[PATCH 2/4] kcov: add interrupt handling self test

[Dmitry Vyukov]

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@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: syzkaller@xxxxxxxxxxxxxxxx

---

In my local testing w/o the previous fix,
it immidiatly produced the following splat:

kcov: running selftest
BUG: TASK stack guard page was hit at ffffc90000147ff8
Oops: stack guard page: 0000 [#1] PREEMPT SMP KASAN PTI
...
 kvm_set_cpu_l1tf_flush_l1d+0x5/0x20
 sysvec_call_function+0x15/0xb0
 asm_sysvec_call_function+0x1a/0x20
 kcov_init+0xe4/0x130
 do_one_initcall+0xbc/0x470
 kernel_init_freeable+0x4fc/0x930
 kernel_init+0x1c/0x2b0
---
 kernel/kcov.c     | 28 ++++++++++++++++++++++++++++
 lib/Kconfig.debug |  9 +++++++++
 2 files changed, 37 insertions(+)

diff --git a/kernel/kcov.c b/kernel/kcov.c
index c3124f6d5536..04136f80042f 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -1057,6 +1057,30 @@ u64 kcov_common_handle(void)
 }
 EXPORT_SYMBOL(kcov_common_handle);
 
+#ifdef CONFIG_KCOV_TEST
+static void __init selftest(void)
+{
+	volatile int i;
+
+	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 ~400 ms 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.
+	 */
+	current->kcov_mode = KCOV_MODE_TRACE_PC;
+	for (i = 0; i < (1 << 28); i++)
+		;
+	current->kcov_mode = 0;
+	pr_err("done running self test\n");
+}
+#endif
+
 static int __init kcov_init(void)
 {
 	int cpu;
@@ -1076,6 +1100,10 @@ static int __init kcov_init(void)
 	 */
 	debugfs_create_file_unsafe("kcov", 0600, NULL, NULL, &kcov_fops);
 
+#ifdef CONFIG_KCOV_TEST
+	selftest();
+#endif
+
 	return 0;
 }
 
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 59b6765d86b8..79836a15b6cb 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2171,6 +2171,15 @@ config KCOV_IRQ_AREA_SIZE
 	  soft interrupts. This specifies the size of those areas in the
 	  number of unsigned long words.
 
+config KCOV_TEST
+	bool "Test CONFIG_KCOV feature"
+	depends on KCOV
+	help
+	  Sanity check for KCOV coverage collection.
+	  Runs built-in self test on boot to detect some common issues.
+
+	  If unsure, say N.
+
 menuconfig RUNTIME_TESTING_MENU
 	bool "Runtime Testing"
 	default y
-- 
2.45.1.467.gbab1589fc0-goog