From: "Jan Beulich" <JBeulich@suse.com>
To: "H.J. Lu" <hjl.tools@gmail.com>
Cc: <kirill.yukhin@intel.com>,"Binutils" <binutils@sourceware.org>
Subject: Re: [PATCH 2/6] x86/MPX: fix address size handling
Date: Thu, 10 Oct 2013 12:27:00 -0000 [thread overview]
Message-ID: <5256B94802000078000FA2E3@nat28.tlf.novell.com> (raw)
In-Reply-To: <CAMe9rOqy=b9y2xtjmQygNooQ-CeVpr2O2SNeP6LNCNg6+Ttq8w@mail.gmail.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
next prev parent reply other threads:[~2013-10-10 12:27 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-08 14:36 [PATCH 0/6] x86: various MPX fixes Jan Beulich
2013-10-08 14:41 ` [PATCH 2/6] x86/MPX: fix address size handling Jan Beulich
2013-10-08 15:15 ` H.J. Lu
2013-10-08 15:20 ` Jan Beulich
2013-10-08 15:32 ` H.J. Lu
2013-10-09 7:30 ` Jan Beulich
2013-10-09 15:45 ` H.J. Lu
2013-10-10 12:27 ` Jan Beulich [this message]
2013-10-10 15:18 ` H.J. Lu
2013-10-08 14:41 ` [PATCH 1/6] x86/MPX: testsuite adjustments Jan Beulich
2013-10-08 14:42 ` [PATCH 3/6] x86/MPX: suppress base/index swapping in Intel mode for bndmk, bndldx, and bndstx Jan Beulich
2013-10-08 15:16 ` H.J. Lu
2013-10-08 15:23 ` Jan Beulich
2013-10-08 15:34 ` H.J. Lu
2013-10-08 16:00 ` Jan Beulich
2013-10-08 16:19 ` H.J. Lu
2013-10-09 7:15 ` acceptance rules (was: Re: [PATCH 3/6] x86/MPX: suppress base/index swapping ...) Jan Beulich
2013-10-09 16:45 ` H.J. Lu
2013-10-08 14:43 ` [PATCH 5/6] x86/MPX: fix operand size handling Jan Beulich
2013-10-08 15:45 ` H.J. Lu
2013-10-09 7:36 ` Jan Beulich
2013-10-09 15:51 ` H.J. Lu
2013-10-10 13:14 ` Jan Beulich
2013-10-10 15:14 ` H.J. Lu
2013-10-12 15:58 ` H.J. Lu
2013-10-12 17:12 ` H.J. Lu
2013-10-08 14:43 ` [PATCH 4/6] x86/MPX: bndmk, bndldx, and bndstx only allow a memory operand Jan Beulich
2013-10-08 15:28 ` H.J. Lu
2013-10-09 7:24 ` Jan Beulich
2013-10-09 15:17 ` H.J. Lu
2013-10-08 14:44 ` [PATCH 6/6] x86/MPX: bndmk, bndldx, and bndstx don't allow RIP-relative addressing Jan Beulich
2013-10-08 16:13 ` H.J. Lu
2013-10-09 7:40 ` Jan Beulich
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