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

Jan Beulich JBeulich@suse.com
Mon May 11 13:22:00 GMT 2015 
>>> On 11.05.15 at 14:40, <hjl.tools@gmail.com> wrote:
> 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

No, it's not - as you can see from the SEGV happing on address
00000527. If the displacement was 32 bits, then the faulting
address would have been 90900527.

> 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?

Not as far as I'm aware.

Jan
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