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=-7.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 A0EAAC433E0 for ; Tue, 30 Jun 2020 16:26:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6E6C7206BE for ; Tue, 30 Jun 2020 16:26:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1593534418; bh=HBFDlrVI3IwkjEQubd8u4VTO99jFn/iXk+OeqTS0Vrs=; h=References:In-Reply-To:From:Date:Subject:To:Cc:List-ID:From; b=fuaka4g0hlKjbEgeNZ2sCLzvcs8jWn65G8X3BtND2wRufyAQFrAprTN0yOOjLkN15 v9xGP1RGRbYioLyoZ+ZlRI6b9WHnZfwt12CM56JAX2EAdJ1Dtvf45nDF7cni1NH9Pi EYMqgx98mLpVRD9+ZcOLK/U74PQKdSR4w/lFgWnM= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387859AbgF3Q05 (ORCPT ); Tue, 30 Jun 2020 12:26:57 -0400 Received: from mail.kernel.org ([198.145.29.99]:45550 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387434AbgF3Q04 (ORCPT ); Tue, 30 Jun 2020 12:26:56 -0400 Received: from mail-ot1-f43.google.com (mail-ot1-f43.google.com [209.85.210.43]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 779D0207F9 for ; Tue, 30 Jun 2020 16:26:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1593534415; bh=HBFDlrVI3IwkjEQubd8u4VTO99jFn/iXk+OeqTS0Vrs=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=LwsGhl86O9T8w4jU3ZMJWc93UJD3XnHILloasfsEwguZkHE5qSYfs2ctj9OSCprig DWExO67eZ8Tincrp0oKr7MckXMlJIgppH/oUElxG8dpdf8lAUazamRtDOT619iGHcx gWS2LH+SyuQN8PyfNYvLrbsRaR8KlokEwlo31d3k= Received: by mail-ot1-f43.google.com with SMTP id 76so4461628otu.9 for ; Tue, 30 Jun 2020 09:26:55 -0700 (PDT) X-Gm-Message-State: AOAM5339whJWO6y+U7bNAq0bvZ5IG5h78dNF/VCZs+s8hmsTBIz84RRk dNWe4+MN0hip1nznUsFawsaKxpGsIEOyziYPzB4= X-Google-Smtp-Source: ABdhPJxbvfITvHp7n7T9uYx6THN1L08zYlFMRXAGpSRr/ip3lvNMV6h+OajI+z7y1DF7oZXFunFw2iDkagNrwZ/0gfo= X-Received: by 2002:a9d:5a12:: with SMTP id v18mr17948123oth.90.1593534414633; Tue, 30 Jun 2020 09:26:54 -0700 (PDT) MIME-Version: 1.0 References: <20200629140928.858507-1-nivedita@alum.mit.edu> <20200629140928.858507-8-nivedita@alum.mit.edu> <202006290907.E5EF18A@keescook> <202006290919.93C759C62@keescook> <20200629165603.GD900899@rani.riverdale.lan> <20200629173735.l3ssrj7m3q5swfup@google.com> <20200629233405.n56yb4xwlgxrt3fn@google.com> In-Reply-To: <20200629233405.n56yb4xwlgxrt3fn@google.com> From: Ard Biesheuvel Date: Tue, 30 Jun 2020 18:26:43 +0200 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v3 7/7] x86/boot: Check that there are no runtime relocations To: Fangrui Song Cc: Kees Cook , Arvind Sankar , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , X86 ML , Nick Desaulniers , Dmitry Golovin , clang-built-linux , Masahiro Yamada , Daniel Kiper , Sedat Dilek , Nathan Chancellor , Arnd Bergmann , "H . J . Lu" , Linux Kernel Mailing List 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, 30 Jun 2020 at 01:34, Fangrui Song wrote: > > On 2020-06-29, Ard Biesheuvel wrote: > >On Mon, 29 Jun 2020 at 19:37, Fangrui Song wrote: > >> > >> On 2020-06-29, Arvind Sankar wrote: > >> >On Mon, Jun 29, 2020 at 09:20:31AM -0700, Kees Cook wrote: > >> >> On Mon, Jun 29, 2020 at 06:11:59PM +0200, Ard Biesheuvel wrote: > >> >> > On Mon, 29 Jun 2020 at 18:09, Kees Cook wrote: > >> >> > > > >> >> > > On Mon, Jun 29, 2020 at 10:09:28AM -0400, Arvind Sankar wrote: > >> >> > > > Add a linker script check that there are no runtime relocations, and > >> >> > > > remove the old one that tries to check via looking for specially-named > >> >> > > > sections in the object files. > >> >> > > > > >> >> > > > Drop the tests for -fPIE compiler option and -pie linker option, as they > >> >> > > > are available in all supported gcc and binutils versions (as well as > >> >> > > > clang and lld). > >> >> > > > > >> >> > > > Signed-off-by: Arvind Sankar > >> >> > > > Reviewed-by: Ard Biesheuvel > >> >> > > > Reviewed-by: Fangrui Song > >> >> > > > --- > >> >> > > > arch/x86/boot/compressed/Makefile | 28 +++----------------------- > >> >> > > > arch/x86/boot/compressed/vmlinux.lds.S | 8 ++++++++ > >> >> > > > 2 files changed, 11 insertions(+), 25 deletions(-) > >> >> > > > >> >> > > Reviewed-by: Kees Cook > >> >> > > > >> >> > > question below ... > >> >> > > > >> >> > > > diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S > >> >> > > > index a4a4a59a2628..a78510046eec 100644 > >> >> > > > --- a/arch/x86/boot/compressed/vmlinux.lds.S > >> >> > > > +++ b/arch/x86/boot/compressed/vmlinux.lds.S > >> >> > > > @@ -42,6 +42,12 @@ SECTIONS > >> >> > > > *(.rodata.