Re: [Qemu-devel] [kvm-unit-tests PATCH v5 10/11] new: add isaac prng library from CCAN

Andrew Jones
2015-07-31T19:22:07Z

On Fri, Jul 31, 2015 at 04:54:00PM +0100, Alex Bennée wrote:
> It's often useful to introduce some sort of random variation when
> testing several racing CPU conditions. Instead of each test implementing
> some half-arsed PRNG bring in a a decent one which has good statistical
> randomness. Obviously it is deterministic for a given seed value which
> is likely the behaviour you want.
>
> I've pulled in the ISAAC library from CCAN:
>
> http://ccodearchive.net/info/isaac.html
>
> I shaved off the float related stuff which is less useful for unit
> testing and re-indented to fit the style. The original license was
> CC0 (Public Domain) which is compatible with the LGPL v2 of
> kvm-unit-tests.
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> CC: Timothy B. Terriberry <tterribe@xiph.org>
> ---
> config/config-arm-common.mak | 1 +
> lib/prng.c | 162 +++++++++++++++++++++++++++++++++++++++++++
> lib/prng.h | 82 ++++++++++++++++++++++
> 3 files changed, 245 insertions(+)
> create mode 100644 lib/prng.c
> create mode 100644 lib/prng.h
>
> diff --git a/config/config-arm-common.mak b/config/config-arm-common.mak
> index 9c7607b..67a9dda 100644
> --- a/config/config-arm-common.mak
> +++ b/config/config-arm-common.mak
> @@ -34,6 +34,7 @@ cflatobjs += lib/devicetree.o
> cflatobjs += lib/virtio.o
> cflatobjs += lib/virtio-mmio.o
> cflatobjs += lib/chr-testdev.o
> +cflatobjs += lib/prng.o
> cflatobjs += lib/arm/io.o
> cflatobjs += lib/arm/setup.o
> cflatobjs += lib/arm/mmu.o
> diff --git a/lib/prng.c b/lib/prng.c
> new file mode 100644
> index 0000000..ebd6df7
> --- /dev/null
> +++ b/lib/prng.c
> @@ -0,0 +1,162 @@
> +/*
> + * Pseudo Random Number Generator
> + *
> + * Lifted from ccan modules ilog/isaac under CC0
> + * - http://ccodearchive.net/info/isaac.html
> + * - http://ccodearchive.net/info/ilog.html
> + *
> + * And lightly hacked to compile under the KVM unit test environment.
> + * This provides a handy RNG for torture tests that want to vary
> + * delays and the like.
> + *
> + */
> +
> +/*Written by Timothy B. Terriberry (tterribe@xiph.org) 1999-2009.
> + CC0 (Public domain) - see LICENSE file for details
> + Based on the public domain implementation by Robert J. Jenkins Jr.*/
> +
> +#include "libcflat.h"
> +
> +#include <string.h>
> +#include "prng.h"
> +
> +#define ISAAC_MASK (0xFFFFFFFFU)
> +
> +/* Extract ISAAC_SZ_LOG bits (starting at bit 2). */
> +static inline uint32_t lower_bits(uint32_t x)
> +{
> + return (x & ((ISAAC_SZ-1) << 2)) >> 2;
> +}
> +
> +/* Extract next ISAAC_SZ_LOG bits (starting at bit ISAAC_SZ_LOG+2). */
> +static inline uint32_t upper_bits(uint32_t y)
> +{
> + return (y >> (ISAAC_SZ_LOG+2)) & (ISAAC_SZ-1);
> +}
> +
> +static void isaac_update(isaac_ctx *_ctx){
> + uint32_t *m;
> + uint32_t *r;
> + uint32_t a;
> + uint32_t b;
> + uint32_t x;
> + uint32_t y;
> + int i;
> + m=_ctx->m;
> + r=_ctx->r;
> + a=_ctx->a;
> + b=_ctx->b+(++_ctx->c);
> + for(i=0;i<ISAAC_SZ/2;i++){
> + x=m[i];
> + a=(a^a<<13)+m[i+ISAAC_SZ/2];
> + m[i]=y=m[lower_bits(x)]+a+b;
