From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:52056 "EHLO mx0b-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725993AbgCEMlV (ORCPT ); Thu, 5 Mar 2020 07:41:21 -0500 Received: from pps.filterd (m0127361.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.42/8.16.0.42) with SMTP id 025CbK2C116813 for ; Thu, 5 Mar 2020 07:41:19 -0500 Received: from e06smtp01.uk.ibm.com (e06smtp01.uk.ibm.com [195.75.94.97]) by mx0a-001b2d01.pphosted.com with ESMTP id 2yfknd3quw-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Thu, 05 Mar 2020 07:41:18 -0500 Received: from localhost by e06smtp01.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 5 Mar 2020 12:41:13 -0000 Date: Thu, 05 Mar 2020 18:11:06 +0530 From: "Naveen N. Rao" Subject: Re: eh_frame confusion References: <3b00b45f-74b5-13e3-9a98-c3d6b3bb7286@rasmusvillemoes.dk> <1583168442.ovqnxu16tp.naveen@linux.ibm.com> <877e01spfa.fsf@mpe.ellerman.id.au> In-Reply-To: <877e01spfa.fsf@mpe.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8BIT Message-Id: <1583411741.zbaz5p86qy.naveen@linux.ibm.com> Sender: linux-kbuild-owner@vger.kernel.org List-ID: To: Linux Kbuild mailing list , LKML , "linuxppc-dev@lists.ozlabs.org" , Rasmus Villemoes , Michael Ellerman Michael Ellerman wrote: > "Naveen N. Rao" writes: >> Rasmus Villemoes wrote: >>> I'm building a ppc32 kernel, and noticed that after upgrading from gcc-7 >>> to gcc-8 all object files now end up having .eh_frame section. For >>> vmlinux, that's not a problem, because they all get discarded in >>> arch/powerpc/kernel/vmlinux.lds.S . However, they stick around in >>> modules, which doesn't seem to be useful - given that everything worked >>> just fine with gcc-7, and I don't see anything in the module loader that >>> handles .eh_frame. >>> >>> The reason I care is that my target has a rather tight rootfs budget, >>> and the .eh_frame section seem to occupy 10-30% of the file size >>> (obviously very depending on the particular module). >>> >>> Comparing the .foo.o.cmd files, I don't see change in options that might >>> explain this (there's a bunch of new -Wno-*, and the -mspe=no spelling >>> is apparently no longer supported in gcc-8). Both before and after, there's >>> >>> -fno-dwarf2-cfi-asm >>> >>> about which gcc's documentation says >>> >>> '-fno-dwarf2-cfi-asm' >>> Emit DWARF unwind info as compiler generated '.eh_frame' section >>> instead of using GAS '.cfi_*' directives. >>> >>> Looking into where that comes from got me even more confused, because >>> both arm and unicore32 say >>> >>> # Never generate .eh_frame >>> KBUILD_CFLAGS += $(call cc-option,-fno-dwarf2-cfi-asm) >>> >>> while the ppc32 case at hand says >>> >>> # FIXME: the module load should be taught about the additional relocs >>> # generated by this. >>> # revert to pre-gcc-4.4 behaviour of .eh_frame >> >> Michael opened a task to look into this recently and I had spent some >> time last week on this. The original commit/discussion adding >> -fno-dwarf2-cfi-asm refers to R_PPC64_REL32 relocations not being >> handled by our module loader: >> http://lkml.kernel.org/r/20090224065112.GA6690@bombadil.infradead.org > > I opened that issue purely based on noticing the wart in the Makefile, > not because I'd actually tested it. > >> However, that is now handled thanks to commit 9f751b82b491d: >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9f751b82b491d > > Haha, written by me, what an idiot. > > So the Makefile hack can presumably be dropped, because the module > loader can handle the relocations. > > And then maybe we also want to turn off the unwind tables, but that > would be a separate patch. > >> I did a test build and a simple module loaded fine, so I think >> -fno-dwarf2-cfi-asm is not required anymore, unless Michael has seen >> some breakages with it. Michael? > > No, as I said above it was just reading the Makefile. Ok, thanks for clarifying. To test, I did 'allmodconfig' builds across three environments: - gcc (Ubuntu 9.2.1-9ubuntu2) 9.2.1 20191008 -- ppc64le - gcc (SUSE Linux) 7.5.0 -- ppc64le - gcc (GCC) 8.2.1 20181215 (Red Hat 8.2.1-6) -- ppc64 (BE) Then, used the below command to list all relocations in the modules: $ find . -name '*.ko' | xargs -n 1 readelf -Wr | grep -v "Relocation " | grep -v "Offset " | cut -d' ' -f4 | sort | uniq R_PPC64_ADDR32 R_PPC64_ADDR64 R_PPC64_ENTRY R_PPC64_REL24 R_PPC64_REL32 R_PPC64_REL64 R_PPC64_TOC R_PPC64_TOC16_HA R_PPC64_TOC16_LO R_PPC64_TOC16_LO_DS All three environments show up similar set of relocations, all of which we handle in the module loader today. If Rasmus/Christophe can confirm that this is true for ppc32 as well, then we should be fine. - Naveen