backports: add prandom_u32_max()

This was added in:
commit f337db64af059c9a94278a8b0ab97d87259ff62f
Author: Daniel Borkmann <dborkman@redhat.com>
Date:   Wed Jan 22 02:29:39 2014 +0100

    random32: add prandom_u32_max and convert open coded users

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

diff --git a/backport/backport-include/linux/random.h b/backport/backport-include/linux/random.h
index e91510609774759865260f8109c88a05125e61fc..dd5f377fd1220b7e451375e9aa486b4ffe61cd2e 100644 (file)
--- a/backport/backport-include/linux/random.h
+++ b/backport/backport-include/linux/random.h
@@ -13,4 +13,24 @@
 void prandom_bytes(void *buf, int bytes);
 #endif
 
+#if LINUX_VERSION_CODE < KERNEL_VERSION(3,14,0)
+/**
+ * prandom_u32_max - returns a pseudo-random number in interval [0, ep_ro)
+ * @ep_ro: right open interval endpoint
+ *
+ * Returns a pseudo-random number that is in interval [0, ep_ro). Note
+ * that the result depends on PRNG being well distributed in [0, ~0U]
+ * u32 space. Here we use maximally equidistributed combined Tausworthe
+ * generator, that is, prandom_u32(). This is useful when requesting a
+ * random index of an array containing ep_ro elements, for example.
+ *
+ * Returns: pseudo-random number in interval [0, ep_ro)
+ */
+#define prandom_u32_max LINUX_BACKPORT(prandom_u32_max)
+static inline u32 prandom_u32_max(u32 ep_ro)
+{
+       return (u32)(((u64) prandom_u32() * ep_ro) >> 32);
+}
+#endif /* LINUX_VERSION_CODE < KERNEL_VERSION(3,14,0) */
+
 #endif /* __BACKPORT_RANDOM_H */