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

[Nick Clifton <nickc@redhat.com>]

Hi Peilin,

> objcopy's --only-keep-debug option has been broken for ELF files since
> commit 8c803a2dd7d3 ("elf_backend_section_flags and
> _bfd_elf_init_private_section_data"):
> 
>    1. binutils/objcopy.c:setup_section() marks non-debug sections as
>       SHT_NOBITS, then calls bfd_copy_private_section_data(), see
>       "mark_nobits";
>    2. If ISEC and OSEC share the same section flags,
>       bfd/elf.c:_bfd_elf_init_private_section_data() restores OSEC's
>       section type back to ISEC's section type, effectively undoing
>       "mark_nobits".
> 
> This makes objcopy's --only-keep-debug a no-op.  As an example, Linux's
> scripts/package/builddeb script depends on this option to generate
> debug-info-only kernel modules.
> 
> Handle this special case by checking OSEC's size in addition to its
> type (similar to bfd/elf.c:elf_fake_sections()).  A non-debug section
> marked as SHT_NOBITS by --only-keep-debug should have a non-zero size.

I have a feeling that this change is going to have wider repercussions
than simply fixing this problem.  It is going to affect those sections
created by elf_fake_sections() for example, and I bet that there will
be other scenarios that have not occurred to me.

Possibly a better fix would be to change objcopy instead so that it can
cope with the behaviour of bfd_copy_private_section_data().  For example,
please could you try out this patch and see if it fixes the problem ?


diff --git a/binutils/objcopy.c b/binutils/objcopy.c
index d16d8ee67e4..14b76d8a65a 100644
--- a/binutils/objcopy.c
+++ b/binutils/objcopy.c
@@ -4180,6 +4180,10 @@ setup_section (bfd *ibfd, sec_ptr isection, void *obfdarg)
        err = _("failed to copy private data");
        goto loser;
      }
+  /* Since isec.flags == osec.flags, bfd_copy_private_section_data()
+     will reset the section type.  */
+  if (make_nobits)
+    elf_section_type (osection) = SHT_NOBITS;

    /* All went well.  */
    return;


> This patch is manually tested only.  Which testsuite(s)
> should I use to test this patch? 

Please run the binutils testsuite.  ie:

   cd <build-dir>/binutils
   make check

Cheers
   Nick