.com> <20200918201436.2932360-4-samitolvanen@google.com> In-Reply-To: <20200918201436.2932360-4-samitolvanen@google.com> From: Nick Desaulniers Date: Mon, 21 Sep 2020 08:53:39 -0700 Message-ID: Subject: Re: [PATCH v3 03/30] x86/boot/compressed: Disable relocation relaxation To: Sami Tolvanen Cc: Masahiro Yamada , Will Deacon , Peter Zijlstra , Steven Rostedt , 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)" , Arvind Sankar Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org It looks like this just hit mainline over the weekend. FWIW. EOM. On Fri, Sep 18, 2020 at 1:14 PM Sami Tolvanen wrote: > > From: Arvind Sankar > > The x86-64 psABI [0] specifies special relocation types > (R_X86_64_[REX_]GOTPCRELX) for indirection through the Global Offset > Table, semantically equivalent to R_X86_64_GOTPCREL, which the linker > can take advantage of for optimization (relaxation) at link time. This > is supported by LLD and binutils versions 2.26 onwards. > > The compressed kernel is position-independent code, however, when using > LLD or binutils versions before 2.27, it must be linked without the -pie > option. In this case, the linker may optimize certain instructions into > a non-position-independent form, by converting foo@GOTPCREL(%rip) to $foo. > > This potential issue has been present with LLD and binutils-2.26 for a > long time, but it has never manifested itself before now: > - LLD and binutils-2.26 only relax > movq foo@GOTPCREL(%rip), %reg > to > leaq foo(%rip), %reg > which is still position-independent, rather than > mov $foo, %reg > which is permitted by the psABI when -pie is not enabled. > - gcc happens to only generate GOTPCREL relocations on mov instructions. > - clang does generate GOTPCREL relocations on non-mov instructions, but > when building the compressed kernel, it uses its integrated assembler > (due to the redefinition of KBUILD_CFLAGS dropping -no-integrated-as), > which has so far defaulted to not generating the GOTPCRELX > relocations. > > Nick Desaulniers reports [1,2]: > A recent change [3] to a default value of configuration variable > (ENABLE_X86_RELAX_RELOCATIONS OFF -> ON) in LLVM now causes Clang's > integrated assembler to emit R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX > relocations. LLD will relax instructions with these relocations based > on whether the image is being linked as position independent or not. > When not, then LLD will relax these instructions to use absolute > addressing mode (R_RELAX_GOT_PC_NOPIC). This causes kernels built with > Clang and linked with LLD to fail to boot. > > Patch series [4] is a solution to allow the compressed kernel to be > linked with -pie unconditionally, but even if merged is unlikely to be > backported. As a simple solution that can be applied to stable as well, > prevent the assembler from generating the relaxed relocation types using > the -mrelax-relocations=no option. For ease of backporting, do this > unconditionally. > > [0] https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/linker-optimization.tex#L65 > [1] https://lore.kernel.org/lkml/20200807194100.3570838-1-ndesaulniers@google.com/ > [2] https://github.com/ClangBuiltLinux/linux/issues/1121 > [3] https://reviews.llvm.org/rGc41a18cf61790fc898dcda1055c3efbf442c14c0 > [4] https://lore.kernel.org/lkml/20200731202738.2577854-1-nivedita@alum.mit.edu/ > > Reported-by: Nick Desaulniers > Signed-off-by: Arvind Sankar > Tested-by: Nick Desaulniers > Tested-by: Sedat Dilek > Reviewed-by: Nick Desaulniers > Cc: stable@vger.kernel.org > --- > arch/x86/boot/compressed/Makefile | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile > index 3962f592633d..ff7894f39e0e 100644 > --- a/arch/x86/boot/compressed/Makefile > +++ b/arch/x86/boot/compressed/Makefile > @@ -43,6 +43,8 @@ KBUILD_CFLAGS += -Wno-pointer-sign > KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=) > KBUILD_CFLAGS += -fno-asynchronous-unwind-tables > KBUILD_CFLAGS += -D__DISABLE_EXPORTS > +# Disable relocation relaxation in case the link is not PIE. > +KBUILD_CFLAGS += $(call as-option,-Wa$(comma)-mrelax-relocations=no) > > KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__ > GCOV_PROFILE := n > -- > 2.28.0.681.g6f77f65b4e-goog > -- Thanks, ~Nick Desaulniers