Comment 5 for bug 1567219

(In reply to comment #2)
> Technology note for anybody that looks at this in the future - the
> implementation of this feature appears to be that each .debug_xxx section is
> replaced by a .zdebug_xxx section that has been zlib compressed.
>
> Note that some binutils tools like objdump will attempt to hide this fact
> and show the original section, decompressing on the fly if you try and read
> it.

binutils/gdb contain small bfd/compress.c utility for handling .zdebug_* sections.