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

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, May 11, 2015 at 4:29 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>> 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.
>

The displacement is still 32-bit, but stack adjustment is
different.  Display it as callq isn't accurate for AMD and
callw is not correct for both AMD and Intel.  I will leave it
as is since I don't think we should add another instruction
for this.

BTW, thanks for letting me know.  I was using 0x66 as
nop prefix for call/jmp PIC relaxation.  I changed it to
0x48 now.  Will 0x48 a problem on AMD?


H.J.


-- 
H.J.