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

["i at maskray dot me" <sourceware-bugzilla@sourceware.org>]

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

--- Comment #8 from Fangrui Song <i at maskray dot me> ---
(In reply to Mark Wielaard from comment #7)
> > 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)

I have seen
https://sourceware.org/git/?p=elfutils.git;a=commit;h=a5b07cdf9c491fb7a4a16598c482c68b718f59b9
(support ZSTD compression algorithm). I am thinking about Operation modifiers
like the compression level, window size, dictionary, etc. A Linux distribution
may have many packages which are not used for debugging, spending additional
time on the producer side is probably a win. Some use cases may prefer a fast
producer.

> > 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

Notes. You are right:) I noticed this when writing
https://maskray.me/blog/2022-10-30-distribution-of-debug-information#minidebuginfo
but I don't have experience with the package, so did not recall it off the top
my head...

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