[PATCH 3/3] selinux: hweight optimization in avtab_read_item

[Jacob Satterfield <jsatterfield.linux@gmail.com>]

avtab_read_item() is a hot function called when reading each rule in a
binary policydb. With the current Fedora policy and refpolicy, this
function is called nearly 100,000 times per policy load.

A single avtab node is only permitted to have a single specifier to
describe the data it holds. As such, a check is performed to make sure
only one specifier is set. Previously this was done via a for-loop.
However, there is already an optimal function for finding the number of
bits set (hamming weight) and on some architectures, dedicated
instructions (popcount) which can be executed much more efficiently.

Even when using -mcpu=generic on a x86-64 Fedora 38 VM, this commit
results in a modest 2-4% speedup for policy loading due to a substantial
reduction in the number of instructions executed.

Signed-off-by: Jacob Satterfield <jsatterfield.linux@gmail.com>
---
 security/selinux/ss/avtab.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/security/selinux/ss/avtab.c b/security/selinux/ss/avtab.c
index b7a11f417f0a..b0e455fb395c 100644
--- a/security/selinux/ss/avtab.c
+++ b/security/selinux/ss/avtab.c
@@ -17,6 +17,7 @@
  *	Tuned number of hash slots for avtab to reduce memory usage
  */
 
+#include <linux/bitops.h>
 #include <linux/kernel.h>
 #include <linux/slab.h>
 #include <linux/errno.h>
@@ -516,11 +517,7 @@ int avtab_read_item(struct avtab *a, void *fp, struct policydb *pol,
 		return -EINVAL;
 	}
 
-	set = 0;
-	for (i = 0; i < ARRAY_SIZE(spec_order); i++) {
-		if (key.specified & spec_order[i])
-			set++;
-	}
+	set = hweight16(key.specified & (AVTAB_XPERMS | AVTAB_TYPE | AVTAB_AV));
 	if (!set || set > 1) {
 		pr_err("SELinux:  avtab:  more than one specifier\n");
 		return -EINVAL;
-- 
2.41.0