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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 2 Mar 2020 17:09:09 -0000 Received: from d06av21.portsmouth.uk.ibm.com (d06av21.portsmouth.uk.ibm.com [9.149.105.232]) by b06cxnps3074.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 022H98mL57344244 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 2 Mar 2020 17:09:08 GMT Received: from d06av21.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 24BCD52051; Mon, 2 Mar 2020 17:09:08 +0000 (GMT) Received: from localhost (unknown [9.199.48.41]) by d06av21.portsmouth.uk.ibm.com (Postfix) with ESMTP id A98FC5204E; Mon, 2 Mar 2020 17:09:07 +0000 (GMT) Date: Mon, 02 Mar 2020 22:39:06 +0530 From: "Naveen N. Rao" Subject: 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> In-Reply-To: <3b00b45f-74b5-13e3-9a98-c3d6b3bb7286@rasmusvillemoes.dk> 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: 8BIT X-TM-AS-GCONF: 00 x-cbid: 20030217-0020-0000-0000-000003AFAB6F X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 20030217-0021-0000-0000-00002207D78C Message-Id: <1583168442.ovqnxu16tp.naveen@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.138,18.0.572 definitions=2020-03-02_06:2020-03-02,2020-03-02 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 malwarescore=0 spamscore=0 lowpriorityscore=0 suspectscore=0 adultscore=0 mlxlogscore=999 clxscore=1015 priorityscore=1501 mlxscore=0 impostorscore=0 phishscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2001150001 definitions=main-2003020115 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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