*) > >> >> > > > _erodata = . ; > >> >> > > > } > >> >> > > > + .rel.dyn : { > >> >> > > > + *(.rel.*) > >> >> > > > + } > >> >> > > > + .rela.dyn : { > >> >> > > > + *(.rela.*) > >> >> > > > + } > >> >> > > > .got : { > >> >> > > > *(.got) > >> >> > > > } > >> >> > > > >> >> > > Should these be marked (INFO) as well? > >> >> > > > >> >> > > >> >> > Given that sections marked as (INFO) will still be emitted into the > >> >> > ELF image, it does not really make a difference to do this for zero > >> >> > sized sections. > >> >> > >> >> Oh, I misunderstood -- I though they were _not_ emitted; I see now what > >> >> you said was not allocated. So, disk space used for the .got.plt case, > >> >> but not memory space used. Sorry for the confusion! > >> >> > >> >> -Kees > >> > >> About output section type (INFO): > >> https://sourceware.org/binutils/docs/ld/Output-Section-Type.html#Output-Section-Type > >> says "These type names are supported for backward compatibility, and are > >> rarely used." > >> > >> If all input section don't have the SHF_ALLOC flag, the output section > >> will not have this flag as well. This type is not useful... > >> > >> If .got and .got.plt were used, they should be considered dynamic > >> relocations which should be part of the loadable image. So they should > >> have the SHF_ALLOC flag. (INFO) will not be applicable anyway. > >> > > > >I don't care deeply either way, but Kees indicated that he would like > >to get rid of the 24 bytes of .got.plt magic entries that we have no > >need for. > > > >In fact, a lot of this mangling is caused by the fact that the linker > >is creating a relocatable binary, and assumes that it is a hosted > >binary that is loaded by a dynamic loader. It would actually be much > >better if the compiler and linker would take -ffreestanding into > >account, and suppress GOT entries, PLTs, dynamic program headers for > >shared libraries altogether. > > Linkers (GNU ld and LLD) don't create .got or .got.plt just because the linker > command line has -pie or -shared. They create .got or .got.plt if there are > specific needs. > > For .got.plt, if there is (1) any .plt/.iplt entry, (2) any .got.plt based > relocation (e.g. R_X86_64_GOTPC32 on x86-64), or (3) if _GLOBAL_OFFSET_TABLE_ is > referenced, .got.plt will be created (both GNU ld and LLD) with usually 3 > entries (for ld.so purposes). > This is not the case for AArch64. There, __GLOBAL_OFFSET_TABLE__ is always emitted, along with the magic .got.plt entries, regardless of the input. As for the input objects - why is '#pragma GCC visibility(hidden)' not the default for -ffreestanding builds? This suppresses any GOT entries emitted by the compiler, but the only way to get this behavior is through the #pragma, which is how we ended up with '-include hidden.h' in a couple of places. IOW, if the toolchain behavior was not 100% geared towards shared executables as it is today, we would not need the hacks that we need to apply to get a relocatable bare metal binary like we need for the KASLR kernel. > If (1) is not satisfied, the created .got.plt is just served as an anchor for > things that want to reference (the distance from GOT base to some point). The > linker will still reserve 3 words but the words are likely not needed. > > I don't think there is a specific need for another option to teach the linker > (GNU ld or LLD) that this is a kernel link. For -ffreestanding builds, cc > -static (ld -no-pie))/-static-pie (-pie) already work quite well. You mean 'ld -static -pie' right? That seems to work. Is that a recent invention?