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[4/5] libselinux: use DJB2a string hash function

Message ID 20230816123845.80171-4-cgzones@googlemail.com (mailing list archive)
State Accepted
Commit f1178a13dc8b
Delegated to: Petr Lautrbach
Headers show
Series [1/5] libsepol: include length squared in hashtab_hash_eval() | expand

Commit Message

Christian Göttsche Aug. 16, 2023, 12:38 p.m. UTC 
The hash table implementation uses `& (SIDTAB_SIZE - 1)` to truncate
generated hashes to the number of buckets.  This operation is equal to
`% SIDTAB_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.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
---
 libselinux/src/avc_sidtab.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)
diff mbox series

Patch

diff --git a/libselinux/src/avc_sidtab.c b/libselinux/src/avc_sidtab.c
index f179d855..e396a938 100644
--- a/libselinux/src/avc_sidtab.c
+++ b/libselinux/src/avc_sidtab.c
@@ -15,16 +15,13 @@ 
 
 static inline unsigned sidtab_hash(const char * key)
 {
-	const char *p;
-	unsigned int size;
-	unsigned int val;
-
-	val = 0;
-	size = strlen(key);
-	for (p = key; (unsigned int)(p - key) < size; p++)
-		val =
-		    (val << 4 | (val >> (8 * sizeof(unsigned int) - 4))) ^ (*p);
-	return val & (SIDTAB_SIZE - 1);
+	unsigned int hash = 5381;
+	unsigned char c;
+
+	while ((c = *(unsigned const char *)key++))
+		hash = ((hash << 5) + hash) ^ c;
+
+	return hash & (SIDTAB_SIZE - 1);
 }
 
 int sidtab_init(struct sidtab *s)