Subject: [PATCH v1 3/5] random: add helpers for random numbers with given floor or range

From: "Jason A. Donenfeld" <[email-masked]>
Date: Fri, 21 Oct 2022 21:44:01 -0400

Now that we have get_random_u32_below(), it's trivial to make inline
helpers to compute get_random_u32_above() and get_random_u32_between(),
which will help clean up open coded loops and manual computations
throughout the tree.

Signed-off-by: Jason A. Donenfeld <[email-masked]>
---
 include/linux/random.h | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/include/linux/random.h b/include/linux/random.h
index 3a82c0a8bc46..92188a74e50e 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -91,6 +91,30 @@ 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_between(u32 floor, u32 ceil)
+{
+	BUILD_BUG_ON_MSG(__builtin_constant_p(floor) && __builtin_constant_p(ceil) &&
+			 floor >= ceil, "get_random_u32_above() must take floor < ceil");
+	return floor + get_random_u32_below(ceil - floor);
+}
+
 /*
  * 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.
-- 
2.38.1