Re: [PATCH dwarves v2 2/4] btf_encoder: Manually lay out updated ELF sections

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Feb 4, 2021 at 10:34 AM Giuliano Procida <gprocida@google.com> wrote:
>
> Hi.
>
> On Thu, 4 Feb 2021 at 04:13, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> >
> > On Mon, Feb 1, 2021 at 9:26 AM Giuliano Procida <gprocida@google.com> wrote:
> > >
> > > pahole -J needs to do the following to an ELF file:
> > >
> > > * add or update the ".BTF" section
> > > * maybe update the section name string table
> > > * update the Section Header Table (SHT)
> > >
> > > libelf either takes full control of layout or requires the user to
> > > specify offset, size and alignment of all new and updated sections and
> > > headers.
> > >
> > > To avoid libelf moving program segments in particular, we position the
> >
> > It's not clear to me what's wrong with libelf handling all the layout.
> > Even if libelf will move program segments around, what's the harm?
> > Does it break anything if we just let libelf do this?
> >
>
> It doesn't hurt the userspace case I care about. I've no idea what it
> means in terms of vmlinux.
>
> However, I wrote that text before I discovered that pahole -J isn't
> actually used to modify kernel images.
>
> One thing I haven't tried is to try to make .BTF loadable but leave
> placement to libelf.

I'd concentrate on getting rid of llvm-objcopy dependency first.
Making .BTF loadable is even riskier change and there is no use case
that relies on that today, so definitely worth to split that out.


>
> > > ".BTF" and section name string table (typically named ".shstrtab")
> > > sections after all others. The SHT always lives at the end of the file.
> > >
> > > Note that the last section in an ELF file is normally the section name
> > > string table and any ".BTF" section will normally be second last.
> > > However, if these sections appear earlier, then we'll waste some space
> > > in the ELF file when we rewrite them.
> > >
> > > Signed-off-by: Giuliano Procida <gprocida@google.com>
> > > ---
> > >  libbtf.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
> > >  1 file changed, 62 insertions(+), 2 deletions(-)
> > >
> >
> > [...]