[Bug gdb/29584] gdb: support zstd compressed .gnu_debugdata

["mark at klomp dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29584

--- Comment #7 from Mark Wielaard <mark at klomp dot org> ---
(In reply to Fangrui Song from comment #5)
> (In reply to Mark Wielaard from comment #4)
> > If we are changing the format, Can't we use standard ELF section compression?
> > That supports zstd these days. Then the .gnu_debugdata can be just a normal
> > ELF section, that is a mini-ELF image, which happens to be zstd compressed.
> 
> We can do that but it likely doesn't provide an overall advantage.
>
> The .gnu_debugdata content is an ELF file, not a regular section.
> We don't benefit from information like ch_type/ch_size/ch_addrline
> in the Chdr header.

I think it does provide an advantage because programs already need to handle
standard compressed Elf sections, so they don't need any special code for
decompressing this (assuming we'll drop the old compression scheme eventually).

> To embed an ELF file (with .symtab, .strtab, and optionally .debug_*), we
> need
> one `objcopy --add-section` command, then one `eu-elfcompress -n` command
> (with
> no binutils equivalent
> https://sourceware.org/bugzilla/show_bug.cgi?id=27452).
> Even eu-elfcompress -n doesn't provide the various zstd options that a
> distribution may toggle.

There is already support in eu-elfcompress, it will show up in the next release
(0.189, hopefully later this month):

  -n, --name=SECTION         SECTION name to (de)compress, SECTION is an
                             extended wildcard pattern (defaults to
                             '.?(z)debug*')
[...]
  -t, --type=TYPE            What type of compression to apply. TYPE can be
                             'none' (decompress), 'zlib' (ELF ZLIB compression,
                             the default, 'zlib-gabi' is an alias), 'zlib-gnu'
                             (.zdebug GNU style compression, 'gnu' is an alias)
                             or 'zstd' (ELF ZSTD compression)

> Also, this process involves two binary manipulation commands on
> the ELF file.
> 
> If we use `zstd a.debug; objcopy -S
> --add-section=.gnu_debugdata=a.debug.zstd a a.stripped`,
> there is just one binary manipulation command, and we can toggle zstd
> options easily.

It is slightly more that that. See debugedit find-debuginfo add_mini how the
section is created:
https://sourceware.org/git/?p=debugedit.git;a=blob;f=scripts/find-debuginfo.in;hb=HEAD#l332

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