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=-4.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_GIT 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 40F0BC433DF for ; Sat, 23 May 2020 12:00:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1A8DC2070A for ; Sat, 23 May 2020 12:00:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590235240; bh=a1EfeTlX8OqgAm4kNKi5ia4j9VRX69XYnUu6nE/MoiI=; h=From:To:Cc:Subject:Date:List-ID:From; b=FfTB3/OD69bLL8ErD30isZzy4HpVgVZFawph971DKGAaV8ojRjBrUsRrGfg7oUDTv eAR6rdz+RvH1deYr1zEadr2XGjdcWEOaxiPR8oABhz8U7bOr23XES++SiBTl+jZ647 pZNgzRLKlCtfEE/g9gKqB2UIuzmaPZn7TjuEu15c= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387795AbgEWMAj (ORCPT ); Sat, 23 May 2020 08:00:39 -0400 Received: from mail.kernel.org ([198.145.29.99]:40900 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728749AbgEWMAj (ORCPT ); Sat, 23 May 2020 08:00:39 -0400 Received: from localhost.localdomain (82-64-249-211.subs.proxad.net [82.64.249.211]) (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 B5E29206C3; Sat, 23 May 2020 12:00:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590235238; bh=a1EfeTlX8OqgAm4kNKi5ia4j9VRX69XYnUu6nE/MoiI=; h=From:To:Cc:Subject:Date:From; b=bHFXmpuCwMc3PgK8aXtgTzlOCn9DMagXiavppfZ2J9d17sdFhBPu5vvaVjO4tf/Iy Dde4pvvfFfr9IR+uepbOX5hkDma0SXAIbeu34uEDc3a8w0r+cBXPaDGkAF7Ma23bvO lRpzr2osX3ecrGGNzu/7vWH6c6tKh+5vgQIJ5hd8= From: Ard Biesheuvel To: linux-efi@vger.kernel.org Cc: x86@kernel.org, linux-kernel@vger.kernel.org, Ard Biesheuvel , Maarten Lankhorst , Linus Torvalds , Arvind Sankar Subject: [PATCH v2 0/3] x86/boot: get rid of GOT entries and associated fixup code Date: Sat, 23 May 2020 14:00:18 +0200 Message-Id: <20200523120021.34996-1-ardb@kernel.org> X-Mailer: git-send-email 2.20.1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Building position independent code using GCC by default results in references to symbols with external linkage to be resolved via GOT entries, which carry the absolute addresses of the symbols, and thus need to be corrected if the load time address of the executable != the link time address. For fully linked binaries, such GOT indirected references are completely useless, and actually make the startup code more complicated than necessary, since these corrections may need to be applied more than once. In fact, we have been very careful to avoid such references in the EFI stub code, since it would require yet another [earlier] pass of GOT fixups which we currently don't implement. Older GCCs were quirky when it came to overriding this behavior using symbol visibility, but now that we have increased the minimum GCC version to 4.6, we can actually start setting the symbol visibility to 'hidden' globally for all symbol references in the decompressor, getting rid of the GOT entirely. This means we can get rid of the GOT fixup code right away, and we can start using ordinary external symbol references in the EFI stub without running the risk of boot regressions. (v2 note: we have already started doing this) CC'ing Linus and Maarten, who were involved in diagnosing an issue related to GOT entries emitted from the EFI stub ~5 years ago. [0] [1] Many thanks to Arvind for the suggestions and the help in testing these changes. Tested on GCC 4.6 + binutils 2.24 (Ubuntu 14.04), and GCC 8 + binutils 2.31 (Debian Buster) Changes since v1 [2]: Rebase only - recent EFI changes have moved all the C code into drivers/firmware/efi/libstub/, which is also built with hidden visibility, and contains an additional objdump pass to detect (and reject) absolute symbol references. Unless anyone objects, I'd like to incorporate these changes into my late EFI PR for v5.8, which will go out in a day or two. Cc: Maarten Lankhorst Cc: Linus Torvalds Cc: Arvind Sankar [0] https://lore.kernel.org/lkml/5405E186.2080406@canonical.com/ [1] https://lore.kernel.org/lkml/CA+55aFxW9PmtjOf9nUQwpU8swsFqJOz8whZXcONo+XFmkSwezg@mail.gmail.com/ [2] https://lore.kernel.org/linux-efi/20200108102304.25800-1-ardb@kernel.org/ Ard Biesheuvel (3): x86/boot/compressed: move .got.plt entries out of the .got section x86/boot/compressed: force hidden visibility for all symbol references x86/boot/compressed: get rid of GOT fixup code arch/x86/boot/compressed/Makefile | 1 + arch/x86/boot/compressed/head_32.S | 22 ++------ arch/x86/boot/compressed/head_64.S | 57 -------------------- arch/x86/boot/compressed/hidden.h | 19 +++++++ arch/x86/boot/compressed/vmlinux.lds.S | 16 ++++-- 5 files changed, 36 insertions(+), 79 deletions(-) create mode 100644 arch/x86/boot/compressed/hidden.h -- 2.20.1