Merge tag 'x86-build-2024-09-17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 build updates from Thomas Gleixner:
 "Updates for KCOV instrumentation on x86:

   - Prevent spurious KCOV coverage in common_interrupt()

   - Fixup the KCOV Makefile directive which got stale due to a source
     file rename

   - Exclude stack unwinding from KCOV as it creates large amounts of
     uninteresting coverage

   - Provide a self test to validate that KCOV coverage of the interrupt
     handling code starts not before preempt count got updated"

* tag 'x86-build-2024-09-17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86: Ignore stack unwinding in KCOV
  module: Fix KCOV-ignored file name
  kcov: Add interrupt handling self test
  x86/entry: Remove unwanted instrumentation in common_interrupt()

1 files changed, 31 insertions, 0 deletions

diff --git a/kernel/kcov.c b/kernel/kcov.c
index 274b6b7c718d..28a6be6e64fd 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>
@@ -1067,6 +1068,32 @@ 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;
@@ -1086,6 +1113,10 @@ static int __init kcov_init(void)
 	 */
 	debugfs_create_file_unsafe("kcov", 0600, NULL, NULL, &kcov_fops);
 
+#ifdef CONFIG_KCOV_SELFTEST
+	selftest();
+#endif
+
 	return 0;
 }