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=-28.2 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,FSL_HELO_FAKE, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_IN_DEF_DKIM_WL 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 4B3B9C4361A for ; Thu, 3 Dec 2020 20:28:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EDDF8221E3 for ; Thu, 3 Dec 2020 20:28:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727125AbgLCU2X (ORCPT ); Thu, 3 Dec 2020 15:28:23 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39502 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726187AbgLCU2X (ORCPT ); Thu, 3 Dec 2020 15:28:23 -0500 Received: from mail-pj1-x1043.google.com (mail-pj1-x1043.google.com [IPv6:2607:f8b0:4864:20::1043]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EFDA4C061A4F for ; Thu, 3 Dec 2020 12:27:42 -0800 (PST) Received: by mail-pj1-x1043.google.com with SMTP id v1so1743759pjr.2 for ; Thu, 03 Dec 2020 12:27:42 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=6c6iTUosRVrmzGiP/7D9+PLxepmiQNzcidmSqRByqjY=; b=P5vIYvPKNrqAHbY+37GxHogE+dsicUpBmvZgrwQHiqxH1dN/QDFkNS4CHeqX0Klynh RPpodRunirGMRa1lbe22VFDlAne/i4zyDGOLxn6tThQN3JMOCkGOe4QMgEsT4rzB2xzU NJ+rv9emMXNv5oCTFgKJk9NONjPuYP5XC7FppqfCfhcBKEPF2Weu5X3QFVLM/OSTbuka RdNracu5rjrrEeZ6ivk71nZ0I9X/HLaNl9ndqba4TQtG7dqrQuvnTZimGzHOceasfjXp uGg7Bp57PhU7d9REcSV+IixxlQibyidOAdEfQqcJC5M6ChGWMGXy21ztAvQWsO0DSa4a tSeg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=6c6iTUosRVrmzGiP/7D9+PLxepmiQNzcidmSqRByqjY=; b=a25u7rdz7wdlQ5rkxpyRkDSHfX+HuyXFqcd1g4XbLDcXN6gHWRSIR/PQAQCdvV3ZBD eIyOG/PUwp5ULK98ciIJuX1BR5f5zNeNMY+K4cjpGMgk2lPfkYwAWIuDohh+zySHDOz/ PVsjLE3bovq0taGompJRBbuI7MYsK6FiISQTlfzf2JIh9Ke1oDOGbLdm/isiQVPAHHer PZ/D5bk4xyVcpG/5W1ZfYEiAgyFzlVxAu/dHtsEF3o0Hf70k0m2mOWartUlg1uBtCBn8 WVONNjbN0wpYQKRH6Kp0HOgVg4YrQPss4qq5uVEUO7QXRi9IpOqab42G4jU6U+YNUT6y aVGQ== X-Gm-Message-State: AOAM530gGe6R6a84FnY4pfmUJzrpBg3TbXbqrtkot1bqvTLMMcyAcIQ3 c+TDLzBqd8E+kt9X2MuTo9BCEQ== X-Google-Smtp-Source: ABdhPJx6QkUBqweo7oQdPA48Ai0FtNZPUHQPt5H2J3HGuEr+QdVmoNFtgjOXybM803OsnuHkeTZtwQ== X-Received: by 2002:a17:902:b7c3:b029:da:76bc:2aa9 with SMTP id v3-20020a170902b7c3b02900da76bc2aa9mr671331plz.21.1607027262306; Thu, 03 Dec 2020 12:27:42 -0800 (PST) Received: from google.com ([2620:15c:2ce:0:a6ae:11ff:fe11:4abb]) by smtp.gmail.com with ESMTPSA id o14sm2008267pgh.1.2020.12.03.12.27.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 03 Dec 2020 12:27:41 -0800 (PST) Date: Thu, 3 Dec 2020 12:27:37 -0800 From: Fangrui Song To: Nick Desaulniers Cc: Arnd Bergmann , linux-arch , LKML , clang-built-linux , Nathan Chancellor , kernel test robot , dwmw@amazon.co.uk, Peter Zijlstra , Kees Cook , Sam Ravnborg , Jakub Jelinek Subject: Re: [PATCH] firmware_loader: Align .builtin_fw to 8 Message-ID: <20201203202737.7c4wrifqafszyd5y@google.com> References: <20201203170529.1029105-1-maskray@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On 2020-12-03, Nick Desaulniers wrote: >On Thu, Dec 3, 2020 at 9:05 AM Fangrui Song wrote: >> >> arm64 references the start address of .builtin_fw (__start_builtin_fw) >> with a pair of R_AARCH64_ADR_PREL_PG_HI21/R_AARCH64_LDST64_ABS_LO12_NC >> relocations. The compiler is allowed to emit the >> R_AARCH64_LDST64_ABS_LO12_NC relocation because struct builtin_fw in >> include/linux/firmware.h is 8-byte aligned. >> >> The R_AARCH64_LDST64_ABS_LO12_NC relocation requires the address to be a >> multiple of 8, which may not be the