Re: [RFC PATCH] bfd: Handle objcopy --only-keep-debug in _bfd_elf_init_private_section_data()

[Peilin Ye <yepeilin.cs@gmail.com>]

On Sat, Jan 29, 2022 at 01:15:40AM -0800, Peilin Ye wrote:
> It seems that --only-keep-debug is broken for Linux kernel .ko modules,
> but it works just fine on tmpdir/striprog (built from testprog.c) used
> by this testsuite...

I figured this out; it's because --only-keep-debug is only broken for
ELF relocatables.

For dynamically linked and statically linked ELF executables, their
non-debug output sections' type is changed 3 times:

  1. In setup_section() "make_nobits", set to SHT_NOBITS;
  2. Then in _bfd_elf_init_private_section_data(), restored back to input
     section's type, as we already discussed;
  3. Finally in assign_file_positions_for_load_sections(), they are set
     once again to SHT_NOBITS:

	for (i = 0; i < m->count; i++)
		if ((m->sections[i]->flags & (SEC_LOAD | SEC_HAS_CONTENTS)) == 0)
	      	/* If we aren't making room for this section, then
		   it must be SHT_NOBITS regardless of what we've
		   set via struct bfd_elf_special_section.  */
			elf_section_type (m->sections[i]) = SHT_NOBITS;
							    ^^^^^^^^^^

For a complete call graph:

copy_object()
  [...]
    setup_section()				/* 1. set to SHT_NOBITS */
      bfd_copy_private_section_data()
        _bfd_elf_copy_private_section_data()
          _bfd_elf_init_private_section_data()	/* 2. restore to ISEC's type */
  [...]
    copy_section()
      bfd_set_section_contents()
        _bfd_elf_set_section_contents()
          _bfd_elf_compute_section_file_positions()
            assign_file_positions_except_relocs()
              if (EXEC_P or DYNAMIC)		/* ELF executables? e.g. tmpdir/striprog */
                assign_file_positions_for_load_sections()	/* 3. set again to SHT_NOBITS */
              else				/* ELF relocatables? e.g. Linux modules */

Frankly, I don't semantically understand what this code is trying to do
here yet.  Since an ELF relocatable is neither EXEC_P nor DYNAMIC,
assign_file_positions_for_load_sections() is not called, therefore its
non-debug OSECs remain the old (ISEC's) type.

In conclusion, I think currently --only-keep-debug is only broken for
ELF relocatables, although I'm not 100% sure.

I will add this information in v3's commit message, and come up with a
better testsuite (probably test this on a .o file or something).

Thanks,
Peilin Ye
<peilin.ye@bytedance.com>