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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