siphash: use one source of truth for siphash permutations

The SipHash family of permutations is currently used in three places:

- siphash.c itself, used in the ordinary way it was intended.
- random32.c, in a construction from an anonymous contributor.
- random.c, as part of its fast_mix function.

Each one of these places reinvents the wheel with the same C code, same
rotation constants, and same symmetry-breaking constants.

This commit tidies things up a bit by placing macros for the
permutations and constants into siphash.h, where each of the three .c
users can access them. It also leaves a note dissuading more users of
them from emerging.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

2 files changed, 35 insertions, 16 deletions

diff --git a/include/linux/prandom.h b/include/linux/prandom.h
index 056d31317e49..a4aadd2dc153 100644
--- a/include/linux/prandom.h
+++ b/include/linux/prandom.h
@@ -10,6 +10,7 @@
 
 #include <linux/types.h>
 #include <linux/percpu.h>
+#include <linux/siphash.h>
 
 u32 prandom_u32(void);
 void prandom_bytes(void *buf, size_t nbytes);
@@ -27,15 +28,10 @@ DECLARE_PER_CPU(unsigned long, net_rand_noise);
  * The core SipHash round function.  Each line can be executed in
  * parallel given enough CPU resources.
  */
-#define PRND_SIPROUND(v0, v1, v2, v3) ( \
-	v0 += v1, v1 = rol64(v1, 13),  v2 += v3, v3 = rol64(v3, 16), \
-	v1 ^= v0, v0 = rol64(v0, 32),  v3 ^= v2,                     \
-	v0 += v3, v3 = rol64(v3, 21),  v2 += v1, v1 = rol64(v1, 17), \
-	v3 ^= v0,                      v1 ^= v2, v2 = rol64(v2, 32)  \
-)
+#define PRND_SIPROUND(v0, v1, v2, v3) SIPHASH_PERMUTATION(v0, v1, v2, v3)
 
-#define PRND_K0 (0x736f6d6570736575 ^ 0x6c7967656e657261)
-#define PRND_K1 (0x646f72616e646f6d ^ 0x7465646279746573)
+#define PRND_K0 (SIPHASH_CONST_0 ^ SIPHASH_CONST_2)
+#define PRND_K1 (SIPHASH_CONST_1 ^ SIPHASH_CONST_3)
 
 #elif BITS_PER_LONG == 32
 /*
@@ -43,14 +39,9 @@ DECLARE_PER_CPU(unsigned long, net_rand_noise);
  * This is weaker, but 32-bit machines are not used for high-traffic
  * applications, so there is less output for an attacker to analyze.
  */
-#define PRND_SIPROUND(v0, v1, v2, v3) ( \
-	v0 += v1, v1 = rol32(v1,  5),  v2 += v3, v3 = rol32(v3,  8), \
-	v1 ^= v0, v0 = rol32(v0, 16),  v3 ^= v2,                     \
-	v0 += v3, v3 = rol32(v3,  7),  v2 += v1, v1 = rol32(v1, 13), \
-	v3 ^= v0,                      v1 ^= v2, v2 = rol32(v2, 16)  \
-)
-#define PRND_K0 0x6c796765
-#define PRND_K1 0x74656462
+#define PRND_SIPROUND(v0, v1, v2, v3) HSIPHASH_PERMUTATION(v0, v1, v2, v3)
+#define PRND_K0 (HSIPHASH_CONST_0 ^ HSIPHASH_CONST_2)
+#define PRND_K1 (HSIPHASH_CONST_1 ^ HSIPHASH_CONST_3)
 
 #else
 #error Unsupported BITS_PER_LONG
diff --git a/include/linux/siphash.h b/include/linux/siphash.h
index cce8a9acc76c..3af1428da559 100644
--- a/include/linux/siphash.h
+++ b/include/linux/siphash.h
@@ -138,4 +138,32 @@ static inline u32 hsiphash(const void *data, size_t len,
 	return ___hsiphash_aligned(data, len, key);
 }
 
+/*
+ * These macros expose the raw SipHash and HalfSipHash permutations.
+ * Do not use them directly! If you think you have a use for them,
+ * be sure to CC the maintainer of this file explaining why.
+ */
+
+#define SIPHASH_PERMUTATION(a, b, c, d) ( \
+	(a) += (b), (b) = rol64((b), 13), (b) ^= (a), (a) = rol64((a), 32), \
+	(c) += (d), (d) = rol64((d), 16), (d) ^= (c), \
+	(a) += (d), (d) = rol64((d), 21), (d) ^= (a), \
+	(c) += (b), (b) = rol64((b), 17), (b) ^= (c), (c) = rol64((c), 32))
+
+#define SIPHASH_CONST_0 0x736f6d6570736575ULL
+#define SIPHASH_CONST_1 0x646f72616e646f6dULL
+#define SIPHASH_CONST_2 0x6c7967656e657261ULL
+#define SIPHASH_CONST_3 0x7465646279746573ULL
+
+#define HSIPHASH_PERMUTATION(a, b, c, d) ( \
+	(a) += (b), (b) = rol32((b), 5), (b) ^= (a), (a) = rol32((a), 16), \
+	(c) += (d), (d) = rol32((d), 8), (d) ^= (c), \
+	(a) += (d), (d) = rol32((d), 7), (d) ^= (a), \
+	(c) += (b), (b) = rol32((b), 13), (b) ^= (c), (c) = rol32((c), 16))
+
+#define HSIPHASH_CONST_0 0U
+#define HSIPHASH_CONST_1 0U
+#define HSIPHASH_CONST_2 0x6c796765U
+#define HSIPHASH_CONST_3 0x74656462U
+
 #endif /* _LINUX_SIPHASH_H */