landlock: Improve bit operations in audit code

Use the BIT() and BIT_ULL() macros in the new audit code instead of
explicit shifts to improve readability.  Use bitmask instead of modulo
operation to simplify code.

Add test_range1_rand15() and test_range2_rand15() KUnit tests to improve
get_id_range() coverage.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20250512093732.1408485-1-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Author: l0kod

security/landlock/audit.c

@@ -437,7 +437,7 @@ void landlock_log_denial(const struct landlock_cred_security *const subject,
 		return;
 
 	/* Checks if the current exec was restricting itself. */
-	if (subject->domain_exec & (1 << youngest_layer)) {
+	if (subject->domain_exec & BIT(youngest_layer)) {
 		/* Ignores denials for the same execution. */
 		if (!youngest_denied->log_same_exec)
 			return;

security/landlock/id.c

@@ -7,6 +7,7 @@
 
 #include <kunit/test.h>
 #include <linux/atomic.h>
+#include <linux/bitops.h>
 #include <linux/random.h>
 #include <linux/spinlock.h>
 
@@ -25,7 +26,7 @@ static void __init init_id(atomic64_t *const counter, const u32 random_32bits)
 	 * Ensures sure 64-bit values are always used by user space (or may
 	 * fail with -EOVERFLOW), and makes this testable.
 	 */
-	init = 1ULL << 32;
+	init = BIT_ULL(32);
 
 	/*
 	 * Makes a large (2^32) boot-time value to limit ID collision in logs
@@ -105,7 +106,7 @@ static u64 get_id_range(size_t number_of_ids, atomic64_t *const counter,
 	 * to get a new ID (e.g. a full landlock_restrict_self() call), and the
 	 * cost of draining all available IDs during the system's uptime.
 	 */
-	random_4bits = random_4bits % (1 << 4);
+	random_4bits &= 0b1111;
 	step = number_of_ids + random_4bits;
 
 	/* It is safe to cast a signed atomic to an unsigned value. */
@@ -144,6 +145,19 @@ static void test_range1_rand1(struct kunit *const test)
 		init + 2);
 }
 
+static void test_range1_rand15(struct kunit *const test)
+{
+	atomic64_t counter;
+	u64 init;
+
+	init = get_random_u32();
+	atomic64_set(&counter, init);
+	KUNIT_EXPECT_EQ(test, get_id_range(1, &counter, 15), init);
+	KUNIT_EXPECT_EQ(
+		test, get_id_range(get_random_u8(), &counter, get_random_u8()),
+		init + 16);
+}
+
 static void test_range1_rand16(struct kunit *const test)
 {
 	atomic64_t counter;
@@ -196,6 +210,19 @@ static void test_range2_rand2(struct kunit *const test)
 		init + 4);
 }
 
+static void test_range2_rand15(struct kunit *const test)
+{
+	atomic64_t counter;
+	u64 init;
+
+	init = get_random_u32();
+	atomic64_set(&counter, init);
+	KUNIT_EXPECT_EQ(test, get_id_range(2, &counter, 15), init);
+	KUNIT_EXPECT_EQ(
+		test, get_id_range(get_random_u8(), &counter, get_random_u8()),
+		init + 17);
+}
+
 static void test_range2_rand16(struct kunit *const test)
 {
 	atomic64_t counter;
@@ -232,10 +259,12 @@ static struct kunit_case __refdata test_cases[] = {
 	KUNIT_CASE(test_init_once),
 	KUNIT_CASE(test_range1_rand0),
 	KUNIT_CASE(test_range1_rand1),
+	KUNIT_CASE(test_range1_rand15),
 	KUNIT_CASE(test_range1_rand16),
 	KUNIT_CASE(test_range2_rand0),
 	KUNIT_CASE(test_range2_rand1),
 	KUNIT_CASE(test_range2_rand2),
+	KUNIT_CASE(test_range2_rand15),
 	KUNIT_CASE(test_range2_rand16),
 	{}
 	/* clang-format on */

security/landlock/syscalls.c

@@ -9,6 +9,7 @@
 
 #include <asm/current.h>
 #include <linux/anon_inodes.h>
+#include <linux/bitops.h>
 #include <linux/build_bug.h>
 #include <linux/capability.h>
 #include <linux/cleanup.h>
@@ -563,7 +564,7 @@ SYSCALL_DEFINE2(landlock_restrict_self, const int, ruleset_fd, const __u32,
 	new_llcred->domain = new_dom;
 
 #ifdef CONFIG_AUDIT
-	new_llcred->domain_exec |= 1 << (new_dom->num_layers - 1);
+	new_llcred->domain_exec |= BIT(new_dom->num_layers - 1);
 #endif /* CONFIG_AUDIT */
 
 	return commit_creds(new_cred);