From patchwork Fri Jul 28 09:09:01 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ard Biesheuvel X-Patchwork-Id: 707712 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id BDFE1C00528 for ; Fri, 28 Jul 2023 09:13:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234250AbjG1JNX (ORCPT ); Fri, 28 Jul 2023 05:13:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34264 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235216AbjG1JMo (ORCPT ); Fri, 28 Jul 2023 05:12:44 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 58B9B3AAF; Fri, 28 Jul 2023 02:11:13 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id E9BF362086; Fri, 28 Jul 2023 09:11:12 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 95B85C433CB; Fri, 28 Jul 2023 09:11:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1690535472; bh=zBQnolwjGWOFrzM6wjiy++8kSbbyRGDO2Rsd3pLzB7M=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=L1aLycPW7CvFhFTw5MZP8oN9kg9Uv+8u0C/1+9rljlD3FXtiwhokexo/7V97Kenwh ns+2xFfJNd1xPY1ZT4e9/A7ix6oHUv6j23bkNSi18sROGv+yNjpbKZi8ahLW5lWP6V hQU/JaFX7oXsb8I5pPAqTOGYGugiouIlBIQhb4/CbS2Gy68pTVlEIH62I+UHlzb7Fu 3sKT663jMxwSWDa/5TyUpe0GA5FvPq3mIX4vNZEXp/QVEoGPms+8ZgEtNuIY3pc1qS 9b8z8wZclf9J65cec/hNOCOmUReSIRq9J6DlJPgJMvuf9i0B5sCCWggzaRtDO/ruoE NXkBsabfnV+7w== From: Ard Biesheuvel To: linux-efi@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Ard Biesheuvel , Evgeniy Baskov , Borislav Petkov , Andy Lutomirski , Dave Hansen , Ingo Molnar , Peter Zijlstra , Thomas Gleixner , Alexey Khoroshilov , Peter Jones , Gerd Hoffmann , Dave Young , Mario Limonciello , Kees Cook , Tom Lendacky , "Kirill A . Shutemov" , Linus Torvalds , Joerg Roedel Subject: [PATCH v7 07/22] x86/decompressor: Use proper sequence to take the address of the GOT Date: Fri, 28 Jul 2023 11:09:01 +0200 Message-Id: <20230728090916.1538550-8-ardb@kernel.org> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230728090916.1538550-1-ardb@kernel.org> References: <20230728090916.1538550-1-ardb@kernel.org> MIME-Version: 1.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=1782; i=ardb@kernel.org; h=from:subject; bh=zBQnolwjGWOFrzM6wjiy++8kSbbyRGDO2Rsd3pLzB7M=; b=owGbwMvMwCFmkMcZplerG8N4Wi2JIeVw68prT2YcUdzpPvnZl6dGzcfCTMuutpabHf4u2b2F+ XuCx8tFHaUsDGIcDLJiiiwCs/++23l6olSt8yxZmDmsTCBDGLg4BWAiDY4M/12udfY1iKo9U9Xe 9EODS0hn+vQkU7X/yxP2v31XIWO+yYuR4cGpmmVeuU88L4onhDP39PP/qy/e3f3q74TWlDn+F6a 0MgMA X-Developer-Key: i=ardb@kernel.org; a=openpgp; fpr=F43D03328115A198C90016883D200E9CA6329909 Precedence: bulk List-ID: X-Mailing-List: linux-efi@vger.kernel.org The 32-bit decompressor does not actually use a global offset table (GOT), but as is common for 32-bit position independent code, it uses the magic symbol _GLOBAL_OFFSET_TABLE_ as an anchor from which to derive the actual runtime addresses of other symbols, using special @GOTOFF symbol references that are resolved at link time, and populated with the distance between the address of the magic _GLOBAL_OFFSET_TABLE_ anchor and the address of the symbol in question. This means _GLOBAL_OFFSET_TABLE_ is the only symbol whose actual runtime address needs to be determined explicitly, which is one of the first things that happens in startup_32. However, it does so by taking the absolute address via the immediate field of an ADD instruction (plus a small offset), which seems to defeat the point. Fortunately, the assembler knows that _GLOBAL_OFFSET_TABLE_ is magic, and emits a special relative R_386_GOTPC relocation instead, and so the resulting code works as expected. However, this is not obvious for someone reading the code, and the use of LEA with an explicit relative addend is more idiomatic so use that instead. Signed-off-by: Ard Biesheuvel --- arch/x86/boot/compressed/head_32.S | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S index 8876ffe30e9a4819..3530465b5b85ccf3 100644 --- a/arch/x86/boot/compressed/head_32.S +++ b/arch/x86/boot/compressed/head_32.S @@ -58,7 +58,7 @@ SYM_FUNC_START(startup_32) leal (BP_scratch+4)(%esi), %esp call 1f 1: popl %edx - addl $_GLOBAL_OFFSET_TABLE_+(.-1b), %edx + leal (_GLOBAL_OFFSET_TABLE_ - 1b)(%edx), %edx /* Load new GDT */ leal gdt@GOTOFF(%edx), %eax