Commit 4c95236a authored by Jason A. Donenfeld's avatar Jason A. Donenfeld
Browse files

prandom: make use of smaller types in prandom_u32_max 



When possible at compile-time, make use of smaller types in
prandom_u32_max(), so that we can use smaller batches from random.c,
which in turn leads to a 2x or 4x performance boost. This makes a
difference, for example, in kfence, which needs a fast stream of small
numbers (booleans).

At the same time, we use the occasion to update the old documentation on
these functions. prandom_u32() and prandom_bytes() have direct
replacements now in random.h, while prandom_u32_max() remains useful as
a prandom.h function, since it's not cryptographically secure by virtue
of not being evenly distributed.

Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Acked-by: default avatarMarco Elver <elver@google.com>
Signed-off-by: default avatarJason A. Donenfeld <Jason@zx2c4.com>
parent 585cd5fe

include/linux/prandom.h

+11 −6
Original line number Diff line number Diff line
@@ -12,11 +12,13 @@ 
#include <linux/percpu.h>

#include <linux/random.h>



/* Deprecated: use get_random_u32 instead. */

static inline u32 prandom_u32(void)

{

	return get_random_u32();

}



/* Deprecated: use get_random_bytes instead. */

static inline void prandom_bytes(void *buf, size_t nbytes)

{

	return get_random_bytes(buf, nbytes);
@@ -37,17 +39,20 @@ void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state); 
 * prandom_u32_max - returns a pseudo-random number in interval [0, ep_ro)

 * @ep_ro: right open interval endpoint

 *

 * Returns a pseudo-random number that is in interval [0, ep_ro). Note

 * that the result depends on PRNG being well distributed in [0, ~0U]

 * u32 space. Here we use maximally equidistributed combined Tausworthe

 * generator, that is, prandom_u32(). This is useful when requesting a

 * random index of an array containing ep_ro elements, for example.

 * Returns a pseudo-random number that is in interval [0, ep_ro). This is

 * useful when requesting a random index of an array containing ep_ro elements,

 * for example. The result is somewhat biased when ep_ro is not a power of 2,

 * so do not use this for cryptographic purposes.

 *

 * Returns: pseudo-random number in interval [0, ep_ro)

 */

static inline u32 prandom_u32_max(u32 ep_ro)

{

	return (u32)(((u64) prandom_u32() * ep_ro) >> 32);

	if (__builtin_constant_p(ep_ro <= 1U << 8) && ep_ro <= 1U << 8)

		return (get_random_u8() * ep_ro) >> 8;

	if (__builtin_constant_p(ep_ro <= 1U << 16) && ep_ro <= 1U << 16)

		return (get_random_u16() * ep_ro) >> 16;

	return ((u64)get_random_u32() * ep_ro) >> 32;

}



/*