Re: [PATCH bpf-next] tools/resolve_btfids: Fix sections with wrong alignment

[Yonghong Song <yhs@fb.com>]

On 8/19/20 11:16 AM, Nick Desaulniers wrote:
> On Wed, Aug 19, 2020 at 10:36 AM Jiri Olsa <jolsa@redhat.com> wrote:
>>
>> On Wed, Aug 19, 2020 at 08:31:51AM -0700, Yonghong Song wrote:
>>>
>>>
>>> On 8/19/20 2:23 AM, Jiri Olsa wrote:
>>>> The data of compressed section should be aligned to 4
>>>> (for 32bit) or 8 (for 64 bit) bytes.
>>>>
>>>> The binutils ld sets sh_addralign to 1, which makes libelf
>>>> fail with misaligned section error during the update as
>>>> reported by Jesper:
>>>>
>>>>      FAILED elf_update(WRITE): invalid section alignment
>>>>
>>>> While waiting for ld fix, we can fix compressed sections
>>>> sh_addralign value manually.
> 
> Is there a bug filed against GNU ld? Link?
> 
>>>>
>>>> Adding warning in -vv mode when the fix is triggered:
>>>>
>>>>     $ ./tools/bpf/resolve_btfids/resolve_btfids -vv vmlinux
>>>>     ...
>>>>     section(36) .comment, size 44, link 0, flags 30, type=1
>>>>     section(37) .debug_aranges, size 45684, link 0, flags 800, type=1
>>>>      - fixing wrong alignment sh_addralign 16, expected 8
>>>>     section(38) .debug_info, size 129104957, link 0, flags 800, type=1
>>>>      - fixing wrong alignment sh_addralign 1, expected 8
>>>>     section(39) .debug_abbrev, size 1152583, link 0, flags 800, type=1
>>>>      - fixing wrong alignment sh_addralign 1, expected 8
>>>>     section(40) .debug_line, size 7374522, link 0, flags 800, type=1
>>>>      - fixing wrong alignment sh_addralign 1, expected 8
>>>>     section(41) .debug_frame, size 702463, link 0, flags 800, type=1
>>>>     section(42) .debug_str, size 1017571, link 0, flags 830, type=1
>>>>      - fixing wrong alignment sh_addralign 1, expected 8
>>>>     section(43) .debug_loc, size 3019453, link 0, flags 800, type=1
>>>>      - fixing wrong alignment sh_addralign 1, expected 8
>>>>     section(44) .debug_ranges, size 1744583, link 0, flags 800, type=1
>>>>      - fixing wrong alignment sh_addralign 16, expected 8
>>>>     section(45) .symtab, size 2955888, link 46, flags 0, type=2
>>>>     section(46) .strtab, size 2613072, link 0, flags 0, type=3
>>>>     ...
>>>>     update ok for vmlinux
>>>>
>>>> Another workaround is to disable compressed debug info data
>>>> CONFIG_DEBUG_INFO_COMPRESSED kernel option.
>>>
>>> So CONFIG_DEBUG_INFO_COMPRESSED is required to reproduce the bug, right?
>>
>> correct
>>
>>>
>>> I turned on CONFIG_DEBUG_INFO_COMPRESSED in my config and got a bunch of
>>> build failures.
>>>
>>> ld: drivers/crypto/virtio/virtio_crypto_algs.o: unable to initialize
>>> decompress status for section .debug_info
>>> ld: drivers/crypto/virtio/virtio_crypto_algs.o: unable to initialize
>>> decompress status for section .debug_info
>>> ld: drivers/crypto/virtio/virtio_crypto_algs.o: unable to initialize
>>> decompress status for section .debug_info
>>> ld: drivers/crypto/virtio/virtio_crypto_algs.o: unable to initialize
>>> decompress status for section .debug_info
>>> drivers/crypto/virtio/virtio_crypto_algs.o: file not recognized: File format
>>> not recognized
>>>
>>> ld: net/llc/llc_core.o: unable to initialize decompress status for section
>>> .debug_info
>>> ld: net/llc/llc_core.o: unable to initialize decompress status for section
>>> .debug_info
>>> ld: net/llc/llc_core.o: unable to initialize decompress status for section
>>> .debug_info
>>> ld: net/llc/llc_core.o: unable to initialize decompress status for section
>>> .debug_info
>>> net/llc/llc_core.o: file not recognized: File format not recognized
>>>
>>> ...
>>>
>>> The 'ld' in my system:
>>>
>>> $ ld -V
>>> GNU ld version 2.30-74.el8
>>>    Supported emulations:
>>>     elf_x86_64
>>>     elf32_x86_64
>>>     elf_i386
>>>     elf_iamcu
>>>     i386linux
>>>     elf_l1om
>>>     elf_k1om
>>>     i386pep
>>>     i386pe
> 
> According to Documentation/process/changes.rst, the minimum supported
> version of GNU binutils for the kernels is 2.23.  Can you upgrade to
> that and confirm that you still observe the issue?  I don't want to
> spend time chasing bugs in old, unsupported versions of GNU binutils,
> especially as Jiri notes, 2.26 is required for
> CONFIG_DEBUG_INFO_COMPRESSED.  We can always strengthen the Kconfig
> check for it.  Otherwise, I'm not familiar with the observed error
> message.

I built a "ld" with latest binutils-gdb repo and I can reproduced
the issue. Indeed applying the patch here fixed the issue. So
I think there is no need to investigate since upstream exhibits
the exact issue described here.

> 
>>> $
>>>
>>> Do you know what is the issue here?
>>
>> mine's: GNU ld version 2.32-31.fc31
>>
>> there's version info in commit:
>>    10e68b02c861 Makefile: support compressed debug info
>>
>>    Compress the debug information using zlib.  Requires GCC 5.0+ or Clang
>>    5.0+, binutils 2.26+, and zlib.
>>
>> cc-ing Nick Desaulniers, author of that patch.. any idea about the error above?
>>
>> thanks,
>> jirka
>>
> 
>