Date:   Wed, 19 Aug 2020 19:27:54 -0700
From:   Fāng-ruì Sòng <maskray@...gle.com>
To:     Yonghong Song <yhs@...com>, Jiri Olsa <jolsa@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Mark Wielaard <mjw@...hat.com>,
        Nick Clifton <nickc@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH bpf-next] tools/resolve_btfids: Fix sections with wrong alignment

> > >    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

I think this is resolve_btfids's bug. GNU ld and LLD are innocent.
These .debug_* sections work fine if their sh_addralign is 1.
When the section flag SHF_COMPRESSED is set, the meaningful alignment
is Elf64_Chdr::ch_addralign, after the header is uncompressed.

On Wed, Aug 19, 2020 at 2:30 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 8/19/20 11:16 AM, Nick Desaulniers wrote:
> > On Wed, Aug 19, 2020 at 10:36 AM Jiri Olsa <jolsa@...hat.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
> >>
> >
> >



-- 
宋方睿

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