From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:17740 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726980AbgCBRJO (ORCPT ); Mon, 2 Mar 2020 12:09:14 -0500 Received: from pps.filterd (m0098417.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.42/8.16.0.42) with SMTP id 022GsX1X136050 for ; Mon, 2 Mar 2020 12:09:13 -0500 Received: from e06smtp05.uk.ibm.com (e06smtp05.uk.ibm.com [195.75.94.101]) by mx0a-001b2d01.pphosted.com with ESMTP id 2yfmqa26fx-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Mon, 02 Mar 2020 12:09:13 -0500 Received: from localhost by e06smtp05.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 2 Mar 2020 17:09:11 -0000 Date: Mon, 02 Mar 2020 22:39:06 +0530 From: "Naveen N. Rao" Subject: eh_frame confusion References: <3b00b45f-74b5-13e3-9a98-c3d6b3bb7286@rasmusvillemoes.dk> In-Reply-To: <3b00b45f-74b5-13e3-9a98-c3d6b3bb7286@rasmusvillemoes.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8BIT Message-Id: <1583168442.ovqnxu16tp.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 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 However, that is now handled thanks to commit 9f751b82b491d: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9f751b82b491d 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? > > but prior to gcc-8, .eh_frame didn't seem to get generated anyway. > > Can .eh_frame sections be discarded for modules (on ppc32 at least), or > is there some magic that makes them necessary when building with gcc-8? As Segher points out, it looks like we need to add -fno-asynchronous-unwind-tables. Most other architectures seem to use that too. - Naveen