get_random_u32_below()

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

diff --git a/include/linux/random.h b/include/linux/random.h
index ea101d53acd6003572ebcddf88db95a1deb1ba7c..3203d13c0247c4ceea823e0bec3ec95d27bf59f0 100644 (file)
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -9,6 +9,7 @@
 #include <unistd.h>
 #include <sys/syscall.h>
 #include <linux/bug.h>
+#include <linux/log2.h>
 
 #ifdef SYS_getrandom
 static inline int getrandom(void *buf, size_t buflen, unsigned int flags)
@@ -40,7 +41,30 @@ static inline type get_random_##type(void)           \
 
 get_random_type(int);
 get_random_type(long);
+get_random_type(u8);
+get_random_type(u16);
 get_random_type(u32);
 get_random_type(u64);
 
+static inline u32 get_random_u32_below(u32 ceil)
+{
+       if (ceil <= 1)
+               return 0;
+       for (;;) {
+               if (ceil <= 1U << 8) {
+                       u32 mult = ceil * get_random_u8();
+                       if (likely(is_power_of_2(ceil) || (u8)mult >= (1U << 8) % ceil))
+                               return mult >> 8;
+               } else if (ceil <= 1U << 16) {
+                       u32 mult = ceil * get_random_u16();
+                       if (likely(is_power_of_2(ceil) || (u16)mult >= (1U << 16) % ceil))
+                               return mult >> 16;
+               } else {
+                       u64 mult = (u64)ceil * get_random_u32();
+                       if (likely(is_power_of_2(ceil) || (u32)mult >= -ceil % ceil))
+                               return mult >> 32;
+               }
+       }
+}
+
 #endif /* _LINUX_RANDOM_H */