From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric W. Biederman Date: Thu, 19 May 2022 12:59:17 -0500 Subject: [PATCH] kexec_file: Drop weak attribute from arch_kexec_apply_relocations[_add] In-Reply-To: (Baoquan He's message of "Thu, 19 May 2022 10:58:19 +0800") References: <20220518181828.645877-1-naveen.n.rao@linux.vnet.ibm.com> <87ee0q7b92.fsf@email.froward.int.ebiederm.org> Message-ID: <87bkvt4d56.fsf@email.froward.int.ebiederm.org> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: kexec@lists.infradead.org Baoquan He writes: > Hi Eric, > > On 05/18/22 at 04:59pm, Eric W. Biederman wrote: >> "Naveen N. Rao" writes: >> >> > Since commit d1bcae833b32f1 ("ELF: Don't generate unused section >> > symbols") [1], binutils (v2.36+) started dropping section symbols that >> > it thought were unused. This isn't an issue in general, but with >> > kexec_file.c, gcc is placing kexec_arch_apply_relocations[_add] into a >> > separate .text.unlikely section and the section symbol ".text.unlikely" >> > is being dropped. Due to this, recordmcount is unable to find a non-weak >> > symbol in .text.unlikely to generate a relocation record against. >> > >> > Address this by dropping the weak attribute from these functions: >> > - arch_kexec_apply_relocations() is not overridden by any architecture >> > today, so just drop the weak attribute. >> > - arch_kexec_apply_relocations_add() is only overridden by x86 and s390. >> > Retain the function prototype for those and move the weak >> > implementation into the header as a static inline for other >> > architectures. >> > >> > [1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=d1bcae833b32f1 >> >> Any chance you can also get machine_kexec_post_load, >> crash_free_reserved_phys_range, arch_kexec_protect_protect_crashkres, >> arch_kexec_unprotect_crashkres, arch_kexec_kernel_image_probe, >> arch_kexec_kernel_image_probe, arch_kimage_file_post_load_cleanup, >> arch_kexec_kernel_verify_sig, and arch_kexec_locate_mem_hole as well. >> >> That is everything in kexec that uses a __weak symbol. If we can't >> count on them working we might as well just get rid of the rest >> preemptively. > > Is there a new rule that __weak is not suggested in kernel any more? > Please help provide a pointer if yes, so that I can learn that. > > In my mind, __weak is very simple and clear as a mechanism to add > ARCH related functionality. You should be able to trace the conversation back for all of the details but if you can't here is the summary. There is a tool that some architectures use called recordmcount. The recordmcount looks for a symbol in a section, and ignores all weak symbols. In certain cases sections become so simple there are only weak symbols. At which point recordmcount fails. Which means in practice __weak symbols are unreliable and don't work to add ARCH related functionality. Given that __weak symbols fail randomly I would much rather have simpler code that doesn't fail. It has never been the case that __weak symbols have been very common in the kernel. I expect they are something like bool that have been gaining traction. Still given that __weak symbols don't work. I don't want them. Eric 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 Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7B570C433EF for ; Thu, 19 May 2022 18:00:19 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4L3yLp0P6Fz3bk1 for ; Fri, 20 May 2022 04:00:18 +1000 (AEST) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=xmission.com (client-ip=166.70.13.233; helo=out03.mta.xmission.com; envelope-from=ebiederm@xmission.com; receiver=) Received: from out03.mta.xmission.com (out03.mta.xmission.com [166.70.13.233]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-256) server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4L3yLJ2461z2xKs for ; Fri, 20 May 2022 03:59:50 +1000 (AEST) Received: from in02.mta.xmission.com ([166.70.13.52]:36212) by out03.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1nrkR5-00ENiG-OH; Thu, 19 May 2022 11:59:43 -0600 Received: from ip68-227-174-4.om.om.cox.net ([68.227.174.4]:38802 helo=email.froward.int.ebiederm.org.xmission.com) by in02.