selinux: Use a better hash function for avtab

This function, based on murmurhash3, has much better distribution than
the original. Using the current default of 2048 buckets, there are many
fewer collisions:

Before:
101421 entries and 2048/2048 buckets used, longest chain length 374
After:
101421 entries and 2048/2048 buckets used, longest chain length 81

The difference becomes much more significant when buckets are increased.
A naive attempt to expand the current function to larger outputs doesn't
yield any significant improvement; so this function is a prerequisite
for increasing the bucket size.

sds:  Adapted from the original patches for libsepol to the kernel.

Signed-off-by: John Brooks <john.brooks@jolla.com>
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <pmoore@redhat.com>

2 files changed, 38 insertions, 5 deletions

diff --git a/security/selinux/ss/avtab.c b/security/selinux/ss/avtab.c
index 3ea0198fc964..b64f2772b030 100644
--- a/security/selinux/ss/avtab.c
+++ b/security/selinux/ss/avtab.c
@@ -25,10 +25,43 @@
 
 static struct kmem_cache *avtab_node_cachep;
 
-static inline int avtab_hash(struct avtab_key *keyp, u16 mask)
+/* Based on MurmurHash3, written by Austin Appleby and placed in the
+ * public domain.
+ */
+static inline int avtab_hash(struct avtab_key *keyp, u32 mask)
 {
-        return ((keyp->target_class + (keyp->target_type << 2) +
-                 (keyp->source_type << 9)) & mask);
+        static const u32 c1 = 0xcc9e2d51;
+        static const u32 c2 = 0x1b873593;
+        static const u32 r1 = 15;
+        static const u32 r2 = 13;
+        static const u32 m  = 5;
+        static const u32 n  = 0xe6546b64;
+
+        u32 hash = 0;
+
+#define mix(input) { \
+        u32 v = input; \
+        v *= c1; \
+        v = (v << r1) | (v >> (32 - r1)); \
+        v *= c2; \
+        hash ^= v; \
+        hash = (hash << r2) | (hash >> (32 - r2)); \
+        hash = hash * m + n; \
+}
+
+        mix(keyp->target_class);
+        mix(keyp->target_type);
+        mix(keyp->source_type);
+
+#undef mix
+
+        hash ^= hash >> 16;
+        hash *= 0x85ebca6b;
+        hash ^= hash >> 13;
+        hash *= 0xc2b2ae35;
+        hash ^= hash >> 16;
+
+        return hash & mask;
 }
 
 static struct avtab_node*
@@ -256,7 +289,7 @@ int avtab_init(struct avtab *h)
 
 int avtab_alloc(struct avtab *h, u32 nrules)
 {
-        u16 mask = 0;
+        u32 mask = 0;
         u32 shift = 0;
         u32 work = nrules;
         u32 nslot = 0;
diff --git a/security/selinux/ss/avtab.h b/security/selinux/ss/avtab.h
index 9318b2b8f6c9..6d794a2eee57 100644
--- a/security/selinux/ss/avtab.h
+++ b/security/selinux/ss/avtab.h
@@ -56,7 +56,7 @@ struct avtab {
         struct flex_array *htable;
         u32 nel;        /* number of elements */
         u32 nslot;      /* number of hash slots */
-        u16 mask;       /* mask to compute hash func */
+        u32 mask;       /* mask to compute hash func */
 
 };