[committed, PATCH] PR binutis/18386: callw with 0x66 prefix incorrectly disassembled in 64-bit mode

[Jan Beulich]

>>> On 11.05.15 at 13:04, <hjl.tools@gmail.com> wrote:
> On Mon, May 11, 2015 at 12:28 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>> On 09.05.15 at 15:52, <hjl.tools@gmail.com> wrote:
>>> The operand size prefix (0x66) is ignored for 32-bit PC-relative call,
>>> jmp and jcc in 64-bit mode.
>>
>> Now that's again a change that should have been discussed
>> before committing: While it is correct for the Intel implementation
>> of x86-64, it isn't for AMD's original (and I'd tend to say that the
>> latter is more correct, since afaik there's nowhere that exceptions
>> are being made for the meaning of the operand size prefix).
> 
> It is also correct for AMD.

Very certainly not, based on practical observation: This code

x86_64:
	mov	rcx, rsp
	.byte 0x66, 0xe8, 0x00, 0x00, 0x90, 0x90
	xchg	rcx, rsp
	ret

yields

Reading symbols from /home/jbeulich/x86-64...done.
(gdb) break x86_64
Breakpoint 1 at 0x400520
(gdb) r
Starting program: /home/jbeulich/x86-64
Failed to read a valid object file image from memory.

Breakpoint 1, 0x0000000000400520 in x86_64 ()
(gdb) x/5i $pc
=> 0x400520 <x86_64>:   mov    rcx,rsp
   0x400523 <x86_64+3>: call   0x527
   0x400527 <x86_64+7>: nop
   0x400528 <x86_64+8>: nop
   0x400529 <x86_64+9>: xchg   rcx,rsp
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x0000000000000527 in ?? ()

with RSP also decremented by just 2.

Jan