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: Marco Elver <elver@xxxxxxxxxx>
Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
include/linux/prandom.h | 17 +++++++++++------
1 file changed, 11 insertions(+), 6 deletions(-)
diff --git a/include/linux/prandom.h b/include/linux/prandom.h
index deace5fb4e62..78db003bc290 100644
--- a/include/linux/prandom.h
+++ b/include/linux/prandom.h
@@ -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;
}
/*
--
2.37.3
