[Bug binutils/30496] New: Add support for PE IMAGE_SCN_MEM_16BIT section characteristic

[pali at kernel dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=30496

            Bug ID: 30496
           Summary: Add support for PE IMAGE_SCN_MEM_16BIT section
                    characteristic
           Product: binutils
           Version: 2.39
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: binutils
          Assignee: unassigned at sourceware dot org
          Reporter: pali at kernel dot org
  Target Milestone: ---

PE for machine type of IMAGE_FILE_MACHINE_I386 (0x014c) can contain
IMAGE_SCN_MEM_16BIT (0x00020000) in section characteristics. When
IMAGE_SCN_MEM_16BIT is specified then section contain 16-bit x86 code.

Microsoft DUMPBIN.EXE recognize this characteristic and show its name as
"Purgeable or 16-Bit". And sections marked with this characteristic disassemble
as 16-bit instead of default 32-bit.

Note that PE IMAGE_SCN_MEM_PURGEABLE characteristic has same value as
IMAGE_SCN_MEM_16BIT = 0x00020000, but my experiments reveal that for MS tools 
IMAGE_SCN_MEM_16BIT seems to be I386 specific and IMAGE_SCN_MEM_PURGEABLE seems
to be M68K specific.

Here is very simple object file test.obj with two functions _test32 and _test16
which are in different sections with and without IMAGE_SCN_MEM_16BIT
characteristics.

$ xxd test.obj
00000000: 4c01 0200 a94f 7364 6a00 0000 0a00 0000  L....Osdj.......
00000010: 0000 0000 2e74 6578 7400 0000 0000 0000  .....text.......
00000020: 0000 0000 0300 0000 6400 0000 0000 0000  ........d.......
00000030: 0000 0000 0000 0000 2010 1060 2e74 6578  ........ ..`.tex
00000040: 7400 0000 0300 0000 0000 0000 0300 0000  t...............
00000050: 6700 0000 0000 0000 0000 0000 0000 0000  g...............
00000060: 2010 1260 33c0 c333 c0c3 2e66 696c 6500   ..`3..3...file.
00000070: 0000 0000 0000 feff 0000 6701 7465 7374  ..........g.test
00000080: 2e63 0000 0000 0000 0000 0000 0000 5f74  .c............_t
00000090: 6573 7433 3200 0000 0000 0000 2000 0200  est32....... ...
000000a0: 2e74 6578 7400 0000 0000 0000 0100 0000  .text...........
000000b0: 0301 0300 0000 0000 0000 0000 0000 0000  ................
000000c0: 0100 0000 5f74 6573 7433 3200 0000 0000  ...._test32.....
000000d0: 0100 2000 0200 5f74 6573 7431 3600 0000  .. ..._test16...
000000e0: 0000 0000 2000 0200 2e74 6578 7400 0000  .... ....text...
000000f0: 0000 0000 0200 0000 0301 0300 0000 0000  ................
00000100: 0000 0000 0000 0000 0100 0000 5f74 6573  ............_tes
00000110: 7431 3600 0000 0000 0200 2000 0200 0400  t16....... .....
00000120: 0000                                     ..



Microsoft DUMPBIN.EXE correcly recognize IMAGE_SCN_MEM_16BIT in _test16 and
disassemble "0x33 0xC0" as "xor ax,ax":

$ DUMPBIN.EXE /DISASM /HEADERS test.obj
...
SECTION HEADER #1
   .text name
       0 physical address
       0 virtual address
       3 size of raw data
      64 file pointer to raw data
       0 file pointer to relocation table
       0 file pointer to line numbers
       0 number of relocations
       0 number of line numbers
60101020 flags
         Code
         Communal; sym= _test32
         1 byte align
         Execute Read

_test32:
  00000000: 33 C0              xor         eax,eax
  00000002: C3                 ret
DUMPBIN : warning LNK4078: multiple ".text" sections found with different
attributes (60121020)

SECTION HEADER #2
   .text name
       3 physical address
       0 virtual address
       3 size of raw data
      67 file pointer to raw data
       0 file pointer to relocation table
       0 file pointer to line numbers
       0 number of relocations
       0 number of line numbers
60121020 flags
         Code
         Communal; sym= _test16
         Purgeable or 16-Bit
         1 byte align
         Execute Read

_test16:
  0002:0000: 33 C0              xor         ax,ax
  0002:0002: C3                 ret

     Summary

           3 .text
           3 .text



But GNU objdump does not recognize it and disassemble "0x33 0xC0" in 32-bit
mode as "xor %eax,%eax":

$ i686-w64-mingw32-objdump -d test.mod.obj

test.obj:     file format pe-i386


Disassembly of section .text:

00000000 <_test32>:
   0:   33 c0                   xor    %eax,%eax
   2:   c3                      ret

Disassembly of section .text:

00000000 <_test16>:
   0:   33 c0                   xor    %eax,%eax
   2:   c3                      ret



Also assembling simple code with .code16 directive with GNU AS does not set
IMAGE_SCN_MEM_16BIT section characteristic.

Test case:

$ cat test-16bit.S
.section .text32, "rx0"
_test32:
  xor %eax, %eax
  ret

.section .text16, "rx0"
.code16
_test16:
  xor %ax, %ax
  ret

$ i686-w64-mingw32-as -o test-16bit.o test-16bit.S
$ i686-w64-mingw32-objdump -d test-16bit.o

test-16bit.o:     file format pe-i386


Disassembly of section .text32:

00000000 <_test32>:
   0:   31 c0                   xor    %eax,%eax
   2:   c3                      ret

Disassembly of section .text16:

00000000 <_test16>:
   0:   31 c0                   xor    %eax,%eax
   2:   c3                      ret


$ DUMPBIN.EXE /DISASM /HEADERS test-16bit.o
...
SECTION HEADER #5
 .text16 name
       0 physical address
       0 virtual address
       3 size of raw data
      DF file pointer to raw data
       0 file pointer to relocation table
       0 file pointer to line numbers
       0 number of relocations
       0 number of line numbers
60100060 flags
         Code
         Initialized Data
         1 byte align
         Execute Read

_test16:
  00000000: 31 C0              xor         eax,eax
  00000002: C3                 ret



It would be really nice if GNU objdump recognize IMAGE_SCN_MEM_16BIT for
IMAGE_FILE_MACHINE_I386 and disassemble code in 16-bit. I know that objdump can
be instructed to disassemble 16-bit code by argument -Maddr16,data16 but
autodetection is better as it can detect "mixed" object files (with contain
both 32-bit and 16-bit x86 code).

Also it would be nice if GNU AS .code16 directive can set IMAGE_SCN_MEM_16BIT.

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