Re: llvm-objdump bpf coredump

[Yonghong Song <yhs@fb.com>]

On 9/18/21 8:41 AM, Vincent Li wrote:
> On Fri, Sep 17, 2021 at 8:00 PM Yonghong Song <yhs@fb.com> wrote:
>>
>>
>>
>> On 9/17/21 6:34 PM, Vincent Li wrote:
>>> Hi,
>>>
>>> I am supposed to file a bug report in https://bugs.llvm.org/   but the
>>> site requires account login, while waiting for my account setup, and
>>> since this is related to BPF, I will try my luck here :)
>>>
>>> This is cilium environment, when I use llvm-objdump to disassemble the
>>> bpf_lxc.o like below, it core dumps
>>>
>>> #llvm-objdump -S -D  /var/run/cilium/state/2799/bpf_lxc.o >
>>> /home/bpf_lxc-5.4.txt
>>
>> Any particular reason you are using '-D' here?
>>
>> "llvm-objdump --help" gives:
>>
>>     --disassemble-all       Display assembler mnemonics for the machine
>> instructions
>>
>>     --disassemble           Display assembler mnemonics for the machine
>> instructions
>>
>>
>>     -D                      Alias for --disassemble-all
>>
>>
>>     -d                      Alias for --disassemble
>>
>> It gives the same description about -D and -d but actually -D tries to
>> disassemble *all* sections and -d tries to disassemble *only* executable
>> sections.
>>
>> The following is an example to disassemble a selftest bpf program
>> bpf_iter_tcp4.o:
>>
>> [yhs@devbig003.ftw2 ~/work/bpf-next/tools/testing/selftests/bpf]
>> llvm-objdump -D bpf_iter_tcp4.o >& ~/log
>> [yhs@devbig003.ftw2 ~/work/bpf-next/tools/testing/selftests/bpf] grep
>> "Disassembly of section" ~/log
>> Disassembly of section .strtab:
>> Disassembly of section iter/tcp:
>> Disassembly of section .reliter/tcp:
>> Disassembly of section license:
>> Disassembly of section .rodata:
>> Disassembly of section .debug_loc:
>> Disassembly of section .debug_abbrev:
>> Disassembly of section .debug_info:
>> Disassembly of section .rel.debug_info:
>> Disassembly of section .debug_ranges:
>> Disassembly of section .debug_str:
>> Disassembly of section .BTF:
>> Disassembly of section .rel.BTF:
>> Disassembly of section .BTF.ext:
>> Disassembly of section .rel.BTF.ext:
>> Disassembly of section .debug_frame:
>> Disassembly of section .rel.debug_frame:
>> Disassembly of section .debug_line:
>> Disassembly of section .rel.debug_line:
>> Disassembly of section .llvm_addrsig:
>> Disassembly of section .symtab:
>> [yhs@devbig003.ftw2 ~/work/bpf-next/tools/testing/selftests/bpf]
>> llvm-objdump -d bpf_iter_tcp4.o >& ~/log
>> [yhs@devbig003.ftw2 ~/work/bpf-next/tools/testing/selftests/bpf] grep
>> "Disassembly of section" ~/log
>> Disassembly of section iter/tcp:
>> [yhs@devbig003.ftw2 ~/work/bpf-next/tools/testing/selftests/bpf]
>>
>> You can see, "-D" tries to disassemble *all* sections and "-d" only
>> tries to disassemble "iter/tcp" section which is the *text* section.
>>
>> I suspect llvm BPF backend may have some loose end which didn't handle
>> well for illegal insn, which I will put up a task so we can fix it.
>>
>> I guess you tries to disassemble for non-text sections because you want
>> to compare contents of sections with text file? If this is the case,
>> you may want to compare section data directly, because disassemble
>> illegal insn will result in "<unknown>" insn which may hide the
>> actual data difference. In any case, it would be good to know what you
>> try to do with "-D" result so we may discuss to find a proper solution.
>>
> 
> Yonghong, thank you for the detail explanation, it is me not clear
> about the usage of
> llvm-objdump, should have read the manual or try different argument
> before this post,
> thought better reporting the crash dump than doing nothing :)

No worry. Yes, crash is not good. We will need to fix it.

