[Bug gdb/20948] --write option to GDB causes segmentation fault

["jozef.l at mittosystems dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=20948

--- Comment #15 from Jozef Lawrynowicz <jozef.l at mittosystems dot com> ---
Created attachment 11237
  --> https://sourceware.org/bugzilla/attachment.cgi?id=11237&action=edit
gdb-write.patch

Further investigation into what can actually be modified when a BFD is
opened for update, revealed that the fix for this is quite simple.

When a BFD is opened for update (i.e. with "r+" flags, or indirectly via
"gdb --write"), abfd->output_has_begun is set to TRUE, which heavily restricts
what you can do to the BFD.

- New sections cannot be created
- The size of sections cannot be modified

This means the ELF header, program headers and sections headers will not
change, so they do not need to be written in _bfd_elf_write_object_contents,
as the original versions already exist in the BFD.

Even though the patch modifies write_object_contents to exit before section
contents are written to the output BFD, any changes to contents made using
bfd_set_section_contents or by setting memory in gdb --write have already been
written, so are present in the executable after closing the BFD.

The patch is attached as gdb-write.patch.

I tested it on current binutils-gdb master for x86_64-pc-linux-gnu and
msp430-elf in a couple of ways.
- Build the attached "modify-bfd.c", and run it with an ELF file as an
  argument, check the string FOOBAR appears in the .comment section.
- Run "gdb --write -batch -ex "set {int}main = 0x1234" FILE", then check the
  contents of memory at main are 0x1234
- Run "gdb --write -ex q FILE", verify the before and after versions of FILE
  are identical.

I used the set of resulting executable and object files left over from the
linker testsuite as input.

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