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([fe80::c143:fac2:85b4:14cb]) by SN6PR1501MB2064.namprd15.prod.outlook.com ([fe80::c143:fac2:85b4:14cb%7]) with mapi id 15.20.4478.025; Tue, 7 Sep 2021 22:24:08 +0000 Subject: Re: [PATCH bpf-next v2] libbpf: ignore .eh_frame sections when parsing elf files To: =?UTF-8?Q?Toke_H=c3=b8iland-J=c3=b8rgensen?= , Alexei Starovoitov CC: Andrii Nakryiko , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Song Liu , John Fastabend , KP Singh , Stanislav Fomichev , bpf References: <20210826120953.11041-1-toke@redhat.com> <87lf4hvrgc.fsf@toke.dk> <87wnnysy6k.fsf@toke.dk> <095f116b-7399-25a5-dca2-145cbd093326@fb.com> <87czpqskac.fsf@toke.dk> <0426a8fb-ee42-cbcc-65e9-45654adf5948@fb.com> <87fsugp48q.fsf@toke.dk> From: Yonghong Song Message-ID: <3bbe1fae-617b-4d1a-8c29-2579d19cd5fb@fb.com> Date: Tue, 7 Sep 2021 15:24:05 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 In-Reply-To: <87fsugp48q.fsf@toke.dk> 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X-MS-Exchange-CrossTenant-AuthSource: SN6PR1501MB2064.namprd15.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 07 Sep 2021 22:24:08.7613 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2 X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: BJ43F4nhxBdxVsx4Mmou2/yzFzRB91LStskKQVAM70/SaGFtfsQaPNIUtLMDgoz2 X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA0PR15MB3965 X-OriginatorOrg: fb.com X-Proofpoint-ORIG-GUID: ULv1ert-j9X6ctKSlEqSWswisDOPHAiZ X-Proofpoint-GUID: ULv1ert-j9X6ctKSlEqSWswisDOPHAiZ Content-Transfer-Encoding: 8bit X-Proofpoint-UnRewURL: 2 URL's were un-rewritten MIME-Version: 1.0 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.391,18.0.790 definitions=2021-09-07_08:2021-09-07,2021-09-07 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 adultscore=0 mlxlogscore=999 phishscore=0 impostorscore=0 spamscore=0 bulkscore=0 mlxscore=0 priorityscore=1501 clxscore=1015 suspectscore=0 malwarescore=0 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2109030001 definitions=main-2109070140 X-FB-Internal: deliver Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On 9/7/21 12:36 PM, Toke Høiland-Jørgensen wrote: > Yonghong Song writes: > >> On 9/2/21 3:08 PM, Toke Høiland-Jørgensen wrote: >>> Yonghong Song writes: >>> >>>> On 9/2/21 12:32 PM, Alexei Starovoitov wrote: >>>>> On Thu, Sep 2, 2021 at 10:08 AM Toke Høiland-Jørgensen wrote: >>>>>> >>>>>> Yonghong Song writes: >>>>>> >>>>>>> On 8/31/21 3:28 AM, Toke Høiland-Jørgensen wrote: >>>>>>>> Andrii Nakryiko writes: >>>>>>>> >>>>>>>>> On Thu, Aug 26, 2021 at 5:10 AM Toke Høiland-Jørgensen wrote: >>>>>>>>>> >>>>>>>>>> When .eh_frame and .rel.eh_frame sections are present in BPF object files, >>>>>>>>>> libbpf produces errors like this when loading the file: >>>>>>>>>> >>>>>>>>>> libbpf: elf: skipping unrecognized data section(32) .eh_frame >>>>>>>>>> libbpf: elf: skipping relo section(33) .rel.eh_frame for section(32) .eh_frame >>>>>>>>>> >>>>>>>>>> It is possible to get rid of the .eh_frame section by adding >>>>>>>>>> -fno-asynchronous-unwind-tables to the compilation, but we have seen >>>>>>>>>> multiple examples of these sections appearing in BPF files in the wild, >>>>>>>>>> most recently in samples/bpf, fixed by: >>>>>>>>>> 5a0ae9872d5c ("bpf, samples: Add -fno- >>>>> /to BPF Clang invocation") >>>>>>>>>> >>>>>>>>>> While the errors are technically harmless, they look odd and confuse users. >>>>>>>>> >>>>>>>>> These warnings point out invalid set of compiler flags used for >>>>>>>>> compiling BPF object files, though. Which is a good thing and should >>>>>>>>> incentivize anyone getting those warnings to check and fix how they do >>>>>>>>> BPF compilation. Those .eh_frame sections shouldn't be present in BPF >>>>>>>>> object files at all, and that's what libbpf is trying to say. >>>>>>>> >>>>>>>> Apart from triggering that