> 
> Vincent
> 
> 
>>>
>>> PLEASE submit a bug report to https://bugs.llvm.org/   and include the
>>> crash backtrace.
>>>
>>> Stack dump:
>>>
>>> 0. Program arguments: llvm-objdump -S -D /var/run/cilium/state/2799/bpf_lxc.o
>>>
>>>    #0 0x00000000006336bc llvm::sys::PrintStackTrace(llvm::raw_ostream&,
>>> int) (/usr/local/bin/llvm-objdump+0x6336bc)
>>>
>>>    #1 0x00000000006318a4 llvm::sys::RunSignalHandlers()
>>> (/usr/local/bin/llvm-objdump+0x6318a4)
>>>
>>>    #2 0x0000000000631efd SignalHandler(int) (/usr/local/bin/llvm-objdump+0x631efd)
>>>
>>>    #3 0x00007f9f39a7fb20 __restore_rt (/lib64/libpthread.so.0+0x12b20)
>>>
>>>    #4 0x0000000000492e8b
>>> llvm::BPFInstPrinter::printMemOperand(llvm::MCInst const*, int,
>>> llvm::raw_ostream&, char const*)
>>> (/usr/local/bin/llvm-objdump+0x492e8b)
>>>
>>>    #5 0x00000000004946c0 llvm::BPFInstPrinter::printInst(llvm::MCInst
>>> const*, unsigned long, llvm::StringRef, llvm::MCSubtargetInfo const&,
>>> llvm::raw_ostream&) (/usr/local/bin/llvm-objdump+0x4946c0)
>>>
>>>    #6 0x0000000000432351 (anonymous
>>> namespace)::BPFPrettyPrinter::printInst(llvm::MCInstPrinter&,
>>> llvm::MCInst const*, llvm::ArrayRef<unsigned char>,
>>> llvm::object::SectionedAddress, llvm::formatted_raw_ostream&,
>>> llvm::StringRef, llvm::MCSubtargetInfo const&, (anonymous
>>> namespace)::SourcePrinter*, llvm::StringRef,
>>> std::vector<llvm::object::RelocationRef,
>>> std::allocator<llvm::object::RelocationRef> >*, (anonymous
>>> namespace)::LiveVariablePrinter&)
>>> (/usr/local/bin/llvm-objdump+0x432351)
>>>
>>>    #7 0x00000000004400b8 disassembleObject(llvm::Target const*,
>>> llvm::object::ObjectFile const*, llvm::MCContext&,
>>> llvm::MCDisassembler*, llvm::MCDisassembler*, llvm::MCInstrAnalysis
>>> const*, llvm::MCInstPrinter*, llvm::MCSubtargetInfo const*,
>>> llvm::MCSubtargetInfo const*, (anonymous namespace)::PrettyPrinter&,
>>> (anonymous namespace)::SourcePrinter&, bool)
>>> (/usr/local/bin/llvm-objdump+0x4400b8)
>>>
>>>    #8 0x000000000044444e disassembleObject(llvm::object::ObjectFile
>>> const*, bool) (/usr/local/bin/llvm-objdump+0x44444e)
>>>
>>>    #9 0x00000000004454e2 dumpObject(llvm::object::ObjectFile*,
>>> llvm::object::Archive const*, llvm::object::Archive::Child const*)
>>> (/usr/local/bin/llvm-objdump+0x4454e2)
>>>
>>> #10 0x0000000000409910 main (/usr/local/bin/llvm-objdump+0x409910)
>>>
>>> #11 0x00007f9f3854c493 __libc_start_main (/lib64/libc.so.6+0x23493)
>>>
>>> #12 0x000000000042384e _start (/usr/local/bin/llvm-objdump+0x42384e)
>>>
>>> Segmentation fault (core dumped)
>>>
>>> I compiled and installed llvm and clang 12.0.1 myself
>>>