linux-kernel - [PATCH 5/6] selinux: improve role transition hashing

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Message-Id: <20230818151220.166215-4-cgzones@googlemail.com>
Date:   Fri, 18 Aug 2023 17:12:16 +0200
From:   Christian Göttsche <cgzones@...glemail.com>
To:     selinux@...r.kernel.org
Cc:     Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 5/6] selinux: improve role transition 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, e.g. with
the Reference Policy on Debian there exists 376 entries, leading to a
size of 512, discarding the top 23 bits.

Use jhash to improve the hash table utilization:

    # current
    roletr:  376 entries and 124/512 buckets used, longest chain length 8, sum of chain length^2 1496

    # patch
    roletr:  376 entries and 266/512 buckets used, longest chain length 4, sum of chain length^2 646

Signed-off-by: Christian Göttsche <cgzones@...glemail.com>
---
 security/selinux/ss/policydb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/security/selinux/ss/policydb.c b/security/selinux/ss/policydb.c
index 932e383bcad6..dd4a9eff61be 100644
--- a/security/selinux/ss/policydb.c
+++ b/security/selinux/ss/policydb.c
@@ -491,7 +491,7 @@ static u32 role_trans_hash(const void *k)
 {
 	const struct role_trans_key *key = k;
 
-	return key->role + (key->type << 3) + (key->tclass << 5);
+	return jhash_3words(key->role, key->type, (u32)key->tclass << 16 | key->tclass, 0);
 }
 
 static int role_trans_cmp(const void *k1, const void *k2)
-- 
2.40.1