From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:42120 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727000AbgCBRcw (ORCPT ); Mon, 2 Mar 2020 12:32:52 -0500 Received: from pps.filterd (m0098393.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.42/8.16.0.42) with SMTP id 022HLmA0111489 for ; Mon, 2 Mar 2020 12:32:51 -0500 Received: from e06smtp04.uk.ibm.com (e06smtp04.uk.ibm.com [195.75.94.100]) by mx0a-001b2d01.pphosted.com with ESMTP id 2yfhqpwgk2-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Mon, 02 Mar 2020 12:32:51 -0500 Received: from localhost by e06smtp04.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 2 Mar 2020 17:32:49 -0000 Date: Mon, 02 Mar 2020 23:02:42 +0530 From: "Naveen N. Rao" Subject: eh_frame confusion References: <3b00b45f-74b5-13e3-9a98-c3d6b3bb7286@rasmusvillemoes.dk> <1583168442.ovqnxu16tp.naveen@linux.ibm.com> In-Reply-To: <1583168442.ovqnxu16tp.naveen@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8BIT Message-Id: <1583169883.zo43kx69lm.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 Naveen N. Rao wrote: > 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. Can you check if the below patch works? I am yet to test this in more detail, but would be good to know the implications for ppc32. - Naveen --- diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile index f35730548e42..5b5bf98b8217 100644 --- a/arch/powerpc/Makefile +++ b/arch/powerpc/Makefile @@ -239,10 +239,7 @@ KBUILD_CFLAGS += $(call cc-option,-mno-vsx) KBUILD_CFLAGS += $(call cc-option,-mno-spe) KBUILD_CFLAGS += $(call cc-option,-mspe=no) -# FIXME: the module load should be taught about the additional relocs -# generated by this. -# revert to pre-gcc-4.4 behaviour of .eh_frame -KBUILD_CFLAGS += $(call cc-option,-fno-dwarf2-cfi-asm) +KBUILD_CFLAGS += $(call cc-option,-fno-asynchronous-unwind-tables) # Never use string load/store instructions as they are # often slow when they are implemented at all diff --git a/arch/powerpc/kernel/vdso32/Makefile b/arch/powerpc/kernel/vdso32/Makefile index e147bbdc12cd..d43b0b18137c 100644 --- a/arch/powerpc/kernel/vdso32/Makefile +++ b/arch/powerpc/kernel/vdso32/Makefile @@ -25,6 +25,7 @@ KCOV_INSTRUMENT := n UBSAN_SANITIZE := n ccflags-y := -shared -fno-common -fno-builtin -nostdlib \ + -fasynchronous-unwind-tables \ -Wl,-soname=linux-vdso32.so.1 -Wl,--hash-style=both asflags-y := -D__VDSO32__ -s diff --git a/arch/powerpc/kernel/vdso64/Makefile b/arch/powerpc/kernel/vdso64/Makefile index 32ebb3522ea1..b2cbb5c49bad 100644 --- a/arch/powerpc/kernel/vdso64/Makefile +++ b/arch/powerpc/kernel/vdso64/Makefile @@ -13,6 +13,7 @@ KCOV_INSTRUMENT := n UBSAN_SANITIZE := n ccflags-y := -shared -fno-common -fno-builtin -nostdlib \ + -fasynchronous-unwind-tables \ -Wl,-soname=linux-vdso64.so.1 -Wl,--hash-style=both asflags-y := -D__VDSO64__ -s