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=-21.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS 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 091E9C433E6 for ; Sat, 16 Jan 2021 17:10:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CE4ED23750 for ; Sat, 16 Jan 2021 17:10:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727749AbhAPRGP (ORCPT ); Sat, 16 Jan 2021 12:06:15 -0500 Received: from mail.kernel.org ([198.145.29.99]:55230 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727022AbhAPQfU (ORCPT ); Sat, 16 Jan 2021 11:35:20 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id C1D19207C5; Sat, 16 Jan 2021 16:34:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1610814879; bh=aH136bRk4cKdS8/17ekFrHN1qCL194ImwlzjMMmIfrk=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=UTwCnHtVRR2JZJ0ZlQUPIfJc5KY1gSl7JfDZBhOIuVmd+R19MeujlQPfpAxVlgWOC t/CFSvzgi84drlgws6O5TJOFoDlzmKaABr8NS9WqVAOvxOUaDaYMToQnzKjRB3B9d2 ci9KPIdfDcx9JBhpFL6aWhN40xHjPs/puojIW/hIRqMvejBz+APwzeYGJDhr+POuiO eE2pOF8Hnrmokvl9HJ7rND64ltu347yiMove3zY/M/AJRUzX44QAOJM39x8mL0//ng vpdyVMSnudrOp3hoA+oER0HnrkQupblsEknW4aOjhEENeyWAxS5oBmZ+7nrAEfTt7w 8YVtk+85leHUQ== Received: by mail-ot1-f46.google.com with SMTP id w3so11793524otp.13; Sat, 16 Jan 2021 08:34:39 -0800 (PST) X-Gm-Message-State: AOAM533Y/qQ0BLuAGtRqPz+61h55FD1uwL43pkJ76aJuinIeQjIhlHeb sFItDnm49AbbLbWLokNmpyvRXuGIyC97kwc3Ev4= X-Google-Smtp-Source: ABdhPJy2VyjRGD23Ym1ssVK/6l47tYsfWMPAWGpQwEzb/frJQCEdtDXfTuYLRevKA2MnCgXof1CL9NsQ/Z/SaP/F0v0= X-Received: by 2002:a05:6830:10d2:: with SMTP id z18mr12489782oto.90.1610814879037; Sat, 16 Jan 2021 08:34:39 -0800 (PST) MIME-Version: 1.0 References: <20210107223424.4135538-1-arnd@kernel.org> In-Reply-To: From: Ard Biesheuvel Date: Sat, 16 Jan 2021 17:34:27 +0100 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index To: Arvind Sankar Cc: Arnd Bergmann , Thomas Gleixner , Ingo Molnar , Borislav Petkov , X86 ML , Nathan Chancellor , Nick Desaulniers , Arnd Bergmann , Darren Hart , Andy Shevchenko , "H. Peter Anvin" , linux-efi , platform-driver-x86@vger.kernel.org, Linux Kernel Mailing List , clang-built-linux Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 15 Jan 2021 at 21:27, Arvind Sankar wrote: > > On Fri, Jan 15, 2021 at 02:07:51PM -0500, Arvind Sankar wrote: > > On Thu, Jan 07, 2021 at 11:34:15PM +0100, Arnd Bergmann wrote: > > > From: Arnd Bergmann > > > > > > When 5-level page tables are enabled, clang triggers a BUILD_BUG_ON(): > > > > > > x86_64-linux-ld: arch/x86/platform/efi/efi_64.o: in function `efi_sync_low_kernel_mappings': > > > efi_64.c:(.text+0x22c): undefined reference to `__compiletime_assert_354' > > > > > > Use the same method as in commit c65e774fb3f6 ("x86/mm: Make PGDIR_SHIFT > > > and PTRS_PER_P4D variable") and change it to MAYBE_BUILD_BUG_ON(), > > > so it only triggers for constant input. > > > > > > Link: https://github.com/ClangBuiltLinux/linux/issues/256 > > > Signed-off-by: Arnd Bergmann > > > --- > > > arch/x86/platform/efi/efi_64.c | 4 ++-- > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c > > > index e1e8d4e3a213..62bb1616b4a5 100644 > > > --- a/arch/x86/platform/efi/efi_64.c > > > +++ b/arch/x86/platform/efi/efi_64.c > > > @@ -137,8 +137,8 @@ void efi_sync_low_kernel_mappings(void) > > > * As with PGDs, we share all P4D entries apart from the one entry > > > * that covers the EFI runtime mapping space. > > > */ > > > - BUILD_BUG_ON(p4d_index(EFI_VA_END) != p4d_index(MODULES_END)); > > > - BUILD_BUG_ON((EFI_VA_START & P4D_MASK) != (EFI_VA_END & P4D_MASK)); > > > + MAYBE_BUILD_BUG_ON(p4d_index(EFI_VA_END) != p4d_index(MODULES_END)); > > > + MAYBE_BUILD_BUG_ON((EFI_VA_START & P4D_MASK) != (EFI_VA_END & P4D_MASK)); > > > > > > pgd_efi = efi_pgd + pgd_index(EFI_VA_END); > > > pgd_k = pgd_offset_k(EFI_VA_END); > > > -- > > > 2.29.2 > > > > > > > I think this needs more explanation as to why clang is triggering this. > > The issue mentions clang not inline p4d_index(), and I guess not > > performing inter-procedural analysis either? > > > > For the second assertion there, everything is always constant AFAICT: > > EFI_VA_START, EFI_VA_END and P4D_MASK are all constants regardless of > > CONFIG_5LEVEL. > > > > For the first assertion, it isn't technically constant, but if > > p4d_index() gets inlined, the compiler should be able to see that the > > two are always equal, even though ptrs_per_p4d is not constant: > > EFI_VA_END >> 39 == MODULES_END >> 39 > > so the masking with ptrs_per_p4d-1 doesn't matter for the comparison. > > > > As a matter of fact, it seems like the four assertions could be combined > > into: > > BUILD_BUG_ON((EFI_VA_END & P4D_MASK) != (MODULES_END & P4D_MASK)); > > BUILD_BUG_ON((EFI_VA_START & P4D_MASK) != (EFI_VA_END & P4D_MASK)); > > instead of separately asserting they're the same PGD entry and the same > > P4D entry. > > > > Thanks. > > I actually don't quite get the MODULES_END check -- Ard, do you know > what that's for? > Maybe Boris remembers? He wrote the original code for the 'new' EFI page table layout. > What we really should be checking is that EFI_VA_START is in the top-most > PGD entry and the top-most P4D entry, since we only copy PGD/P4D entries > before EFI_VA_END, but not after EFI_VA_START. So the checks should > really be > BUILD_BUG_ON(((EFI_VA_START - 1) & P4D_MASK) != (-1ul & P4D_MASK)); > BUILD_BUG_ON(((EFI_VA_START - 1) & P4D_MASK) != (EFI_VA_END & P4D_MASK)); > imo. I guess that's what using MODULES_END is effectively checking, but > it would be clearer to check it directly. This obviously needs a comment, but checking that everything lives in the top 512 GB of the kernel VA space seems sufficient to me,