case if .builtin_fw is empty. >> Unconditionally align .builtin_fw to fix the linker error. >> >> Fixes: 5658c76 ("firmware: allow firmware files to be built into kernel image") >> Link: https://github.com/ClangBuiltLinux/linux/issues/1204 >> Reported-by: kernel test robot >> Signed-off-by: Fangrui Song >> --- >> include/asm-generic/vmlinux.lds.h | 1 + >> 1 file changed, 1 insertion(+) >> >> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h >> index b2b3d81b1535..3cd4bd1193ab 100644 >> --- a/include/asm-generic/vmlinux.lds.h >> +++ b/include/asm-generic/vmlinux.lds.h >> @@ -459,6 +459,7 @@ >> } \ >> \ >> /* Built-in firmware blobs */ \ >> + ALIGN_FUNCTION(); \ > >Thanks for the patch! > >I'm going to repeat my question from the above link >(https://github.com/ClangBuiltLinux/linux/issues/1204#issuecomment-737610582) >just in case it's not naive: > >ALIGN_FUNCTION() C preprocessor macro seems to be used to realign >code, while STRUCT_ALIGN() seems to be used to realign data. It looks >to me like only data is put into .builtin_fw. If these relocations >require an alignment of 8, than multiples of 8 should also be fine >(STRUCT_ALIGN in 32 for all toolchain version, except gcc 4.9 which is >64; both are multiples of 8 though). It looks like only structs are >placed in .builtin_fw; ie. data. In that case, I worry that using >ALIGN_FUNCTION/8 might actually be under-aligning data in this >section. Regarding STRUCT_ALIGN (32 for GCC>4.9) in include/asm-generic/vmlinux.lds.h, it is probably not suitable for .builtin_fw * Its comment is a bit unclear. It probably should mention that the 32-byte overalignment is only for global structure variables which are at least 32 byte large. But this is just my observation. Adding a GCC maintainer to comment on this. * Even if GCC does overalign defined global struct variables, it is unlikely that GCC will leverage this property for undefined `extern struct builtin_fw __start_builtin_fw[]` (drivers/base/firmware_loader/main.c) To make .builtin_fw aligned, I agree that ALIGN_FUNCTION() is probably a misuse. Maybe I should just use `. = ALIGN(8)` if the kernel linker script prefers `. = ALIGN(8)` to an output section alignment (https://sourceware.org/binutils/docs/ld/Output-Section-Description.html#Output-Section-Description https://lld.llvm.org/ELF/linker_script.html#output-section-alignment) >Though, in https://github.com/ClangBuiltLinux/linux/issues/1204#issuecomment-737625134 >you're comment: > >>> In GNU ld, the empty .builtin_fw is removed > >So that's a difference in behavior between ld.bfd and ld.lld, which is >fine, but it makes me wonder whether we should instead or additionally >be discarding this section explicitly via linker script when >CONFIG_FW_LOADER is not set? Short answer: No, we should not discard .builtin_fw .builtin_fw : AT(ADDR(.builtin_fw) - LOAD_OFFSET) { __start_builtin_fw = .; ... } In LLD, either a section reference (`ADDR(.builtin_fw)`) or a non-PROVIDE symbol assignment __start_builtin_fw makes the section non-discardable. It can be argued that discarding an output section with a symbol assignment (GNU ld) is strange because the symbol (st_shndx) will be defined relative to an arbitrary unrelated section. Retaining the section can avoid some other issues. >> .builtin_fw : AT(ADDR(.builtin_fw) - LOAD_OFFSET) { \ >> __start_builtin_fw = .; \ >> KEEP(*(.builtin_fw)) \ >> -- >> 2.29.2.576.ga3fc446d84-goog >> > > >-- >Thanks, >~Nick Desaulniers