From 5464c961865e049279dc696a93fbaf08b8bc39ce Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Wed, 11 Nov 2020 14:44:35 +0100
Subject: [PATCH] random-util: add random_u64_range() that acquires a random
 number from a certain range, unbiased

So far we have been quite sloppy with this and ignored modulus and range
bias. Let's do something about, and add the option to do better.
---
 src/basic/random-util.c | 19 +++++++++++++++++++
 src/basic/random-util.h |  2 ++
 2 files changed, 21 insertions(+)

diff --git a/src/basic/random-util.c b/src/basic/random-util.c
index c831f06dac..eca8187571 100644
--- a/src/basic/random-util.c
+++ b/src/basic/random-util.c
@@ -494,3 +494,22 @@ int random_write_entropy(int fd, const void *seed, size_t size, bool credit) {
 
         return 1;
 }
+
+int random_u64_range(uint64_t m) {
+        uint64_t x, remainder;
+
+        /* Generates a random number in the range 0…m-1, unbiased. (Java's algorithm) */
+
+        if (m == 0) /* Let's take m == 0 as special case to return an integer from the full range */
+                return random_u64();
+        if (m == 1)
+                return 0;
+
+        remainder = UINT64_MAX % m;
+
+        do {
+                x = random_u64();
+        } while (x >= UINT64_MAX - remainder);
+
+        return x % m;
+}
diff --git a/src/basic/random-util.h b/src/basic/random-util.h
index f661fc093a..1d5fb60fa2 100644
--- a/src/basic/random-util.h
+++ b/src/basic/random-util.h
@@ -40,3 +40,5 @@ int rdrand(unsigned long *ret);
 size_t random_pool_size(void);
 
 int random_write_entropy(int fd, const void *seed, size_t size, bool credit);
+
+int random_u64_range(uint64_t max);