kcov: Add interrupt handling self test

Add a boot self test that can catch sprious coverage from interrupts.
The coverage callback filters out interrupt code, but only after the
handler updates preempt count. Some code periodically leaks out
of that section and leads to spurious coverage.
Add a best-effort (but simple) test that is likely to catch such bugs.
If the test is enabled on CI systems that use KCOV, they should catch
any issues fast.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexander Potapenko <glider@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Link: https://lore.kernel.org/all/7662127c97e29da1a748ad1c1539dd7b65b737b2.1718092070.git.dvyukov@google.com

1 files changed, 31 insertions, 0 deletions

diff --git a/kernel/kcov.c b/kernel/kcov.c
index f0a69d402066..d9d4a0c04185 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -11,6 +11,7 @@
 #include <linux/fs.h>
 #include <linux/hashtable.h>
 #include <linux/init.h>
+#include <linux/jiffies.h>
 #include <linux/kmsan-checks.h>
 #include <linux/mm.h>
 #include <linux/preempt.h>
@@ -1058,6 +1059,32 @@ u64 kcov_common_handle(void)
 }
 EXPORT_SYMBOL(kcov_common_handle);
 
+#ifdef CONFIG_KCOV_SELFTEST
+static void __init selftest(void)
+{
+	unsigned long start;
+
+	pr_err("running self test\n");
+	/*
+	 * Test that interrupts don't produce spurious coverage.
+	 * The coverage callback filters out interrupt code, but only
+	 * after the handler updates preempt count. Some code periodically
+	 * leaks out of that section and leads to spurious coverage.
+	 * It's hard to call the actual interrupt handler directly,
+	 * so we just loop here for a bit waiting for a timer interrupt.
+	 * We set kcov_mode to enable tracing, but don't setup the area,
+	 * so any attempt to trace will crash. Note: we must not call any
+	 * potentially traced functions in this region.
+	 */
+	start = jiffies;
+	current->kcov_mode = KCOV_MODE_TRACE_PC;
+	while ((jiffies - start) * MSEC_PER_SEC / HZ < 300)
+		;
+	current->kcov_mode = 0;
+	pr_err("done running self test\n");
+}
+#endif
+
 static int __init kcov_init(void)
 {
 	int cpu;
@@ -1077,6 +1104,10 @@ static int __init kcov_init(void)
 	 */
 	debugfs_create_file_unsafe("kcov", 0600, NULL, NULL, &kcov_fops);
 
+#ifdef CONFIG_KCOV_SELFTEST
+	selftest();
+#endif
+
 	return 0;
 }