siphash: Clean up hash finalisation with posthash_final() function

The POSTAMBLE macro implements the finalisation steps of SipHash.  It
relies on some variables in the environment, including returning the final
hash value that way.  That isn't great hygeine.

In addition the PREAMBLE macro takes a length parameter which is used only
to initialize the 'b' value that's not used until the finalisation and is
also sometimes modified in a non-obvious way by the callers.

The 'b' value is always composed from the total length of the hash input
plus up to 7 bytes of "tail" data - that is the remainder of the hash input
after a multiple of 8 bytes has been consumed.

Simplify all this by replacing the POSTAMBLE macro with a siphash_final()
function which takes the length and tail data as parameters and returns the
final hash value.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

siphash.c

@@ -51,16 +51,16 @@
  *
  */
 
+#include <stddef.h>
 #include <stdint.h>
 
 #include "siphash.h"
 
 #define ROTL(x, b) (uint64_t)(((x) << (b)) | ((x) >> (64 - (b))))
 
-#define PREAMBLE(len)							  \
+#define PREAMBLE							  \
 	uint64_t v[4] = { 0x736f6d6570736575ULL, 0x646f72616e646f6dULL,	  \
 			  0x6c7967656e657261ULL, 0x7465646279746573ULL }; \
-	uint64_t b = (uint64_t)(len) << 56;				  \
 	int __i;							  \
 									  \
 	do {								  \
@@ -103,13 +103,21 @@ static inline void siphash_feed(uint64_t *v, uint64_t in)
 	v[0] ^= in;
 }
 
-#define POSTAMBLE							  \
+/**
-	do {								  \
+ * siphash_final - Finalize SipHash calculations
-		siphash_feed(v, b);					  \
+ * @v:		siphash state (4 x 64-bit integers)
-		v[2] ^= 0xff;						  \
+ * @len:	Total length of input data
-		sipround(v, 4);						  \
+ * @tail:	Final data for the hash (<= 7 bytes)
-		b = (v[0] ^ v[1]) ^ (v[2] ^ v[3]);			  \
+ */
-	} while (0)
+static inline uint64_t siphash_final(uint64_t *v, size_t len, uint64_t tail)
+{
+	uint64_t b = (uint64_t)(len) << 56 | tail;
+
+	siphash_feed(v, b);
+	v[2] ^= 0xff;
+	sipround(v, 4);
+	return v[0] ^ v[1] ^ v[2] ^ v[3];
+}
 
 /**
  * siphash_8b() - Table index or timestamp offset for TCP over IPv4 (8 bytes in)
@@ -132,11 +140,11 @@ __attribute__((optimize("-fno-strict-aliasing")))
 /* cppcheck-suppress unusedFunction */
 uint64_t siphash_8b(const uint8_t *in, const uint64_t *k)
 {
-	PREAMBLE(8);
+	PREAMBLE;
 	siphash_feed(v, *(uint64_t *)in);
-	POSTAMBLE;
 
-	return b;
+
+	return siphash_final(v, 8, 0);
 }
 
 /**
@@ -153,12 +161,10 @@ uint64_t siphash_12b(const uint8_t *in, const uint64_t *k)
 {
 	uint32_t *in32 = (uint32_t *)in;
 
-	PREAMBLE(12);
+	PREAMBLE;
 	siphash_feed(v, (uint64_t)(*(in32 + 1)) << 32 | *in32);
-	b |= *(in32 + 2);
-	POSTAMBLE;
 
-	return b;
+	return siphash_final(v, 12, *(in32 + 2));
 }
 
 /**
@@ -175,15 +181,12 @@ uint64_t siphash_20b(const uint8_t *in, const uint64_t *k)
 	uint32_t *in32 = (uint32_t *)in;
 	int i;
 
-	PREAMBLE(20);
+	PREAMBLE;
 
 	for (i = 0; i < 2; i++, in32 += 2)
 		siphash_feed(v, (uint64_t)(*(in32 + 1)) << 32 | *in32);
 
-	b |= *in32;
+	return siphash_final(v, 20, *in32);
-	POSTAMBLE;
-
-	return b;
 }
 
 /**
@@ -201,14 +204,12 @@ uint64_t siphash_32b(const uint8_t *in, const uint64_t *k)
 	uint64_t *in64 = (uint64_t *)in;
 	int i;
 
-	PREAMBLE(32);
+	PREAMBLE;
 
 	for (i = 0; i < 4; i++, in64++)
 		siphash_feed(v, *in64);
 
-	POSTAMBLE;
+	return siphash_final(v, 32, 0);
-
-	return b;
 }
 
 /**
@@ -225,13 +226,10 @@ uint64_t siphash_36b(const uint8_t *in, const uint64_t *k)
 	uint32_t *in32 = (uint32_t *)in;
 	int i;
 
-	PREAMBLE(36);
+	PREAMBLE;
 
 	for (i = 0; i < 4; i++, in32 += 2)
 		siphash_feed(v, (uint64_t)(*(in32 + 1)) << 32 | *in32);
 
-	b |= *in32;
+	return siphash_final(v, 36, *in32);
-	POSTAMBLE;
-
-	return b;
 }