> + r[i]=b=m[upper_bits(y)]+x;
> + x=m[++i];
> + a=(a^a>>6)+m[i+ISAAC_SZ/2];
> + m[i]=y=m[lower_bits(x)]+a+b;
> + r[i]=b=m[upper_bits(y)]+x;
> + x=m[++i];
> + a=(a^a<<2)+m[i+ISAAC_SZ/2];
> + m[i]=y=m[lower_bits(x)]+a+b;
> + r[i]=b=m[upper_bits(y)]+x;
> + x=m[++i];
> + a=(a^a>>16)+m[i+ISAAC_SZ/2];
> + m[i]=y=m[lower_bits(x)]+a+b;
> + r[i]=b=m[upper_bits(y)]+x;
> + }
> + for(i=ISAAC_SZ/2;i<ISAAC_SZ;i++){
> + x=m[i];
> + a=(a^a<<13)+m[i-ISAAC_SZ/2];
> + m[i]=y=m[lower_bits(x)]+a+b;
> + r[i]=b=m[upper_bits(y)]+x;
> + x=m[++i];
> + a=(a^a>>6)+m[i-ISAAC_SZ/2];
> + m[i]=y=m[lower_bits(x)]+a+b;
> + r[i]=b=m[upper_bits(y)]+x;
> + x=m[++i];
> + a=(a^a<<2)+m[i-ISAAC_SZ/2];
> + m[i]=y=m[lower_bits(x)]+a+b;
> + r[i]=b=m[upper_bits(y)]+x;
> + x=m[++i];
> + a=(a^a>>16)+m[i-ISAAC_SZ/2];
> + m[i]=y=m[lower_bits(x)]+a+b;
> + r[i]=b=m[upper_bits(y)]+x;
> + }
> + _ctx->b=b;
> + _ctx->a=a;
> + _ctx->n=ISAAC_SZ;
> +}
> +
> +static void isaac_mix(uint32_t _x[8]){
> + static const unsigned char SHIFT[8]={11,2,8,16,10,4,8,9};
> + int i;
> + for(i=0;i<8;i++){
> + _x[i]^=_x[(i+1)&7]<<SHIFT[i];
> + _x[(i+3)&7]+=_x[i];
> + _x[(i+1)&7]+=_x[(i+2)&7];
> + i++;
> + _x[i]^=_x[(i+1)&7]>>SHIFT[i];
> + _x[(i+3)&7]+=_x[i];
> + _x[(i+1)&7]+=_x[(i+2)&7];
> + }
> +}
> +
> +
> +void isaac_init(isaac_ctx *_ctx,const unsigned char *_seed,int _nseed){
> + _ctx->a=_ctx->b=_ctx->c=0;
> + memset(_ctx->r,0,sizeof(_ctx->r));
> + isaac_reseed(_ctx,_seed,_nseed);
> +}
> +
> +void isaac_reseed(isaac_ctx *_ctx,const unsigned char *_seed,int _nseed){
> + uint32_t *m;
> + uint32_t *r;
> + uint32_t x[8];
> + int i;
> + int j;
> + m=_ctx->m;
> + r=_ctx->r;
> + if(_nseed>ISAAC_SEED_SZ_MAX)_nseed=ISAAC_SEED_SZ_MAX;
> + for(i=0;i<_nseed>>2;i++){
> + r[i]^=(uint32_t)_seed[i<<2|3]<<24|(uint32_t)_seed[i<<2|2]<<16|
> + (uint32_t)_seed[i<<2|1]<<8|_seed[i<<2];
> + }
> + _nseed-=i<<2;
> + if(_nseed>0){
> + uint32_t ri;
> + ri=_seed[i<<2];
> + for(j=1;j<_nseed;j++)ri|=(uint32_t)_seed[i<<2|j]<<(j<<3);
> + r[i++]^=ri;
> + }
> + x[0]=x[1]=x[2]=x[3]=x[4]=x[5]=x[6]=x[7]=0x9E3779B9U;
> + for(i=0;i<4;i++)isaac_mix(x);
> + for(i=0;i<ISAAC_SZ;i+=8){
> + for(j=0;j<8;j++)x[j]+=r[i+j];
> + isaac_mix(x);
> + memcpy(m+i,x,sizeof(x));
> + }
> + for(i=0;i<ISAAC_SZ;i+=8){
> + for(j=0;j<8;j++)x[j]+=m[i+j];
> + isaac_mix(x);
> + memcpy(m+i,x,sizeof(x));
> + }
> + isaac_update(_ctx);
> +}
> +
> +uint32_t isaac_next_uint32(isaac_ctx *_ctx){
> + if(!_ctx->n)isaac_update(_ctx);
> + return _ctx->r[--_ctx->n];
> +}
> +
> +uint32_t isaac_next_uint(isaac_ctx *_ctx,uint32_t _n){
> + uint32_t r;
> + uint32_t v;
> + uint32_t d;
> + do{
> + r=isaac_next_uint32(_ctx);
> + v=r%_n;
> + d=r-v;
> + }
> + while(((d+_n-1)&ISAAC_MASK)<d);
> + return v;
> +}
> diff --git a/lib/prng.h b/lib/prng.h
> new file mode 100644
> index 0000000..bf5776d
> --- /dev/null
> +++ b/lib/prng.h
> @@ -0,0 +1,82 @@
> +/*
> + * PRNG Header
> + */
> +#ifndef __PRNG_H__
> +#define __PRNG_H__
> +
> +# include <stdint.h>
> +
> +
> +
> +typedef struct isaac_ctx isaac_ctx;
> +
> +
> +
> +/*This value may be lowered to reduce memory usage on embedded platforms, at
> + the cost of reducing security and increasing bias.
