[PATCH v4 2/3] prandom: Add __always_inline version of prandom_u32_state()

[Ryan Roberts]

We will shortly use prandom_u32_state() to implement kstack offset
randomization and some arches need to call it from non-instrumentable
context. So let's implement prandom_u32_state() as an out-of-line
wrapper around a new __always_inline prandom_u32_state_inline(). kstack
offset randomization will use this new version.

Acked-by: Mark Rutland <mark.rutland at arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts at arm.com>
---
 include/linux/prandom.h | 20 ++++++++++++++++++++
 lib/random32.c          |  8 +-------
 2 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/include/linux/prandom.h b/include/linux/prandom.h
index ff7dcc3fa105..801188680a29 100644
--- a/include/linux/prandom.h
+++ b/include/linux/prandom.h
@@ -17,6 +17,26 @@ struct rnd_state {
 	__u32 s1, s2, s3, s4;
 };
 
+/**
+ * prandom_u32_state_inline - seeded pseudo-random number generator.
+ * @state: pointer to state structure holding seeded state.
+ *
+ * This is used for pseudo-randomness with no outside seeding.
+ * For more random results, use get_random_u32().
+ * For use only where the out-of-line version, prandom_u32_state(), cannot be
+ * used (e.g. noinstr code).
+ */
+static __always_inline u32 prandom_u32_state_inline(struct rnd_state *state)
+{
+#define TAUSWORTHE(s, a, b, c, d) ((s & c) << d) ^ (((s << a) ^ s) >> b)
+	state->s1 = TAUSWORTHE(state->s1,  6U, 13U, 4294967294U, 18U);
+	state->s2 = TAUSWORTHE(state->s2,  2U, 27U, 4294967288U,  2U);
+	state->s3 = TAUSWORTHE(state->s3, 13U, 21U, 4294967280U,  7U);
+	state->s4 = TAUSWORTHE(state->s4,  3U, 12U, 4294967168U, 13U);
+
+	return (state->s1 ^ state->s2 ^ state->s3 ^ state->s4);
+}
+
 u32 prandom_u32_state(struct rnd_state *state);
 void prandom_bytes_state(struct rnd_state *state, void *buf, size_t nbytes);
 void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state);
diff --git a/lib/random32.c b/lib/random32.c
index 24e7acd9343f..2a02d82e91bc 100644
--- a/lib/random32.c
+++ b/lib/random32.c
@@ -51,13 +51,7 @@
  */
 u32 prandom_u32_state(struct rnd_state *state)
 {
-#define TAUSWORTHE(s, a, b, c, d) ((s & c) << d) ^ (((s << a) ^ s) >> b)
-	state->s1 = TAUSWORTHE(state->s1,  6U, 13U, 4294967294U, 18U);
-	state->s2 = TAUSWORTHE(state->s2,  2U, 27U, 4294967288U,  2U);
-	state->s3 = TAUSWORTHE(state->s3, 13U, 21U, 4294967280U,  7U);
-	state->s4 = TAUSWORTHE(state->s4,  3U, 12U, 4294967168U, 13U);
-
-	return (state->s1 ^ state->s2 ^ state->s3 ^ state->s4);
+	return prandom_u32_state_inline(state);
 }
 EXPORT_SYMBOL(prandom_u32_state);
 
-- 
2.43.0