diff options
Diffstat (limited to 'drivers/char')
| -rw-r--r-- | drivers/char/random.c | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/drivers/char/random.c b/drivers/char/random.c index 69754155300e..6f323344d0b9 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -160,6 +160,7 @@ EXPORT_SYMBOL(wait_for_random_bytes); * u8 get_random_u8() * u16 get_random_u16() * u32 get_random_u32() + * u32 get_random_u32_below(u32 ceil) * u64 get_random_u64() * unsigned long get_random_long() * @@ -510,6 +511,27 @@ DEFINE_BATCHED_ENTROPY(u16) DEFINE_BATCHED_ENTROPY(u32) DEFINE_BATCHED_ENTROPY(u64) +u32 __get_random_u32_below(u32 ceil) +{ + /* + * This is the slow path for variable ceil. It is still fast, most of + * the time, by doing traditional reciprocal multiplication and + * opportunistically comparing the lower half to ceil itself, before + * falling back to computing a larger bound, and then rejecting samples + * whose lower half would indicate a range indivisible by ceil. The use + * of `-ceil % ceil` is analogous to `2^32 % ceil`, but is computable + * in 32-bits. + */ + u64 mult = (u64)ceil * get_random_u32(); + if (unlikely((u32)mult < ceil)) { + u32 bound = -ceil % ceil; + while (unlikely((u32)mult < bound)) + mult = (u64)ceil * get_random_u32(); + } + return mult >> 32; +} +EXPORT_SYMBOL(__get_random_u32_below); + #ifdef CONFIG_SMP /* * This function is called when the CPU is coming up, with entry |