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

andrey.konovalov
2022-01-24T19:04:44Z

- 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,