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=-22.6 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,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 56C87C433DF for ; Sat, 15 Aug 2020 22:15:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2C98420657 for ; Sat, 15 Aug 2020 22:15:45 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="ALUgM3Fc" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728563AbgHOWPo (ORCPT ); Sat, 15 Aug 2020 18:15:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49996 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728047AbgHOWPm (ORCPT ); Sat, 15 Aug 2020 18:15:42 -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 5F3D5C03D1C2 for ; Sat, 15 Aug 2020 13:57:01 -0700 (PDT) Received: by mail-pg1-x544.google.com with SMTP id o13so6174030pgf.0 for ; Sat, 15 Aug 2020 13:57:01 -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=UtBjDqPvO0XxPgS4w2mz5FIgUERc0EMLvaI4Ab10ZQQ=; b=ALUgM3FcF2qEVoH7TniGeuZI28YG4mu8n0qX2RGR2POGI/b0FKkpQKy8WDD0tC0dT1 CpoWwlyn4FknyQJQhjOx2qfR6ReXiG3WbYHm/KzYPyZZwIh9C4cM7npX0ICIF8YbXAQX l47yl2FUm9LZPzZluz0jEV4QWZlX9+lzjyFb8DJ6H1xbel7UPK9MrAVDE2K1Yxr+YkGP RW+zt6+uTntEs24vQyJfLU/9fG9lMHHTC4KhjVirZ9rgmofCC9BfFQDjfBAc4GCX7CJT siFosB/+mUhX/nK7lcm35Sp+8LJHyQDwk4ykhd8VxF1FigK0VPwjc7QQAagi2OGc4XHF X5ww== 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=UtBjDqPvO0XxPgS4w2mz5FIgUERc0EMLvaI4Ab10ZQQ=; b=eJ1rUN8e8xfMil0mHK52prO/f0oED1uEJwQe4J/HKD4Qm2fdRI1M4x3uFTsMO+Q7HR RWy7+BPVAfke0I2IjJReAdBHN3ifUUqfziRAf10sEBngHpGZWLDumOdD1lm+/VThIfmn 5aEaAxL7DW9LlgtQlBwgiEWRwAaF0VTtVCuHpyu1aBocqkx61WeNy3BIKlm/t0nVHnsB m/4W2MSdgR/gK1RT7Gum1mw+tYPxAgp917PHX3mV7iwRmroCFJOK+bnUPeUqOvooWaYN w8Ph5HuNRl9mEVW0RLdeOKcquAVQk1TSgbCUX/ZfJOi3sL9QehvVPOlcqa427M0ExadU y9iQ== X-Gm-Message-State: AOAM533x8n4OdU3SNQHvfts5aaRZmc5zlxIlllVnaFmy96ZNr21m5tQt fijHsjxPaz3eEwOrRz03URxuiAfoJSyB1K2/lO5QDQ== X-Google-Smtp-Source: ABdhPJzczVhCaED7MWfeMK93Dcd77/zRrJQu9LHftmA8LrAopKmTRRIolOVSp11IYSfyOljqyR6hqYHY6bjzXsn0foE= X-Received: by 2002:a62:8303:: with SMTP id h3mr6288262pfe.169.1597525020265; Sat, 15 Aug 2020 13:57:00 -0700 (PDT) MIME-Version: 1.0 References: <20200812004158.GA1447296@rani.riverdale.lan> <20200812004308.1448603-1-nivedita@alum.mit.edu> In-Reply-To: From: Nick Desaulniers Date: Sat, 15 Aug 2020 13:56:49 -0700 Message-ID: Subject: Re: [PATCH v2] x86/boot/compressed: Disable relocation relaxation To: Ingo Molnar , Borislav Petkov , Thomas Gleixner Cc: Arvind Sankar , Fangrui Song , clang-built-linux , e5ten.arch@gmail.com, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , "H. Peter Anvin" , Masahiro Yamada , Ard Biesheuvel , Kees Cook , LKML , "# 3.4.x" , Sedat Dilek 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 Hi Ingo, I saw you picked up Arvind's other series into x86/boot. Would you mind please including this, as well? Our CI is quite red for x86... EOM On Sat, Aug 15, 2020 at 8:49 AM Sedat Dilek wrote: > > On Wed, Aug 12, 2020 at 2:43 AM Arvind Sankar wrote: > > > > 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/ > > > > Signed-off-by: Arvind Sankar > > Reported-by: Nick Desaulniers > > Reviewed-by: Nick Desaulniers > > Tested-by: Nick Desaulniers > > Cc: stable@vger.kernel.org > > Signed-off-by: Arvind Sankar > > Thanks for the patch. > > Tested-by: Sedat Dilek > > - Sedat - > > [1] https://github.com/ClangBuiltLinux/linux/issues/1120#issuecomment-674409705 > > > --- > > 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 > > -- > > 2.26.2 > > > > -- > > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@googlegroups.com. > > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20200812004308.1448603-1-nivedita%40alum.mit.edu. -- Thanks, ~Nick Desaulniers