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

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Date:	Fri, 26 Apr 2013 15:14:16 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Kees Cook <keescook@...omium.org>
CC:	linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Jarkko Sakkinen <jarkko.sakkinen@...el.com>,
	Matthew Garrett <mjg@...hat.com>,
	Matt Fleming <matt.fleming@...el.com>,
	Eric Northup <digitaleric@...gle.com>,
	Dan Rosenberg <drosenberg@...curity.com>,
	Julien Tinnes <jln@...gle.com>, Will Drewry <wad@...omium.org>
Subject: Re: [PATCH 2/6] x86: kaslr: move CPU flags out of cpucheck

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));

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