From 7f576b2593a978451416424e75f69ad1e3ae4efe Mon Sep 17 00:00:00 2001 From: "Jason A. Donenfeld" Date: Wed, 19 Oct 2022 23:19:35 -0600 Subject: [PATCH] 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 --- drivers/char/random.c | 18 +++++++++++++++++- include/linux/random.h | 25 +++++++++++++++++++++++++ 2 files changed, 42 insertions(+), 1 deletion(-) diff --git a/drivers/char/random.c b/drivers/char/random.c index 6f323344d0b9..f5868dddbb61 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -161,6 +161,8 @@ EXPORT_SYMBOL(wait_for_random_bytes); * u16 get_random_u16() * u32 get_random_u32() * u32 get_random_u32_below(u32 ceil) + * u32 get_random_u32_above(u32 floor) + * u32 get_random_u32_inclusive(u32 floor, u32 ceil) * u64 get_random_u64() * unsigned long get_random_long() * @@ -522,7 +524,21 @@ u32 __get_random_u32_below(u32 ceil) * of `-ceil % ceil` is analogous to `2^32 % ceil`, but is computable * in 32-bits. */ - u64 mult = (u64)ceil * get_random_u32(); + u32 rand = get_random_u32(); + u64 mult; + + /* + * This function is technically undefined for ceil == 0, and in fact + * for the non-underscored constant version in the header, we build bug + * on that. But for the non-constant case, it's convenient to have that + * evaluate to being a straight call to get_random_u32(), so that + * get_random_u32_inclusive() can work over its whole range without + * undefined behavior. + */ + if (unlikely(!ceil)) + return rand; + + mult = (u64)ceil * rand; if (unlikely((u32)mult < ceil)) { u32 bound = -ceil % ceil; while (unlikely((u32)mult < bound)) 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.