On February 23, 2017 6:09:29 AM PST, Peter Zijlstra <pet...@infradead.org>
wrote:
>On Thu, Feb 23, 2017 at 02:28:13PM +0100, Peter Zijlstra wrote:
>> + * Since various instruction decoders disagree on the length of UD1,
>> + * we cannot use it either. So use UD0 for WARN.
>> + *
>> + * (binutils knows about "ud1" but {en,de}codes it as 2 bytes,
>whereas
>> + * our kernel decoder thinks it takes a ModRM byte, which seems
>consistent
>> + * with various things like the Intel SDM instruction encoding
>rules)
>> + */
>> +
>> +#define ASM_UD0 ".byte 0x0f, 0xff"
>> +#define ASM_UD1 ".byte 0x0f, 0xb9" /* + ModRM */
>> +#define ASM_UD2 ".byte 0x0f, 0x0b"
>
>http://repo.or.cz/nasm.git/blob/HEAD:/x86/insns.dat
>
>has:
>
>1378 UD0 void [ 0f ff]
> 186,UNDOC
>1379 UD1 void [ 0f b9]
> 186,UNDOC
>1380 UD2B void [ 0f b9]
> 186,UNDOC,ND
>1381 UD2 void [ 0f 0b]
> 186
>1382 UD2A void [ 0f 0b]
> 186,ND
>
>which seems to use the 2 byte version of UD1.
>
>hpa, any input?
Well, it only matters if the instruction extends past a segment boundary or
page. However, the CPU instruction decoder will consume a modrm for UD1, and
so using just the two opcode bytes may cause a #PF or #GP when a #UD was
intended.
This was documented very recently and Nasm hasn't caught up yet (hence the
UNDOC flag.)
--
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