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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: Marco Elver <elver@google.com> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
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@ -12,11 +12,13 @@
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#include <linux/percpu.h>
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#include <linux/random.h>
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/* Deprecated: use get_random_u32 instead. */
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static inline u32 prandom_u32(void)
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{
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return get_random_u32();
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}
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/* Deprecated: use get_random_bytes instead. */
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static inline void prandom_bytes(void *buf, size_t nbytes)
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{
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return get_random_bytes(buf, nbytes);
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@ -37,17 +39,20 @@ void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state);
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* prandom_u32_max - returns a pseudo-random number in interval [0, ep_ro)
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* @ep_ro: right open interval endpoint
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*
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* Returns a pseudo-random number that is in interval [0, ep_ro). Note
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* that the result depends on PRNG being well distributed in [0, ~0U]
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* u32 space. Here we use maximally equidistributed combined Tausworthe
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* generator, that is, prandom_u32(). This is useful when requesting a
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* random index of an array containing ep_ro elements, for example.
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* Returns a pseudo-random number that is in interval [0, ep_ro). This is
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* useful when requesting a random index of an array containing ep_ro elements,
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* for example. The result is somewhat biased when ep_ro is not a power of 2,
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* so do not use this for cryptographic purposes.
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*
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* Returns: pseudo-random number in interval [0, ep_ro)
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*/
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static inline u32 prandom_u32_max(u32 ep_ro)
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{
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return (u32)(((u64) prandom_u32() * ep_ro) >> 32);
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if (__builtin_constant_p(ep_ro <= 1U << 8) && ep_ro <= 1U << 8)
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return (get_random_u8() * ep_ro) >> 8;
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if (__builtin_constant_p(ep_ro <= 1U << 16) && ep_ro <= 1U << 16)
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return (get_random_u16() * ep_ro) >> 16;
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return ((u64)get_random_u32() * ep_ro) >> 32;
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}
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/*
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