warning, what effect does this have, though? >>>>>>>> The programs seem to work just fine (as evidenced by the fact that >>>>>>>> samples/bpf has been built this way for years, for instance)... >>>>>>>> >>>>>>>> Also, how is a user supposed to go from that cryptic error message to >>>>>>>> figuring out that it has something to do with compiler flags? >>>>>>>> >>>>>>>>> I don't know exactly in which situations that .eh_frame section is >>>>>>>>> added, but looking at our selftests (and now samples/bpf as well), >>>>>>>>> where we use -target bpf, we don't need >>>>>>>>> -fno-asynchronous-unwind-tables at all. >>>>>>>> >>>>>>>> This seems to at least be compiler-dependent. We ran into this with >>>>>>>> bpftool as well (for the internal BPF programs it loads whenever it >>>>>>>> runs), which already had '-target bpf' in the Makefile. We're carrying >>>>>>>> an internal RHEL patch adding -fno-asynchronous-unwind-tables to the >>>>>>>> bpftool build to fix this... >>>>>>> >>>>>>> I haven't seen an instance of .eh_frame as well with -target bpf. >>>>>>> Do you have a reproducible test case? I would like to investigate >>>>>>> what is the possible cause and whether we could do something in llvm >>>>>>> to prevent its generatin. Thanks! >>>>>> >>>>>> We found this in the RHEL builds of bpftool. I don't think we're doing >>>>>> anything special, other than maybe building with a clang version that's >>>>>> a few versions behind: >>>>>> >>>>>> # clang --version >>>>>> clang version 11.0.0 (Red Hat 11.0.0-1.module+el8.4.0+8598+a071fcd5) >>>>>> Target: x86_64-unknown-linux-gnu >>>>>> Thread model: posix >>>>>> InstalledDir: /usr/bin >>>>>> >>>>>> So I suppose it may resolve itself once we upgrade LLVM? >>>>> >>>>> That's odd. I don't think I've seen this issue even with clang 11 >>>>> (but I built it myself). >>>> >>>> I cannot reproduce it by self with self built llvm (11, 12, 13, 14). >>>> But I can reproduce it with an upstream built llvm12. >>>> >>>> /bin/clang \ >>>> -I. \ >>>> -I/home/yhs/work/bpf-next/tools/include/uapi/ \ >>>> -I/home/yhs/work/bpf-next/tools/lib/bpf/ \ >>>> -I/home/yhs/work/bpf-next/tools/lib \ >>>> -g -O2 -Wall -target bpf -c skeleton/pid_iter.bpf.c -o >>>> pid_iter.bpf.o && llvm-strip -g pid_iter.bpf.o >>>> GEN pid_iter.skel.h >>>> libbpf: elf: skipping unrecognized data section(11) .eh_frame >>>> libbpf: elf: skipping relo section(12) .rel.eh_frame for section(11) >>>> .eh_frame >>> >>> Ah, that's interesting! >>> >>>>> If there is a fix indeed let's backport it to llvm 11. The user >>>>> experience matters. >>>>> It could be llvm configuration too. >>>>> I'm guessing some build flags might influence default settings >>>>> for unwind tables. >>>>> >>>>> Yonghong, can we make bpf backend to ignore needsUnwindTableEntry ? >>>> >>>> Sure. I will try to get upstream build flags, reproduce and fix it >>>> in llvm. >> >> I did some investigation and this is due to centos private patch: >> https://git.centos.org/rpms/clang/blob/b99d8d4a38320329e10570f308c3e2d8cf295c78/f/SOURCES/0002-PATCH-clang-Make-funwind-tables-the-default-on-all-a.patch >> >> In upstream, the original llvm-project source is patched with >> several private patches before building the rpm. >> https://koji.mbox.centos.org/pkgs/packages/clang/12.0.1/1.module_el8.5.0+892+54d791e1/data/logs/x86_64/build.log >> >> The above private patch enables unwind-table (.eh_frame section) >> by default for ALL architectures and bpf is a victim of this. > > Ah, doh! I had no idea we were doing this :/ > >> I filed a redhat bugzilla bug to fix their private patch. >> >> https://bugzilla.redhat.com/show_bug.cgi?id=2002024 >> >> Hopefully future newer compiler build won't have this issue. > > Thank you for finding the root cause of this! I'll follow up internally > and make sure we get this fixed... Thanks! Hopefully this can be resolved soon. > > -Toke >