binfmt_elf: Avoid total_mapping_size for ET_EXEC

Partially revert commit 5f501d55

 ("binfmt_elf: reintroduce using
MAP_FIXED_NOREPLACE"), which applied the ET_DYN "total_mapping_size"
logic also to ET_EXEC.

At least ia64 has ET_EXEC PT_LOAD segments that are not virtual-address
contiguous (but _are_ file-offset contiguous). This would result in a
giant mapping attempting to cover the entire span, including the virtual
address range hole, and well beyond the size of the ELF file itself,
causing the kernel to refuse to load it. For example:

$ readelf -lW /usr/bin/gcc
...
Program Headers:
  Type Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   ...
...
  LOAD 0x000000 0x4000000000000000 0x4000000000000000 0x00b5a0 0x00b5a0 ...
  LOAD 0x00b5a0 0x600000000000b5a0 0x600000000000b5a0 0x0005ac 0x000710 ...
...
       ^^^^^^^^ ^^^^^^^^^^^^^^^^^^                    ^^^^^^^^ ^^^^^^^^

File offset range     : 0x000000-0x00bb4c
			0x00bb4c bytes

Virtual address range : 0x4000000000000000-0x600000000000bcb0
			0x200000000000bcb0 bytes

Remove the total_mapping_size logic for ET_EXEC, which reduces the
ET_EXEC MAP_FIXED_NOREPLACE coverage to only the first PT_LOAD (better
than nothing), and retains it for ET_DYN.

Ironically, this is the reverse of the problem that originally caused
problems with MAP_FIXED_NOREPLACE: overlapping PT_LOAD segments. Future
work could restore full coverage if load_elf_binary() were to perform
mappings in a separate phase from the loading (where it could resolve
both overlaps and holes).

Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Reported-by: matoro <matoro_bugzilla_kernel@matoro.tk>
Fixes: 5f501d55 ("binfmt_elf: reintroduce using MAP_FIXED_NOREPLACE")
Link: https://lore.kernel.org/r/a3edd529-c42d-3b09-135c-7e98a15b150f@leemhuis.info


Tested-by: matoro <matoro_mailinglist_kernel@matoro.tk>
Link: https://lore.kernel.org/lkml/ce8af9c13bcea9230c7689f3c1e0e2cd@matoro.tk


Tested-By: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/lkml/49182d0d-708b-4029-da5f-bc18603440a6@physik.fu-berlin.de


Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>