Re: [PATCH] i386: Check invalid (%dx) usage

[Jan Beulich <jbeulich@suse.com>]

On 09.11.2022 21:24, H.J. Lu wrote:
> On Tue, Nov 8, 2022 at 11:21 PM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 08.11.2022 22:06, H.J. Lu wrote:
>>> On Mon, Nov 7, 2022 at 11:34 PM Jan Beulich <jbeulich@suse.com> wrote:
>>>> On 07.11.2022 20:58, H.J. Lu wrote:
>>>>> On Mon, Nov 7, 2022 at 3:44 AM Jan Beulich <jbeulich@suse.com> wrote:
>>>>>> x86: restrict use of (%dx)
>>>>>>
>>>>>> PR gas/29751
>>>>>> The AT&T mode special case operand (%dx) is valid to use only with
>>>>>> instructions nominally expecting %dx to specify an I/O port address.
>>>>>> Prefix the respective checking with an opcode check. Keep that as
>>>>>> simple as possible by recognizing that opcodes 0x64 and 0x66 (which
>>>>>
>>>>> Since current_templates doesn't point to the matched instruction,
>>>>> checking current_templates looks like abuse.  I don't think error
>>>>> messages should be a concern here.
>>>>
>>>> We use current_templates in similar ways in quite a number of places,
>>>> when match_templates() hasn't run yet.
>>>
>>> Since the first template isn't the selected one, your check allows
>>> the invalid opcodes.
>>
>> I guess I don't understand, but I guess I'll also give up. Which
> 
> Your proposed change does
> 
> current_templates->start->base_opcode | 0x8a) == 0xee
> 
> to allow opcode 0xe4 and (%dx) is allowed for non-I/O opcodes.

0xe4 is very much an I/O opcode, merely one not allowing for (%dx).
This solely is to ...

>> template the check is done against doesn't really matter here, as
>> long as it's one with the correct mnemonic. We could of course
>> also re-order templates to have ones allowing for %dx first, but
>> I view any such ordering dependencies as fragile.
>>
> 
> That is true.

... avoid introducing yet another ordering dependency.

Jan