> + Quoting Bob Jenkins: "The current best guess is that bias is detectable after
> + 2**37 values for [ISAAC_SZ_LOG]=3, 2**45 for 4, 2**53 for 5, 2**61 for 6,
> + 2**69 for 7, and 2**77 values for [ISAAC_SZ_LOG]=8."*/
> +#define ISAAC_SZ_LOG (8)
> +#define ISAAC_SZ (1<<ISAAC_SZ_LOG)
> +#define ISAAC_SEED_SZ_MAX (ISAAC_SZ<<2)
> +
> +
> +
> +/*ISAAC is the most advanced of a series of pseudo-random number generators
> + designed by Robert J. Jenkins Jr. in 1996.
> + http://www.burtleburtle.net/bob/rand/isaac.html
> + To quote:
> + No efficient method is known for deducing their internal states.
> + ISAAC requires an amortized 18.75 instructions to produce a 32-bit value.
> + There are no cycles in ISAAC shorter than 2**40 values.
> + The expected cycle length is 2**8295 values.*/
> +struct isaac_ctx{
> + unsigned n;
> + uint32_t r[ISAAC_SZ];
> + uint32_t m[ISAAC_SZ];
> + uint32_t a;
> + uint32_t b;
> + uint32_t c;
> +};
> +
> +
> +/**
> + * isaac_init - Initialize an instance of the ISAAC random number generator.
> + * @_ctx: The instance to initialize.
> + * @_seed: The specified seed bytes.
> + * This may be NULL if _nseed is less than or equal to zero.
> + * @_nseed: The number of bytes to use for the seed.
> + * If this is greater than ISAAC_SEED_SZ_MAX, the extra bytes are
> + * ignored.
> + */
> +void isaac_init(isaac_ctx *_ctx,const unsigned char *_seed,int _nseed);
> +
> +/**
> + * isaac_reseed - Mix a new batch of entropy into the current state.
> + * To reset ISAAC to a known state, call isaac_init() again instead.
> + * @_ctx: The instance to reseed.
> + * @_seed: The specified seed bytes.
> + * This may be NULL if _nseed is zero.
> + * @_nseed: The number of bytes to use for the seed.
> + * If this is greater than ISAAC_SEED_SZ_MAX, the extra bytes are
> + * ignored.
> + */
> +void isaac_reseed(isaac_ctx *_ctx,const unsigned char *_seed,int _nseed);
> +/**
> + * isaac_next_uint32 - Return the next random 32-bit value.
> + * @_ctx: The ISAAC instance to generate the value with.
> + */
> +uint32_t isaac_next_uint32(isaac_ctx *_ctx);
> +/**
> + * isaac_next_uint - Uniform random integer less than the given value.
> + * @_ctx: The ISAAC instance to generate the value with.
> + * @_n: The upper bound on the range of numbers returned (not inclusive).
> + * This must be greater than zero and less than 2**32.
> + * To return integers in the full range 0...2**32-1, use
> + * isaac_next_uint32() instead.
> + * Return: An integer uniformly distributed between 0 and _n-1 (inclusive).
> + */
> +uint32_t isaac_next_uint(isaac_ctx *_ctx,uint32_t _n);
> +
> +#endif
> --
> 2.5.0
>
>

Agreed that (pseudo) random numbers could be useful in unit tests.

Acked-by: Andrew Jones <drjones@redhat.com>