[PATCH] Fix overlapping sections in ELF files when a memory segment has no contents

[Jozef Lawrynowicz]

On targets which have small page size, or no concept of page size, memory
segments in the output file may require little or no alignment.
An optimization in bfd/elf.c:assign_file_positions_for_load_sections, to save
some space in the output file when a segment has no contents, can cause sections
to overlap on these targets.
This was exposed when running the "simple objcopy of executable" test for
msp430-elf with the -mlarge target flag, which failed due to different
section and segment offsets between the original ELF file and the ELF file
output from objcopy.

Here are the relevant parts of the "readelf -a" output of the linked executable
from the above test:

======
Section Headers:
   [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
...
   [ 2] .rodata           PROGBITS        00002000 000134 000090 00   A  0   0  2
...
   [ 7] .bss              NOBITS          00000646 000136 00002c 00  WA  0   0  2
...
Program Headers:
   Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
   LOAD           0x000000 0x00000510 0x00000510 0x00134 0x00162 RW  0x4
   LOAD           0x000134 0x00002000 0x00002000 0x00090 0x00090 R   0x4
...
  Section to Segment mapping:
   Segment Sections...
    00     .bss
    01     .rodata
======
The first segment contains only .bss. .data has its VMA in this segment, but its
LMA is elsewhere, which is partly why this segment has size, despite its
sections having no contents. This segment also contains some ELF headers which
is where the rest of the file size comes from.

As can be seen in the above readelf output, the offset of the .bss section
(0x136), is not within the memory segment that's supposed to contain it. This
segment ends at 0x134. When objcopy copies this ELF file, the new ELF file has
both the .rodata section offset, and the file offset for the segment containing
.rodata, set to 0x138. File size for the segment containing .bss is also
incremented to 0x136.

The attached patch fixes the cause of the overlapping sections, by incrementing
the segment filesize, and not decrementing output file offset, if it appears the
next segment would start before the end of the current segment.

Testing shows no regressions from the binutils, gas and ld testsuites on
x86_64-pc-linux-gnu and msp430-elf. This patch fixes the "simple objcopy of
executable" binutils test for msp430-elf with the -mlarge target flag.

If the patch is acceptable, I would appreciate if someone would commit it for
me as I don't have write access.

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