[PATCH 6/6] selinux: improve symtab string hashing

[Christian Göttsche]

The number of buckets is calculated by performing a binary AND against
the mask of the hash table, which is one less than its size (which is a
power of two).  This leads to all top bits being discarded, requiring
for short or similar inputs a hash function with a good avalanche
effect.

Use djb2a:

    # current
    common prefixes:  7 entries and 5/8 buckets used, longest chain length 2, sum of chain length^2 11
    classes:  134 entries and 100/256 buckets used, longest chain length 5, sum of chain length^2 234
    roles:  15 entries and 6/16 buckets used, longest chain length 5, sum of chain length^2 57
    types:  4448 entries and 3016/8192 buckets used, longest chain length 41, sum of chain length^2 14922
    users:  7 entries and 3/8 buckets used, longest chain length 3, sum of chain length^2 17
    bools:  306 entries and 221/512 buckets used, longest chain length 4, sum of chain length^2 524
    levels:  1 entries and 1/1 buckets used, longest chain length 1, sum of chain length^2 1
    categories:  1024 entries and 400/1024 buckets used, longest chain length 4, sum of chain length^2 2740

    # patch
    common prefixes:  7 entries and 5/8 buckets used, longest chain length 2, sum of chain length^2 11
    classes:  134 entries and 101/256 buckets used, longest chain length 3, sum of chain length^2 210
    roles:  15 entries and 9/16 buckets used, longest chain length 3, sum of chain length^2 31
    types:  4448 entries and 3459/8192 buckets used, longest chain length 5, sum of chain length^2 6778
    users:  7 entries and 5/8 buckets used, longest chain length 3, sum of chain length^2 13
    bools:  306 entries and 236/512 buckets used, longest chain length 5, sum of chain length^2 470
    levels:  1 entries and 1/1 buckets used, longest chain length 1, sum of chain length^2 1
    categories:  1024 entries and 518/1024 buckets used, longest chain length 7, sum of chain length^2 2992

Signed-off-by: Christian Göttsche <cgzones@xxxxxxxxxxxxxx>
---
 security/selinux/ss/symtab.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/security/selinux/ss/symtab.c b/security/selinux/ss/symtab.c
index 43d7f0319ccd..b6761b96eee4 100644
--- a/security/selinux/ss/symtab.c
+++ b/security/selinux/ss/symtab.c
@@ -11,16 +11,14 @@
 
 static unsigned int symhash(const void *key)
 {
-	const char *p, *keyp;
-	unsigned int size;
-	unsigned int val;
-
-	val = 0;
-	keyp = key;
-	size = strlen(keyp);
-	for (p = keyp; (p - keyp) < size; p++)
-		val = (val << 4 | (val >> (8*sizeof(unsigned int)-4))) ^ (*p);
-	return val;
+	/* djb2a */
+	unsigned int hash = 5381;
+	unsigned char c;
+
+	while ((c = *(const unsigned char *)key++))
+		hash = ((hash << 5) + hash) ^ c;
+
+	return hash;
 }
 
 static int symcmp(const void *key1, const void *key2)
-- 
2.40.1