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Date:   Wed, 19 Aug 2020 11:16:17 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Yonghong Song <yhs@...com>, Jiri Olsa <jolsa@...nel.org>
Cc:     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>,
        Fangrui Song <maskray@...gle.com>
Subject: Re: [PATCH bpf-next] tools/resolve_btfids: Fix sections with wrong alignment

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.

> > $
> >
> > 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
>


-- 
Thanks,
~Nick Desaulniers

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