selinux: improve symtab string hashing

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@googlemail.com>
[PM: line length fixes in the commit message]
Signed-off-by: Paul Moore <paul@paul-moore.com>

diff --git a/security/selinux/ss/symtab.c b/security/selinux/ss/symtab.c
index c04f8d447873bbff4be117ef244363c67a4ff4de..832660fd84a96c2554daddd1763fc1cbfb36c497 100644 (file)
--- a/security/selinux/ss/symtab.c
+++ b/security/selinux/ss/symtab.c
@@ -12,17 +12,17 @@
 
 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
+        * Public domain from cdb v0.75
+        */
+       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)