test_kprobes: Add recursed kprobe test case

Add a recursed kprobe test case to the KUnit test module for kprobes.
This will probe a function which is called from the pre_handler and
post_handler itself. If the kprobe is correctly implemented, the recursed
kprobe handlers will be skipped and the number of skipped kprobe will
be counted on kprobe::nmissed.

Link: https://lore.kernel.org/all/167414238758.2301956.258548940194352895.stgit@devnote3/Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

lib/test_kprobes.c

@@ -14,6 +14,7 @@
 
 static u32 rand1, preh_val, posth_val;
 static u32 (*target)(u32 value);
+static u32 (*recursed_target)(u32 value);
 static u32 (*target2)(u32 value);
 static struct kunit *current_test;
 
@@ -27,18 +28,27 @@ static noinline u32 kprobe_target(u32 value)
 	return (value / div_factor);
 }
 
+static noinline u32 kprobe_recursed_target(u32 value)
+{
+	return (value / div_factor);
+}
+
 static int kp_pre_handler(struct kprobe *p, struct pt_regs *regs)
 {
 	KUNIT_EXPECT_FALSE(current_test, preemptible());
-	preh_val = (rand1 / div_factor);
+
+	preh_val = recursed_target(rand1);
 	return 0;
 }
 
 static void kp_post_handler(struct kprobe *p, struct pt_regs *regs,
 		unsigned long flags)
 {
+	u32 expval = recursed_target(rand1);
+
 	KUNIT_EXPECT_FALSE(current_test, preemptible());
-	KUNIT_EXPECT_EQ(current_test, preh_val, (rand1 / div_factor));
+	KUNIT_EXPECT_EQ(current_test, preh_val, expval);
+
 	posth_val = preh_val + div_factor;
 }
 
@@ -136,6 +146,29 @@ static void test_kprobes(struct kunit *test)
 	unregister_kprobes(kps, 2);
 }
 
+static struct kprobe kp_missed = {
+	.symbol_name = "kprobe_recursed_target",
+	.pre_handler = kp_pre_handler,
+	.post_handler = kp_post_handler,
+};
+
+static void test_kprobe_missed(struct kunit *test)
+{
+	current_test = test;
+	preh_val = 0;
+	posth_val = 0;
+
+	KUNIT_EXPECT_EQ(test, 0, register_kprobe(&kp_missed));
+
+	recursed_target(rand1);
+
+	KUNIT_EXPECT_EQ(test, 2, kp_missed.nmissed);
+	KUNIT_EXPECT_NE(test, 0, preh_val);
+	KUNIT_EXPECT_NE(test, 0, posth_val);
+
+	unregister_kprobe(&kp_missed);
+}
+
 #ifdef CONFIG_KRETPROBES
 static u32 krph_val;
 
@@ -336,6 +369,7 @@ static int kprobes_test_init(struct kunit *test)
 {
 	target = kprobe_target;
 	target2 = kprobe_target2;
+	recursed_target = kprobe_recursed_target;
 	stacktrace_target = kprobe_stacktrace_target;
 	internal_target = kprobe_stacktrace_internal_target;
 	stacktrace_driver = kprobe_stacktrace_driver;
@@ -346,6 +380,7 @@ static int kprobes_test_init(struct kunit *test)
 static struct kunit_case kprobes_testcases[] = {
 	KUNIT_CASE(test_kprobe),
 	KUNIT_CASE(test_kprobes),
+	KUNIT_CASE(test_kprobe_missed),
 #ifdef CONFIG_KRETPROBES
 	KUNIT_CASE(test_kretprobe),
 	KUNIT_CASE(test_kretprobes),