tests: use PRNG to generate data for tests

This replaces the multiples of a prime sequence used for the per-CPU
helper tests with a slightly more self-documenting PRNG interface in
both the test kernel module and Python test scaffolding. It will be used
more in upcoming tests.

Signed-off-by: Omar Sandoval <[email protected]>

Author: osandov

tests/linux_kernel/__init__.py

@@ -104,6 +104,18 @@ def setUpClass(cls):
 )
 
 
+# PRNG used by the test kernel module.
+def prng32(seed):
+    seed = seed.encode("ascii")
+    assert len(seed) == 4
+    x = int.from_bytes(seed, "big")
+    while True:
+        x ^= (x << 13) & 0xFFFFFFFF
+        x ^= x >> 17
+        x ^= (x << 5) & 0xFFFFFFFF
+        yield x
+
+
 def wait_until(fn, *args, **kwds):
     TIMEOUT = 5
     deadline = time.monotonic() + TIMEOUT

tests/linux_kernel/helpers/test_percpu.py

@@ -5,6 +5,7 @@
 from drgn.helpers.linux.percpu import per_cpu, per_cpu_ptr
 from tests.linux_kernel import (
     LinuxKernelTestCase,
+    prng32,
     skip_unless_have_test_kmod,
     smp_enabled,
 )
@@ -25,18 +26,14 @@ def test_per_cpu(self):
 
     @skip_unless_have_test_kmod
     def test_per_cpu_module_static(self):
-        expected = prime = self.prog["drgn_test_percpu_static_prime"]
-        for cpu in for_each_possible_cpu(self.prog):
-            expected *= prime
+        for cpu, expected in zip(for_each_possible_cpu(self.prog), prng32("PCPU")):
             self.assertEqual(
                 per_cpu(self.prog["drgn_test_percpu_static"], cpu), expected
             )
 
     @skip_unless_have_test_kmod
     def test_per_cpu_module_dynamic(self):
-        expected = prime = self.prog["drgn_test_percpu_dynamic_prime"]
-        for cpu in for_each_possible_cpu(self.prog):
-            expected *= prime
+        for cpu, expected in zip(for_each_possible_cpu(self.prog), prng32("pcpu")):
             self.assertEqual(
                 per_cpu_ptr(self.prog["drgn_test_percpu_dynamic"], cpu)[0], expected
             )

tests/linux_kernel/kmod/drgn_test.c

@@ -29,6 +29,23 @@
 #define HAVE_XARRAY 0
 #endif
 
+// Convert a 4-character string to a seed for drgn_test_prng32().
+static inline u32 drgn_test_prng32_seed(const char *s)
+{
+	BUG_ON(strlen(s) != 4);
+	return ((u32)s[0] << 24) | ((u32)s[1] << 16) | ((u32)s[2] << 8) | (u32)s[3];
+}
+
+// x must not be 0; the return value is never 0.
+static u32 drgn_test_prng32(u32 x)
+{
+	// Xorshift RNG with a period of 2^32 - 1.
+	x ^= x << 13;
+	x ^= x >> 17;
+	x ^= x << 5;
+	return x;
+}
+
 // list
 
 LIST_HEAD(drgn_test_empty_list);
@@ -157,26 +174,23 @@ static void drgn_test_mm_exit(void)
 // percpu
 
 DEFINE_PER_CPU(unsigned int, drgn_test_percpu_static);
-const unsigned int drgn_test_percpu_static_prime = 0xa45dcfc3U;
 unsigned int __percpu *drgn_test_percpu_dynamic;
-const unsigned int drgn_test_percpu_dynamic_prime = 0x6d80a613U;
 
 static int drgn_test_percpu_init(void)
 {
 	int cpu;
-	unsigned int static_prime = drgn_test_percpu_static_prime;
-	unsigned int dynamic_prime = drgn_test_percpu_dynamic_prime;
+	u32 static_seed = drgn_test_prng32_seed("PCPU");
+	u32 dynamic_seed = drgn_test_prng32_seed("pcpu");
 
 	drgn_test_percpu_dynamic = alloc_percpu(unsigned int);
 	if (!drgn_test_percpu_dynamic)
 		return -ENOMEM;
-	// Initialize the per-cpu variables to powers of a random prime number
-	// which are extremely unlikely to appear anywhere else.
+	// Initialize the per-cpu variables with a PRNG sequence.
 	for_each_possible_cpu(cpu) {
-		static_prime *= drgn_test_percpu_static_prime;
-		per_cpu(drgn_test_percpu_static, cpu) = static_prime;
-		dynamic_prime *= drgn_test_percpu_dynamic_prime;
-		*per_cpu_ptr(drgn_test_percpu_dynamic, cpu) = dynamic_prime;
+		static_seed = drgn_test_prng32(static_seed);
+		per_cpu(drgn_test_percpu_static, cpu) = static_seed;
+		dynamic_seed = drgn_test_prng32(dynamic_seed);
+		*per_cpu_ptr(drgn_test_percpu_dynamic, cpu) = dynamic_seed;
 	}
 	return 0;
 }