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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Thu, 5 Mar 2020 12:41:10 -0000 Received: from b06wcsmtp001.portsmouth.uk.ibm.com (b06wcsmtp001.portsmouth.uk.ibm.com [9.149.105.160]) by b06cxnps4074.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 025Cf9I551576978 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 5 Mar 2020 12:41:09 GMT Received: from b06wcsmtp001.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E2949A4067; Thu, 5 Mar 2020 12:41:08 +0000 (GMT) Received: from b06wcsmtp001.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 25C51A4054; Thu, 5 Mar 2020 12:41:08 +0000 (GMT) Received: from localhost (unknown [9.199.53.44]) by b06wcsmtp001.portsmouth.uk.ibm.com (Postfix) with ESMTP; Thu, 5 Mar 2020 12:41:07 +0000 (GMT) Date: Thu, 05 Mar 2020 18:11:06 +0530 From: "Naveen N. Rao" Subject: Re: eh_frame confusion To: Linux Kbuild mailing list , LKML , "linuxppc-dev@lists.ozlabs.org" , Rasmus Villemoes , Michael Ellerman 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 User-Agent: astroid/v0.15-13-gb675b421 (https://github.com/astroidmail/astroid) Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable X-TM-AS-GCONF: 00 x-cbid: 20030512-4275-0000-0000-000003A8A160 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 20030512-4276-0000-0000-000038BDB137 Message-Id: <1583411741.zbaz5p86qy.naveen@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.138, 18.0.572 definitions=2020-03-05_03:2020-03-05, 2020-03-05 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 suspectscore=0 mlxscore=0 malwarescore=0 adultscore=0 bulkscore=0 impostorscore=0 mlxlogscore=999 clxscore=1015 lowpriorityscore=0 spamscore=0 priorityscore=1501 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2001150001 definitions=main-2003050082 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: , Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" 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 tha= t >>> handles .eh_frame. >>>=20 >>> 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). >>>=20 >>> Comparing the .foo.o.cmd files, I don't see change in options that migh= t >>> explain this (there's a bunch of new -Wno-*, and the -mspe=3Dno spellin= g >>> is apparently no longer supported in gcc-8). Both before and after, the= re's >>>=20 >>> -fno-dwarf2-cfi-asm >>>=20 >>> about which gcc's documentation says >>>=20 >>> '-fno-dwarf2-cfi-asm' >>> Emit DWARF unwind info as compiler generated '.eh_frame' section >>> instead of using GAS '.cfi_*' directives. >>>=20 >>> Looking into where that comes from got me even more confused, because >>> both arm and unicore32 say >>>=20 >>> # Never generate .eh_frame >>> KBUILD_CFLAGS +=3D $(call cc-option,-fno-dwarf2-cfi-asm) >>>=20 >>> while the ppc32 case at hand says >>>=20 >>> # 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=20 >> time last week on this. The original commit/discussion adding=20 >> -fno-dwarf2-cfi-asm refers to R_PPC64_REL32 relocations not being=20 >> handled by our module loader: >> http://lkml.kernel.org/r/20090224065112.GA6690@bombadil.infradead.org >=20 > I opened that issue purely based on noticing the wart in the Makefile, > not because I'd actually tested it. >=20 >> However, that is now handled thanks to commit 9f751b82b491d: >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commi= t/?id=3D9f751b82b491d >=20 > Haha, written by me, what an idiot. >=20 > So the Makefile hack can presumably be dropped, because the module > loader can handle the relocations. >=20 > And then maybe we also want to turn off the unwind tables, but that > would be a separate patch. >=20 >> I did a test build and a simple module loaded fine, so I think=20 >> -fno-dwarf2-cfi-asm is not required anymore, unless Michael has seen=20 >> some breakages with it. Michael? >=20 > No, as I said above it was just reading the Makefile. Ok, thanks for clarifying. To test, I did 'allmodconfig' builds across=20 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 " | g= rep -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=20 we handle in the module loader today. If Rasmus/Christophe can confirm that this is true for ppc32 as well,=20 then we should be fine. - Naveen