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Re: [PATCH 2/6] x86/MPX: fix address size handling

On Tue, Oct 8, 2013 at 8:20 AM, Jan Beulich <> wrote:
>>>> On 08.10.13 at 17:15, "H.J. Lu" <> wrote:
>> On Tue, Oct 8, 2013 at 7:41 AM, Jan Beulich <> wrote:
>>> While address overrides are ignored in 64-bit mode (and hence shouldn't
>>> result in an error), trying to use 16-bit addressing is documented to
>>> result in #UD, and hence the assembler should reject the attempt.
>>> gas/
>>> 2013-10-08  Jan Beulich <>
>>>         * tc-i386.c (md_assemble): Alter address size checking for MPX
>>>         instructions.
>>> --- 2013-10-07/gas/config/tc-i386.c
>>> +++ 2013-10-07/gas/config/tc-i386.c
>>> @@ -3549,10 +3549,15 @@ md_assemble (char *line)
>>>    if (i.bnd_prefix && !
>>>      as_bad (_("expecting valid branch instruction after `bnd'"));
>>> -  if (
>>> -      && flag_code == CODE_64BIT
>>> -      && i.prefix[ADDR_PREFIX])
>>> -    as_bad (_("32-bit address isn't allowed in 64-bit MPX instructions."));
>> It is done on purpose.  When 32-bit address prefix in 64-bit is ignored,
>> MPX doesn't work correctly for x32.
> I don't understand: It _is_ being ignored by the hardware as per
> the documentation. So x32 need to get along with that. Maybe
> an example would help, so I could understand why you think
> this _needs_ to be an error...

X32 won't work with MPX since hardware assumes pointer
size is always 64 bit in 64-bit mode with or without address
size prefix.  MPX depends on correct pointer size to work.
I don't want people to use MPX in x32 by accident.


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