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1nrkR3-006e82-PA; Thu, 19 May 2022 11:59:43 -0600 From: "Eric W. Biederman" To: Baoquan He References: <20220518181828.645877-1-naveen.n.rao@linux.vnet.ibm.com> <87ee0q7b92.fsf@email.froward.int.ebiederm.org> Date: Thu, 19 May 2022 12:59:17 -0500 In-Reply-To: (Baoquan He's message of "Thu, 19 May 2022 10:58:19 +0800") Message-ID: <87bkvt4d56.fsf@email.froward.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1nrkR3-006e82-PA; ; ; mid=<87bkvt4d56.fsf@email.froward.int.ebiederm.org>; ; ; hst=in02.mta.xmission.com; ; ; ip=68.227.174.4; ; ; frm=ebiederm@xmission.com; ; ; spf=softfail X-XM-AID: U2FsdGVkX1/rSxyKigaHMoSccMUFZ8spyBuVsjtCT6w= X-SA-Exim-Connect-IP: 68.227.174.4 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH] kexec_file: Drop weak attribute from arch_kexec_apply_relocations[_add] X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kexec@lists.infradead.org, linux-kernel@vger.kernel.org, "Naveen N. Rao" , linuxppc-dev@lists.ozlabs.org, Andrew Morton Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Baoquan He writes: > Hi Eric, > > On 05/18/22 at 04:59pm, Eric W. Biederman wrote: >> "Naveen N. Rao" writes: >> >> > Since commit d1bcae833b32f1 ("ELF: Don't generate unused section >> > symbols") [1], binutils (v2.36+) started dropping section symbols that >> > it thought were unused. This isn't an issue in general, but with >> > kexec_file.c, gcc is placing kexec_arch_apply_relocations[_add] into a >> > separate .text.unlikely section and the section symbol ".text.unlikely" >> > is being dropped. Due to this, recordmcount is unable to find a non-weak >> > symbol in .text.unlikely to generate a relocation record against. >> > >> > Address this by dropping the weak attribute from these functions: >> > - arch_kexec_apply_relocations() is not overridden by any architecture >> > today, so just drop the weak attribute. >> > - arch_kexec_apply_relocations_add() is only overridden by x86 and s390. >> > Retain the function prototype for those and move the weak >> > implementation into the header as a static inline for other >> > architectures. >> > >> > [1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=d1bcae833b32f1 >> >> Any chance you can also get machine_kexec_post_load, >> crash_free_reserved_phys_range, arch_kexec_protect_protect_crashkres, >> arch_kexec_unprotect_crashkres, arch_kexec_kernel_image_probe, >> arch_kexec_kernel_image_probe, arch_kimage_file_post_load_cleanup, >> arch_kexec_kernel_verify_sig, and arch_kexec_locate_mem_hole as well. >> >> That is everything in kexec that uses a __weak symbol. If we can't >> count on them working we might as well just get rid of the rest >> preemptively. > > Is there a new rule that __weak is not suggested in kernel any more? > Please help provide a pointer if yes, so that I can learn that. > > In my mind, __weak is very simple and clear as a mechanism to add > ARCH related functionality. You should be able to trace the conversation back for all of the details but if you can't here is the summary. There is a tool that some architectures use called recordmcount. The recordmcount looks for a symbol in a section, and ignores all weak symbols. In certain cases sections become so simple there are only weak symbols. At which point recordmcount fails. Which means in practice __weak symbols are unreliable and don't work to add ARCH related functionality. Given that __weak symbols fail randomly I would much rather have simpler code that doesn't fail. It has never been the case that __weak symbols have been very common in the kernel. I expect they are something like bool that have been gaining traction. Still given that __weak symbols don't work. I don't want them. Eric 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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5090DC433F5 for ; Thu, 19 May 2022 17:59:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243473AbiESR7t (ORCPT ); Thu, 19 May 2022 13:59:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43258 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231908AbiESR7q (ORCPT ); Thu, 19 May 2022 13:59:46 -0400 Received: from out03.mta.xmission.com (out03.mta.xmission.com [166.70.13.233]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 31941D4101 for ; Thu, 19 May 2022 10:59:46 -0700 (PDT) Received: from in02.mta.xmission.com ([166.70.13.52]:36212) by out03.