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[Bug ld/4694] New: ld corrupts a.out text sections

From: vincent dot riviere at freesbee dot fr
Subject: [Bug ld/4694] New: ld corrupts a.out text sections
Date: 24 Jun 2007 19:40:38 -0000

On a.out targets, when ld links an executable containing no symbols and no 
relocations, the last byte of the last section is overwritten with a zero byte.
However, it is rarely a problem, because most executables contains relocations, 
and the sections are padded with filler bytes.

This scenario shows how to reproduce this bug on a m68k-netbsd target.
I found that on this target, linking an executable containing only a few 
instructions will result in a text section padded to exactly 8160 bytes.
Thus, I generate a text section having exactly this size, to avoid padding.

$ cat nop.s
        .rept 4080
$ as nop.s -o nop.o
$ ld nop.o -o nop -s
$ objdump -d nop |tail
    3fec:       4e71            nop
    3fee:       4e71            nop
    3ff0:       4e71            nop
    3ff2:       4e71            nop
    3ff4:       4e71            nop
    3ff6:       4e71            nop
    3ff8:       4e71            nop
    3ffa:       4e71            nop
    3ffc:       4e71            nop
    3ffe:       4e00            047000

The last byte is 0 instead of 0x71

I found the cause of this problem at the end of the file bfd/aoutx.h
in the function NAME (aout, final_link)

  /* Write out the string table, unless there are no symbols.  */
  if (abfd->symcount > 0)
      if (bfd_seek (abfd, obj_str_filepos (abfd), SEEK_SET) != 0
          || ! emit_stringtab (abfd, aout_info.strtab))
        goto error_return;
  else if (obj_textsec (abfd)->reloc_count == 0
           && obj_datasec (abfd)->reloc_count == 0)
      bfd_byte b;
      file_ptr pos;

      b = 0;
      pos = obj_datasec (abfd)->filepos + exec_hdr (abfd)->a_data - 1;
      if (bfd_seek (abfd, pos, SEEK_SET) != 0
          || bfd_bwrite (&b, (bfd_size_type) 1, abfd) != 1)
        goto error_return;

The else case is the cause of this problem.
Removing it fixes the bug.
But that code may be useful for something ?

           Summary: ld corrupts a.out text sections
           Product: binutils
           Version: 2.18 (HEAD)
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ld
        AssignedTo: unassigned at sources dot redhat dot com
        ReportedBy: vincent dot riviere at freesbee dot fr
                CC: bug-binutils at gnu dot org
GCC target triplet: m68k-netbsd


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