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 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 645B4C433DF for ; Sun, 24 May 2020 15:11:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 410DF207DA for ; Sun, 24 May 2020 15:11:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590333078; bh=DxNPBs9W2qmds37BflWLrkbxXBiJz9adX3QQ0BftLtE=; h=References:In-Reply-To:From:Date:Subject:To:Cc:List-ID:From; b=Gv97RmNhU0yaXpKVijMwu3T0bTLDeRWUXqTrQhwEVpADftx/O+AFJrD1NEOjsU2Or pyLE+NiZWedVkC8rcTZnwzS6ZHhBNaDKW+SvAWqkqEIRlAIy2//ul0gEqC+5UTAxOP XbFJ9RqlhFdqUxBKWWzra8A9/kc0xH9/B0CQuomM= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729911AbgEXPLR (ORCPT ); Sun, 24 May 2020 11:11:17 -0400 Received: from mail.kernel.org ([198.145.29.99]:33366 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729016AbgEXPLQ (ORCPT ); Sun, 24 May 2020 11:11:16 -0400 Received: from mail-il1-f181.google.com (mail-il1-f181.google.com [209.85.166.181]) (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 0D18C20826; Sun, 24 May 2020 15:11:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590333076; bh=DxNPBs9W2qmds37BflWLrkbxXBiJz9adX3QQ0BftLtE=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=bcHNTZPZ8gRUXZdbxgnzJaow9mLWrOupi93xteqM2EdQ2Thqy+nZzraMBBlp7N862 P9JQppthsBR6Kdl2xVJX9yjhrkGdFkLY62nl/Y1sU4bE2CEyiVqNJ2gb8qaV+Y8hlL gr2IB2CvJKokP6/O4fd+HCFmWekXzjCCZFEW2X7Y= Received: by mail-il1-f181.google.com with SMTP id w18so15264797ilm.13; Sun, 24 May 2020 08:11:16 -0700 (PDT) X-Gm-Message-State: AOAM532MSy+WeEdlgziwpJ304MHIKpa9v4+rNz4pMFcRBcLUDqgqRX7w r4+VLvQBeHwszEHikPvmqM1mze5wYYKGCKtMRbQ= X-Google-Smtp-Source: ABdhPJyb+MO6pVLnvSVLWO9gykKsPt7sfB7BW7pCtiXfLqYvS/cDUv1xDtN7RV16rBcKCBww+AdckiktBNjJyJnTlrQ= X-Received: by 2002:a92:8555:: with SMTP id f82mr20413491ilh.279.1590333075352; Sun, 24 May 2020 08:11:15 -0700 (PDT) MIME-Version: 1.0 References: <20200523120021.34996-1-ardb@kernel.org> <20200523120021.34996-2-ardb@kernel.org> <20200524150841.GA11617@gmail.com> In-Reply-To: <20200524150841.GA11617@gmail.com> From: Ard Biesheuvel Date: Sun, 24 May 2020 17:11:03 +0200 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2 1/3] x86/boot/compressed: move .got.plt entries out of the .got section To: Ingo Molnar Cc: linux-efi , X86 ML , Linux Kernel Mailing List , Maarten Lankhorst , Linus Torvalds , Arvind Sankar 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 Sun, 24 May 2020 at 17:08, Ingo Molnar wrote: > > > * Ard Biesheuvel wrote: > > > The .got.plt section contains the part of the GOT which is used by PLT > > entries, and which gets updated lazily by the dynamic loader when > > function calls are dispatched through those PLT entries. > > > > On fully linked binaries such as the kernel proper or the decompressor, > > this never happens, and so in practice, the .got.plt section consists > > only of the first 3 magic entries that are meant to point at the _DYNAMIC > > section and at the fixup routine in the loader. However, since we don't > > use a dynamic loader, those entries are never populated or used. > > > > This means that treating those entries like ordinary GOT entries, and > > updating their values based on the actual placement of the executable in > > memory is completely pointless, and we can just ignore the .got.plt > > section entirely, provided that it has no additional entries beyond > > the first 3 ones. > > > > So add an assertion in the linker script to ensure that this assumption > > holds, and move the contents out of the [_got, _egot) memory range that > > is modified by the GOT fixup routines. > > > > Signed-off-by: Ard Biesheuvel > > --- > > arch/x86/boot/compressed/vmlinux.lds.S | 13 ++++++++++++- > > 1 file changed, 12 insertions(+), 1 deletion(-) > > > > diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S > > index 0dc5c2b9614b..ce3fdfb93b57 100644 > > --- a/arch/x86/boot/compressed/vmlinux.lds.S > > +++ b/arch/x86/boot/compressed/vmlinux.lds.S > > @@ -44,10 +44,13 @@ SECTIONS > > } > > .got : { > > _got = .; > > - KEEP(*(.got.plt)) > > KEEP(*(.got)) > > _egot = .; > > } > > + .got.plt : { > > + KEEP(*(.got.plt)) > > + } > > + > > .data : { > > _data = . ; > > *(.data) > > @@ -75,3 +78,11 @@ SECTIONS > > . = ALIGN(PAGE_SIZE); /* keep ZO size page aligned */ > > _end = .; > > } > > + > > +#ifdef CONFIG_X86_64 > > +ASSERT (SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, > > + "Unexpected GOT/PLT entries detected!") > > +#else > > +ASSERT (SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0xc, > > + "Unexpected GOT/PLT entries detected!") > > +#endif > > Cool debugging check! > > Minor consistent-style nit: > > s/ASSERT ( > /ASSERT( > ok > Optional: maybe even merge these on a single line, as a special exception to the col80 rule? > > #ifdef CONFIG_X86_64 > ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, "Unexpected GOT/PLT entries detected!") > #else > ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0xc, "Unexpected GOT/PLT entries detected!") > #endif > > But fine either way. > SIZEOF(.got.plt) <= 0x18 could be used as well, given that the section is either empty, or has at least the 3 magic entries.