diff --git a/Userland/Libraries/LibC/stdlib.cpp b/Userland/Libraries/LibC/stdlib.cpp index cddf548c25b..47dd242bb03 100644 --- a/Userland/Libraries/LibC/stdlib.cpp +++ b/Userland/Libraries/LibC/stdlib.cpp @@ -1039,9 +1039,22 @@ void arc4random_buf(void* buffer, size_t buffer_size) uint32_t arc4random_uniform(uint32_t max_bounds) { - // XXX: Should actually apply special rules for uniformity; avoid what is - // called "modulo bias". - return arc4random() % max_bounds; + // If we try to divide all 2**32 numbers into groups of "max_bounds" numbers, we may end up + // with a group around 2**32-1 that is a bit too small. For this reason, the implementation + // `arc4random() % max_bounds` would be insufficient. Here we compute the last number of the + // last "full group". Note that if max_bounds is a divisor of UINT32_MAX, + // then we end up with UINT32_MAX: + const uint32_t max_usable = UINT32_MAX - (static_cast(UINT32_MAX) + 1) % max_bounds; + uint32_t random_value = arc4random(); + for (int i = 0; i < 20 && random_value > max_usable; ++i) { + // By chance we picked a value from the incomplete group. Note that this group has size at + // most 2**31-1, so picking this group has a chance of less than 50%. + // In practice, this means that for the worst possible input, there is still only a + // once-in-a-million chance to get to iteration 20. In theory we should be able to loop + // forever. Here we prefer marginally imperfect random numbers over weird runtime behavior. + random_value = arc4random(); + } + return random_value % max_bounds; } char* realpath(const char* pathname, char* buffer)