[PATCH 26/27] random: factor out a __limit_random_u32_below helper

[Christoph Hellwig]

Factor out the guts of __get_random_u32_below into a new helper,
so that callers with their own prng state can reuse this code.

Signed-off-by: Christoph Hellwig <hch at lst.de>
---
 drivers/char/random.c  | 26 +++++++++++++++-----------
 include/linux/random.h |  1 +
 2 files changed, 16 insertions(+), 11 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 7ff4d29911fd..23b5addf02fb 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -544,18 +544,16 @@ 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.
+ */
+u32 __limit_random_u32_below(u32 ceil, u32 rand)
 {
-	/*
-	 * 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.
-	 */
-	u32 rand = get_random_u32();
 	u64 mult;
 
 	/*
@@ -577,6 +575,12 @@ u32 __get_random_u32_below(u32 ceil)
 	}
 	return mult >> 32;
 }
+EXPORT_SYMBOL_GPL(__limit_random_u32_below);
+
+u32 __get_random_u32_below(u32 ceil)
+{
+	return __limit_random_u32_below(ceil, get_random_u32());
+}
 EXPORT_SYMBOL(__get_random_u32_below);
 
 #ifdef CONFIG_SMP
diff --git a/include/linux/random.h b/include/linux/random.h
index 8a8064dc3970..54401dd53f68 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -50,6 +50,7 @@ static inline unsigned long get_random_long(void)
 #endif
 }
 
+u32 __limit_random_u32_below(u32 ceil, u32 rand);
 u32 __get_random_u32_below(u32 ceil);
 
 /*
-- 
2.47.3