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libsepol: use DJB2a string hash function 

The hash table implementation uses `& (h->size - 1)` to truncate
generated hashes to the number of buckets.  This operation is equal to
`% h->size` if and only if the size is a power of two (which seems to be
always the case).  One property of the binary and with a power of two
(and probably a small one <=2048) is all higher bits are discarded.
Thus a hash function is needed with a good avalanche effect, which the
current one is not.

Benchmark of building Reference Policy:

    # Current
    Benchmark 1: /tmp/destdir/usr/bin/checkpolicy -c 33 -U deny -S -O -E policy.conf -o policy.33
      Time (mean ± σ):      2.521 s ±  0.025 s    [User: 2.442 s, System: 0.076 s]
      Range (min … max):    2.467 s …  2.550 s    10 runs

    # Patch
    Benchmark 1: /tmp/destdir/usr/bin/checkpolicy -c 33 -U deny -S -O -E policy.conf -o policy.33
      Time (mean ± σ):      2.385 s ±  0.031 s    [User: 2.303 s, System: 0.081 s]
      Range (min … max):    2.353 s …  2.446 s    10 runs

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Acked-by: James Carter <jwcart2@gmail.com>
This commit is contained in:
Christian Göttsche 2023-08-16 14:38:42 +02:00 committed by James Carter
parent 26cec7caa7
commit d03d506a30
1 changed files with 6 additions and 10 deletions
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16
libsepol/src/symtab.c
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@ -17,17 +17,13 @@
ignore_unsigned_overflow_
static unsigned int symhash(hashtab_t h, const_hashtab_key_t key)
{
const char *p, *keyp;
size_t size;
unsigned int val;
unsigned int hash = 5381;
unsigned char c;
val = 0;
keyp = (const char *)key;
size = strlen(keyp);
for (p = keyp; ((size_t) (p - keyp)) < size; p++)
val =
(val << 4 | (val >> (8 * sizeof(unsigned int) - 4))) ^ (*p);
return val & (h->size - 1);
while ((c = *(unsigned const char *)key++))
hash = ((hash << 5) + hash) ^ c;
return hash & (h->size - 1);
}
static int symcmp(hashtab_t h