Re: [PATCH dwarves 0/4] BTF ELF writing changes

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Hi.

On Wed, 27 Jan 2021 at 14:06, Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> wrote:
>
> Em Mon, Jan 25, 2021 at 01:06:21PM +0000, Giuliano Procida escreveu:
> > Hi.
> >
> > This follows on from my change to improve the error handling around
> > llvm-objcopy in libbtf.c.
> >
> > Note on recipients: Please let me know if I should adjust To or CC.
> >
> > Note on style: I've generally placed declarations as allowed by C99,
> > closest to point of use. Let me know if you'd prefer otherwise.
> >
> > 1. Improve ELF error reporting
>
> applied
>
> > 2. Add .BTF section using libelf
> >
> > This shows the minimal amount of code needed to drive libelf. However,
> > it leaves layout up to libelf, which is almost certainly not wanted.
> >
> > As an unexpcted side-effect, vmlinux is larger than before. It seems
> > llvm-objcopy likes to trim down .strtab.
>
> We have to test this thoroughly, I'm adding support to gcc's -gdwarf-5
> DW_AT_data_bit_offset, I think I should get that done and release 1.20,
> if some bug is still left on that new code, we can just fallback to
> -gdwarf-4.
>
> Then get back to the last 2 patches in your series, ok?
>

That's fine.

I've spent a little time digging into what llvm-objcopy (11.0.0) is
doing. It turns out it will rewrite an ELF file even if you just do
llvm-objcopy --dump-section .strtab=/dev/null elf_file, and even if
the file is not writable.

Our AOSP kernels have a lot symbols filtered out of the symbol table
and perhaps this is what makes such a difference to the size of
.strtab after llvm-objcopy has done its thing. I will try on a vanilla
kernel.

Giuliano.

> - Arnaldo
>
> > 3. Manually lay out updated ELF sections
> >
> > This does full layout of new and updated ELF sections. If the update
> > ELF sections were not the last ones in the file by offset, then it can
> > leave gaps between sections.
> >
> > 4. Align .BTF section to 8 bytes
> >
> > This was my original aim.
> >
> > Regards.
> >
> > Giuliano Procida (4):
> >   btf_encoder: Improve ELF error reporting
> >   btf_encoder: Add .BTF section using libelf
> >   btf_encoder: Manually lay out updated ELF sections
> >   btf_encoder: Align .BTF section to 8 bytes
> >
> >  libbtf.c | 222 +++++++++++++++++++++++++++++++++++++++++++------------
> >  1 file changed, 175 insertions(+), 47 deletions(-)
> >
> > --
> > 2.30.0.280.ga3ce27912f-goog
> >
>
> --
>
> - Arnaldo



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