[PATCH 3/9] x86: correct MPX insn w/o base or index encoding in 16-bit mode

H.J. Lu hjl.tools@gmail.com
Wed Mar 4 13:26:00 GMT 2020


On Wed, Mar 4, 2020 at 4:58 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 04.03.2020 12:54, H.J. Lu wrote:
> > On Wed, Mar 4, 2020 at 3:50 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 04.03.2020 12:45, H.J. Lu wrote:
> >>> On Wed, Mar 4, 2020 at 1:38 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>> --- a/gas/config/tc-i386.c
> >>>> +++ b/gas/config/tc-i386.c
> >>>> @@ -10297,6 +10297,21 @@ i386_addressing_mode (void)
> >>>>
> >>>>    if (i.prefix[ADDR_PREFIX])
> >>>>      addr_mode = flag_code == CODE_32BIT ? CODE_16BIT : CODE_32BIT;
> >>>> +  else if (flag_code == CODE_16BIT
> >>>> +          && current_templates->start->cpu_flags.bitfield.cpumpx
> >>>> +          /* Avoid replacing the "16-bit addressing not allowed" diagnostic
> >>>> +             from md_assemble() by "is not a valid base/index expression"
> >>>> +             when there is a base and/or index.  */
> >>>> +          && !i.types[this_operand].bitfield.baseindex)
> >>>> +    {
> >>>> +      /* MPX insn memory operands with neither base nor index must be forced
> >>>> +        to use 32-bit addressing in 16-bit mode.  */
> >>>> +      addr_mode = CODE_32BIT;
> >>>> +      i.prefix[ADDR_PREFIX] = ADDR_PREFIX_OPCODE;
> >>>> +      ++i.prefixes;
> >>>> +      gas_assert (!i.types[this_operand].bitfield.disp16);
> >>>> +      gas_assert (!i.types[this_operand].bitfield.disp32);
> >>>> +    }
> >>>>    else
> >>>>      {
> >>>
> >>> Since MPX isn't available in 16-bit mode, should they be disallowed?
> >>
> >> How is it not available? As per my understanding, one just needs
> >> to use 32-bit addressing.
> >
> > 0x67 prefix is special for MPX.  It can't be used as address prefix on MPX
> > instructions.
>
> It not only can, but is required to be in 16-bit mode. Let me quote
> BNDMK's SDM page:
>
> Protected Mode Exceptions
> #UD If the LOCK prefix is used.
> If ModRM.r/m encodes BND4-BND7 when Intel MPX is enabled.
> If 67H prefix is not used and CS.D=0.
> If 67H prefix is used and CS.D=1.
>
> Real-Address Mode Exceptions
> #UD If the LOCK prefix is used.
> If ModRM.r/m encodes BND4-BND7 when Intel MPX is enabled.
> If 16-bit addressing is used.
>
> Virtual-8086 Mode Exceptions
> #UD If the LOCK prefix is used.
> If ModRM.r/m encodes BND4-BND7 when Intel MPX is enabled.
> If 16-bit addressing is used.
>
> It is quite clear to me from this that (a) MPX is allowed
> in 16-bit mode (and even in all forms of it, other than
> e.g. VEX/EVEX-encoded insns) and (b) the 67 prefix acts
> as a normal address size override there. Its use simply is
> mandatory in 16-bit mode.
>
> >>> Given that MPX has been deprecated, I prefer an error here.
> >>
> >> The use of "here" is confusing - just for the broken case (no
> >> base/index), or for MPX insns in general? (Asking just in case
> >> my understanding expressed above is wrong.)
> >
> > flag_code == CODE_16BIT && current_templates->start->cpu_flags.bitfield.cpumpx
> >
> > should be an error.
>
> As per above, I see no reason for such behavior.
>

Patch is OK.

Thanks.

-- 
H.J.



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