Re: [PATCH 2/6] x86: kaslr: move CPU flags out of cpucheck

[Kees Cook]

On Fri, Apr 26, 2013 at 3:14 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> On 04/26/2013 02:47 PM, H. Peter Anvin wrote:
>> On 04/26/2013 12:03 PM, Kees Cook wrote:
>>> +
>>> +static inline void cpuid(u32 id, u32 *a, u32 *b, u32 *c, u32 *d)
>>> +{
>>> +    /* Handle x86_32 PIC using ebx. */
>>> +    asm volatile("movl %%ebx, %%edi \n\t"
>>> +                 "cpuid             \n\t"
>>> +                 "xchgl %%edi, %%ebx\n\t"
>>> +                : "=a" (*a),
>>> +                  "=D" (*b),
>>> +                  "=c" (*c),
>>> +                  "=d" (*d)
>>> +                : "a" (id)
>>> +    );
>>> +}
>>
>> Please don't constrain registers unnecessarily.
>>
>> You can use "=r" there and let gcc assign whatever free register it pleases.
>>
>> You can also limit that to only:
>>
>> #if defined(__i386__) && defined(__PIC__)
>>
>
> How is this for a "beauty":
>
>
> #if defined(__i386__) && defined (__PIC__)
> # define EBX_REG "=r"
> #else
> # define EBX_REG "=b"
> #endif
>
>   asm volatile(".ifnc %%ebx,%3 ; movl %%ebx,%3 ; .endif ; "
>                "cpuid ; "
>                ".ifnc %%ebx,%3 ; xchgl %%ebx,%3 ; .endif"
>                : "=a" (*a), "=c" (*c), "=d" (*d),
>                  EBX_REG (*b)
>                : "a" (leaf), "c" (subleaf));
>

Oh, very nice on the ifnc and register define! Is the leaf/subleaf
stuff needed there? That piece doesn't make sense to me.

-Kees

--
Kees Cook
Chrome OS Security
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