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selinux: improve symtab string hashing

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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>
This commit is contained in:
Christian Göttsche 2024-03-15 19:14:04 +01:00 • committed by Paul Moore
parent 0fd0b4fefa
commit 32db469edf
1 changed files with 10 additions and 10 deletions
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20
security/selinux/ss/symtab.c
View file

@ -12,17 +12,17 @@
static unsigned int symhash(const void *key)
{
const char *p, *keyp;
unsigned int size;
unsigned int val;
/*
* djb2a
* Public domain from cdb v0.75
*/
unsigned int hash = 5381;
unsigned char c;
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;
while ((c = *(const unsigned char *)key++))
hash = ((hash << 5) + hash) ^ c;
return hash;
}
static int symcmp(const void *key1, const void *key2)