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

["Jan Beulich" <JBeulich at suse dot com>]

>>> On 09.10.13 at 17:45, "H.J. Lu" <hjl.tools@gmail.com> wrote:
> On Wed, Oct 9, 2013 at 12:30 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>> On 08.10.13 at 17:32, "H.J. Lu" <hjl.tools@gmail.com> wrote:
>>> On Tue, Oct 8, 2013 at 8:20 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>>>> On 08.10.13 at 17:15, "H.J. Lu" <hjl.tools@gmail.com> wrote:
>>>>> On Tue, Oct 8, 2013 at 7:41 AM, Jan Beulich <JBeulich@suse.com> 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 <jbeulich@suse.com>
>>>>>>
>>>>>>         * 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 && !i.tm.opcode_modifier.bndprefixok)
>>>>>>      as_bad (_("expecting valid branch instruction after `bnd'"));
>>>>>>
>>>>>> -  if (i.tm.cpu_flags.bitfield.cpumpx
>>>>>> -      && 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.
>>
>> This seems even more odd - why would x32 be excluded from
>> using MPX? Again - an example might help, as my understanding
>> so far was that the implicit zero extension of results of 32-bit
>> operations should guarantee the half width pointers to be quite
>> fine to use as full width values (i.e. in other memory operands
>> I don't see why you would want to use 32-bit addressing either,
>> except when the wraparound case matters, as might e.g. be
>> the case with EIP-relative addressing).
> 
> In 64-bit mode, bndldx and bndstx ignore the lower 3 bits of
> the address of a pointer, which is OK when pointers are
> 64-bit aligned. X32 runs in 64-bit mode. But pointers are
> 32-bit aligned.  That means 2 pointers may point to the same
> bound table entry.  That is why MPX won't work for x32 and
> assembler shouldn't allow it.

Here you're making assumptions that you can't control. For
example, there's nothing preventing anyone to create a
compiler/library pair that guarantees objects to be at least 8
bytes apart. And that's leaving aside that even in full 64-bit
mode there can be multiple objects within an 8-byte range.

As said before - the assembler should not enforce any kind of
policy.

Jan