Re: [PATCH 4/8] drivers/char/random: Add get_random_mod() functions

[Matt Mackall]

On Mon, 2011-03-14 at 14:26 -0400, George Spelvin wrote:
> This is a function for generating random numbers modulo small
> integers, with uniform distribution and parsimonious use of seed
> material.

This actually looks pretty reasonable, ignoring the scary API foundation
it's built on. But as popular as rand() % m constructs are with
programmers, it's better to design things so as to avoid the modulus
entirely. We've done pretty well at that so far, so I'd rather not have
such a thing in the kernel.

> ---
>  drivers/char/random.c  |   63 ++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/random.h |   14 ++++++++++
>  2 files changed, 77 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 113508e..fc36a98 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1626,6 +1626,8 @@ EXPORT_SYMBOL(secure_dccp_sequence_number);
>   */
>  struct cpu_random {
>  	u32 hash[4];
> +	u32 lim, x;
> +	int avail;	/* Trailing bytes of hash[] available for seed */
>  };
>  DEFINE_PER_CPU(struct cpu_random, get_random_int_data);
>  static u32 __get_random_int(u32 *hash)
> @@ -1646,10 +1648,71 @@ unsigned int get_random_int(void)
>  	struct cpu_random *r = &get_cpu_var(get_random_int_data);
>  	u32 ret = __get_random_int(r->hash);
>  
> +	r->avail = 8;
>  	put_cpu_var(r);
>  	return ret;
>  }
>  
> +struct cpu_random *
> +get_random_mod_start(void)
> +{
> +	struct cpu_random *r = &get_cpu_var(get_random_int_data);
> +
> +	if (r->x >= r->lim) {
> +		r->x = 0;
> +		r->lim = 1;
> +		r->avail = 0;
> +	}
> +	return r;
> +}
> +
> +/*
> + * Return a random 0 <= x < m.  This is exacctly uniformly distributed,
> + * which "random() % m" is not, and it is economical with seed entropy.
> + * For example, this can shuffle 27 elements (27! > 2^93) with only
> + * one call to half_md4_transform.
> + *
> + * This is limited to 24-bit moduli m; larger values risk overflow.
> + */
> +unsigned
> +get_random_mod(struct cpu_random *r, unsigned m)
> +{
> +	unsigned x = r->x, lim = r->lim;
> +
> +	BUG_ON(m >= 0x1000000);
> +	do {
> +		BUG_ON(x >= lim);
> +
> +		/* Ensure lim >= m */
> +		while (lim < m) {
> +			/* Invoke underlying random bit source, if needed. */
> +			if (!r->avail) {
> +				/* Generate 12 more bytes of seed */
> +				(void)__get_random_int(r->hash);
> +				r->avail = 12;
> +			}
> +			/* Add one byte of seed material. */
> +			x = (x << 8) |
> +				((u8 *)r->hash)[sizeof r->hash - r->avail--];
> +			lim <<= 8;
> +		}
> +		/* Now check for uniformity, and loop if necessary. */
> +		r->lim = lim / m;
> +		lim %= m;
> +	} while (unlikely(x < lim));
> +
> +	x -= lim;
> +	/* We now have 0 <= x < m * r->lim, so x % m is uniform */
> +	r->x = x / m;	/* Remainder available for future use */
> +	return x % m;
> +}
> +
> +void
> +get_random_mod_stop(struct cpu_random *r)
> +{
> +	put_cpu_var(r);
> +}
> +
>  /*
>   * randomize_range() returns a start address such that
>   *
> diff --git a/include/linux/random.h b/include/linux/random.h
> index fb7ab9d..2e1c227 100644
> --- a/include/linux/random.h
> +++ b/include/linux/random.h
> @@ -75,6 +75,20 @@ extern const struct file_operations random_fops, urandom_fops;
>  unsigned int get_random_int(void);
>  unsigned long randomize_range(unsigned long start, unsigned long end, unsigned long len);
>  
> +
> +/*
> + * These functions generate a sequence of values modulo a small integer m.
> + * They are intended for shuffling operations.  "m" must be no more
> + * than 24 bits, or they will BUG().  (Rather than suffering an internal
> + * overflow.)
> + * They use per-CPU data, so preemption is disabled in the _start
> + * function and re-enabled in _stop.
> + */
> +struct cpu_random;	/* Opaque to acllers of this interface */
> +struct cpu_random *get_random_mod_start(void);
> +unsigned get_random_mod(struct cpu_random *r, unsigned m);
> +void get_random_mod_stop(struct cpu_random *r);
> +
>  u32 random32(void);
>  void srandom32(u32 seed);
>  


-- 
Mathematics is the supreme nostalgia of our time.


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