[PATCH v6 34/39] kasan: clean up feature flags for HW_TAGS mode

[andrey.konovalov@linux.dev]

From: Andrey Konovalov <andreyknvl@google.com>

- Untie kasan_init_hw_tags() code from the default values of
  kasan_arg_mode and kasan_arg_stacktrace.

- Move static_branch_enable(&kasan_flag_enabled) to the end of
  kasan_init_hw_tags_cpu().

- Remove excessive comments in kasan_arg_mode switch.

- Add new comments.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

---

Changes v4->v5:
- Add this patch.
---
 mm/kasan/hw_tags.c | 38 +++++++++++++++++++++-----------------
 mm/kasan/kasan.h   |  2 +-
 2 files changed, 22 insertions(+), 18 deletions(-)

diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
index 6509809dd5d8..6a3146d1ccc5 100644
--- a/mm/kasan/hw_tags.c
+++ b/mm/kasan/hw_tags.c
@@ -42,16 +42,22 @@ static enum kasan_arg kasan_arg __ro_after_init;
 static enum kasan_arg_mode kasan_arg_mode __ro_after_init;
 static enum kasan_arg_stacktrace kasan_arg_stacktrace __initdata;
 
-/* Whether KASAN is enabled at all. */
+/*
+ * Whether KASAN is enabled at all.
+ * The value remains false until KASAN is initialized by kasan_init_hw_tags().
+ */
 DEFINE_STATIC_KEY_FALSE(kasan_flag_enabled);
 EXPORT_SYMBOL(kasan_flag_enabled);
 
-/* Whether the selected mode is synchronous/asynchronous/asymmetric.*/
+/*
+ * Whether the selected mode is synchronous, asynchronous, or asymmetric.
+ * Defaults to KASAN_MODE_SYNC.
+ */
 enum kasan_mode kasan_mode __ro_after_init;
 EXPORT_SYMBOL_GPL(kasan_mode);
 
 /* Whether to collect alloc/free stack traces. */
-DEFINE_STATIC_KEY_FALSE(kasan_flag_stacktrace);
+DEFINE_STATIC_KEY_TRUE(kasan_flag_stacktrace);
 
 /* kasan=off/on */
 static int __init early_kasan_flag(char *arg)
@@ -127,7 +133,11 @@ void kasan_init_hw_tags_cpu(void)
 	 * as this function is only called for MTE-capable hardware.
 	 */
 
-	/* If KASAN is disabled via command line, don't initialize it. */
+	/*
+	 * If KASAN is disabled via command line, don't initialize it.
+	 * When this function is called, kasan_flag_enabled is not yet
+	 * set by kasan_init_hw_tags(). Thus, check kasan_arg instead.
+	 */
 	if (kasan_arg == KASAN_ARG_OFF)
 		return;
 
@@ -154,42 +164,36 @@ void __init kasan_init_hw_tags(void)
 	if (kasan_arg == KASAN_ARG_OFF)
 		return;
 
-	/* Enable KASAN. */
-	static_branch_enable(&kasan_flag_enabled);
-
 	switch (kasan_arg_mode) {
 	case KASAN_ARG_MODE_DEFAULT:
-		/*
-		 * Default to sync mode.
-		 */
-		fallthrough;
+		/* Default is specified by kasan_mode definition. */
+		break;
 	case KASAN_ARG_MODE_SYNC:
-		/* Sync mode enabled. */
 		kasan_mode = KASAN_MODE_SYNC;
 		break;
 	case KASAN_ARG_MODE_ASYNC:
-		/* Async mode enabled. */
 		kasan_mode = KASAN_MODE_ASYNC;
 		break;
 	case KASAN_ARG_MODE_ASYMM:
-		/* Asymm mode enabled. */
 		kasan_mode = KASAN_MODE_ASYMM;
 		break;
 	}
 
 	switch (kasan_arg_stacktrace) {
 	case KASAN_ARG_STACKTRACE_DEFAULT:
-		/* Default to enabling stack trace collection. */
-		static_branch_enable(&kasan_flag_stacktrace);
+		/* Default is specified by kasan_flag_stacktrace definition. */
 		break;
 	case KASAN_ARG_STACKTRACE_OFF:
-		/* Do nothing, kasan_flag_stacktrace keeps its default value. */
+		static_branch_disable(&kasan_flag_stacktrace);
 		break;
 	case KASAN_ARG_STACKTRACE_ON:
 		static_branch_enable(&kasan_flag_stacktrace);
 		break;
 	}
 
+	/* KASAN is now initialized, enable it. */
+	static_branch_enable(&kasan_flag_enabled);
+
 	pr_info("KernelAddressSanitizer initialized (hw-tags, mode=%s, stacktrace=%s)\n",
 		kasan_mode_info(),
 		kasan_stack_collection_enabled() ? "on" : "off");
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 020f3e57a03f..efda13a9ce6a 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -12,7 +12,7 @@
 #include <linux/static_key.h>
 #include "../slab.h"
 
-DECLARE_STATIC_KEY_FALSE(kasan_flag_stacktrace);
+DECLARE_STATIC_KEY_TRUE(kasan_flag_stacktrace);
 
 enum kasan_mode {
 	KASAN_MODE_SYNC,
-- 
2.25.1