From 7f576b2593a978451416424e75f69ad1e3ae4efe Mon Sep 17 00:00:00 2001 From: "Jason A. Donenfeld" Date: Wed, 19 Oct 2022 23:19:35 -0600 Subject: random: add helpers for random numbers with given floor or range Now that we have get_random_u32_below(), it's nearly trivial to make inline helpers to compute get_random_u32_above() and get_random_u32_inclusive(), which will help clean up open coded loops and manual computations throughout the tree. One snag is that in order to make get_random_u32_inclusive() operate on closed intervals, we have to do some (unlikely) special case handling if get_random_u32_inclusive(0, U32_MAX) is called. The least expensive way of doing this is actually to adjust the slowpath of get_random_u32_below() to have its undefined 0 result just return the output of get_random_u32(). We can make this basically free by calling get_random_u32() before the branch, so that the branch latency gets interleaved. Cc: stable@vger.kernel.org # to ease future backports that use this api Reviewed-by: Kees Cook Signed-off-by: Jason A. Donenfeld --- include/linux/random.h | 25 +++++++++++++++++++++++++ 1 file changed, 25 insertions(+) (limited to 'include/linux/random.h') diff --git a/include/linux/random.h b/include/linux/random.h index 3a82c0a8bc46..bd954ecbef90 100644 --- a/include/linux/random.h +++ b/include/linux/random.h @@ -91,6 +91,31 @@ static inline u32 get_random_u32_below(u32 ceil) } } +/* + * Returns a random integer in the interval (floor, U32_MAX], with uniform + * distribution, suitable for all uses. Fastest when floor is a constant, but + * still fast for variable floor as well. + */ +static inline u32 get_random_u32_above(u32 floor) +{ + BUILD_BUG_ON_MSG(__builtin_constant_p(floor) && floor == U32_MAX, + "get_random_u32_above() must take floor < U32_MAX"); + return floor + 1 + get_random_u32_below(U32_MAX - floor); +} + +/* + * Returns a random integer in the interval [floor, ceil], with uniform + * distribution, suitable for all uses. Fastest when floor and ceil are + * constant, but still fast for variable floor and ceil as well. + */ +static inline u32 get_random_u32_inclusive(u32 floor, u32 ceil) +{ + BUILD_BUG_ON_MSG(__builtin_constant_p(floor) && __builtin_constant_p(ceil) && + (floor > ceil || ceil - floor == U32_MAX), + "get_random_u32_inclusive() must take floor <= ceil"); + return floor + get_random_u32_below(ceil - floor + 1); +} + /* * On 64-bit architectures, protect against non-terminated C string overflows * by zeroing out the first byte of the canary; this leaves 56 bits of entropy. -- cgit v1.2.3