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1nrkR5-00ENiG-OH; Thu, 19 May 2022 11:59:43 -0600 Received: from ip68-227-174-4.om.om.cox.net ([68.227.174.4]:38802 helo=email.froward.int.ebiederm.org.xmission.com) by in02.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1nrkR3-006e82-PA; Thu, 19 May 2022 11:59:43 -0600 From: "Eric W. Biederman" To: Baoquan He Cc: "Naveen N. Rao" , Michael Ellerman , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, Andrew Morton References: <20220518181828.645877-1-naveen.n.rao@linux.vnet.ibm.com> <87ee0q7b92.fsf@email.froward.int.ebiederm.org> Date: Thu, 19 May 2022 12:59:17 -0500 In-Reply-To: (Baoquan He's message of "Thu, 19 May 2022 10:58:19 +0800") Message-ID: <87bkvt4d56.fsf@email.froward.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1nrkR3-006e82-PA;;;mid=<87bkvt4d56.fsf@email.froward.int.ebiederm.org>;;;hst=in02.mta.xmission.com;;;ip=68.227.174.4;;;frm=ebiederm@xmission.com;;;spf=softfail X-XM-AID: U2FsdGVkX1/rSxyKigaHMoSccMUFZ8spyBuVsjtCT6w= X-SA-Exim-Connect-IP: 68.227.174.4 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH] kexec_file: Drop weak attribute from arch_kexec_apply_relocations[_add] X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Baoquan He writes: > Hi Eric, > > On 05/18/22 at 04:59pm, Eric W. Biederman wrote: >> "Naveen N. Rao" writes: >> >> > Since commit d1bcae833b32f1 ("ELF: Don't generate unused section >> > symbols") [1], binutils (v2.36+) started dropping section symbols that >> > it thought were unused. This isn't an issue in general, but with >> > kexec_file.c, gcc is placing kexec_arch_apply_relocations[_add] into a >> > separate .text.unlikely section and the section symbol ".text.unlikely" >> > is being dropped. Due to this, recordmcount is unable to find a non-weak >> > symbol in .text.unlikely to generate a relocation record against. >> > >> > Address this by dropping the weak attribute from these functions: >> > - arch_kexec_apply_relocations() is not overridden by any architecture >> > today, so just drop the weak attribute. >> > - arch_kexec_apply_relocations_add() is only overridden by x86 and s390. >> > Retain the function prototype for those and move the weak >> > implementation into the header as a static inline for other >> > architectures. >> > >> > [1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=d1bcae833b32f1 >> >> Any chance you can also get machine_kexec_post_load, >> crash_free_reserved_phys_range, arch_kexec_protect_protect_crashkres, >> arch_kexec_unprotect_crashkres, arch_kexec_kernel_image_probe, >> arch_kexec_kernel_image_probe, arch_kimage_file_post_load_cleanup, >> arch_kexec_kernel_verify_sig, and arch_kexec_locate_mem_hole as well. >> >> That is everything in kexec that uses a __weak symbol. If we can't >> count on them working we might as well just get rid of the rest >> preemptively. > > Is there a new rule that __weak is not suggested in kernel any more? > Please help provide a pointer if yes, so that I can learn that. > > In my mind, __weak is very simple and clear as a mechanism to add > ARCH related functionality. You should be able to trace the conversation back for all of the details but if you can't here is the summary. There is a tool that some architectures use called recordmcount. The recordmcount looks for a symbol in a section, and ignores all weak symbols. In certain cases sections become so simple there are only weak symbols. At which point recordmcount fails. Which means in practice __weak symbols are unreliable and don't work to add ARCH related functionality. Given that __weak symbols fail randomly I would much rather have simpler code that doesn't fail. It has never been the case that __weak symbols have been very common in the kernel. I expect they are something like bool that have been gaining traction. Still given that __weak symbols don't work. I don't want them. Eric