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Re: [PATCH 2/6] x86/MPX: fix address size handling
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Jan Beulich <JBeulich at suse dot com>
- Cc: kirill dot yukhin at intel dot com, Binutils <binutils at sourceware dot org>
- Date: Wed, 9 Oct 2013 08:45:15 -0700
- Subject: Re: [PATCH 2/6] x86/MPX: fix address size handling
- Authentication-results: sourceware.org; auth=none
- References: <5254349502000078000F9A3D at nat28 dot tlf dot novell dot com> <525435BC02000078000F9A51 at nat28 dot tlf dot novell dot com> <CAMe9rOrZuQRgcaUNgex=MDFjKYxsJimK1aCMqzEyM2XbZXCo4w at mail dot gmail dot com> <52543EE302000078000F9AF4 at nat28 dot tlf dot novell dot com> <CAMe9rOo82eK4oS8YAPKS0DDRktyiCUc16OqxDH4ujihKjmA7vA at mail dot gmail dot com> <5255221C02000078000F9DD2 at nat28 dot tlf dot novell dot com>
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" <firstname.lastname@example.org> 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" <email@example.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.
>>>>> 2013-10-08 Jan Beulich <firstname.lastname@example.org>
>>>>> * tc-i386.c (md_assemble): Alter address size checking for MPX
>>>>> --- 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.