kasan: clean up feature flags for HW_TAGS mode

 - 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.

Link: https://lkml.kernel.org/r/76ebb340265be57a218564a497e1f52ff36a3879.1643047180.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Acked-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
index 6509809..6a3146d 100644 (file)
--- 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 020f3e5..efda13a 100644 (file)
--- 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,