Fix PR gdb/20948: --write option to GDB causes segmentation fault

When opening a BFD for update, as gdb --write does, modifications to
anything but the contents of sections is restricted.

Do not try to write back any ELF headers in this case.

bfd/ChangeLog
2018-09-24  Jozef Lawrynowicz  <jozef.l@mittosystems.com>

	PR gdb/20948
	* elf.c (_bfd_elf_write_object_contents): Return from function
	early if abfd->direction == both_direction.

gdb/testsuite/ChangeLog
2018-09-24  Jozef Lawrynowicz  <jozef.l@mittosystems.com>

	PR gdb/20948
	* gdb.base/write_mem.exp: New test.
	* gdb.base/write_mem.c: Likewise.

1 files changed, 12 insertions, 0 deletions

diff --git a/bfd/elf.c b/bfd/elf.c
index 02d605c..5320ae2 100644
--- a/bfd/elf.c
+++ b/bfd/elf.c
@@ -6391,6 +6391,18 @@ _bfd_elf_write_object_contents (bfd *abfd)
   if (! abfd->output_has_begun
       && ! _bfd_elf_compute_section_file_positions (abfd, NULL))
     return FALSE;
+  /* Do not rewrite ELF data when the BFD has been opened for update.
+     abfd->output_has_begun was set to TRUE on opening, so creation of new
+     sections, and modification of existing section sizes was restricted.
+     This means the ELF header, program headers and section headers can't have
+     changed.
+     If the contents of any sections has been modified, then those changes have
+     already been written to the BFD.  */
+  else if (abfd->direction == both_direction)
+    {
+      BFD_ASSERT (abfd->output_has_begun);
+      return TRUE;
+    }
 
   i_shdrp = elf_elfsections (abfd);