[PATCH v3 02/12] kasan: unify static kasan_flag_enabled across modes

[Sabyrzhan Tasbolatov <snovitoll@gmail.com>]

Historically, the runtime static key kasan_flag_enabled existed only for
CONFIG_KASAN_HW_TAGS mode. Generic and SW_TAGS modes either relied on
architecture-specific kasan_arch_is_ready() implementations or evaluated
KASAN checks unconditionally, leading to code duplication.

This patch implements two-level approach:

1. kasan_enabled() - controls if KASAN is enabled at all (compile-time)
2. kasan_shadow_initialized() - tracks shadow memory
   initialization (runtime)

For architectures that select ARCH_DEFER_KASAN: kasan_shadow_initialized()
uses a static key that gets enabled when shadow memory is ready.

For architectures that don't: kasan_shadow_initialized() returns
IS_ENABLED(CONFIG_KASAN) since shadow is ready from the start.

This provides:
- Consistent interface across all KASAN modes
- Runtime control only where actually needed
- Compile-time constants for optimal performance where possible
- Clear separation between "KASAN configured" vs "shadow ready"

Also adds kasan_init_generic() function that enables the shadow flag and
handles initialization for Generic mode, and updates SW_TAGS and HW_TAGS
to use the unified kasan_shadow_enable() function.

Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217049
Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
---
Changes in v3:
- Only architectures that need deferred KASAN get runtime overhead
- Added kasan_shadow_initialized() for shadow memory readiness tracking
- kasan_enabled() now provides compile-time check for KASAN configuration
---
 include/linux/kasan-enabled.h | 34 ++++++++++++++++++++++++++--------
 include/linux/kasan.h         |  6 ++++++
 mm/kasan/common.c             |  9 +++++++++
 mm/kasan/generic.c            | 11 +++++++++++
 mm/kasan/hw_tags.c            |  9 +--------
 mm/kasan/sw_tags.c            |  2 ++
 6 files changed, 55 insertions(+), 16 deletions(-)

diff --git a/include/linux/kasan-enabled.h b/include/linux/kasan-enabled.h
index 6f612d69ea0..fa99dc58f95 100644
--- a/include/linux/kasan-enabled.h
+++ b/include/linux/kasan-enabled.h
@@ -4,32 +4,50 @@
 
 #include <linux/static_key.h>
 
-#ifdef CONFIG_KASAN_HW_TAGS
+/* Controls whether KASAN is enabled at all (compile-time check). */
+static __always_inline bool kasan_enabled(void)
+{
+	return IS_ENABLED(CONFIG_KASAN);
+}
 
+#ifdef CONFIG_ARCH_DEFER_KASAN
+/*
+ * Global runtime flag for architectures that need deferred KASAN.
+ * Switched to 'true' by the appropriate kasan_init_*()
+ * once KASAN is fully initialized.
+ */
 DECLARE_STATIC_KEY_FALSE(kasan_flag_enabled);
 
-static __always_inline bool kasan_enabled(void)
+static __always_inline bool kasan_shadow_initialized(void)
 {
 	return static_branch_likely(&kasan_flag_enabled);
 }
 
-static inline bool kasan_hw_tags_enabled(void)
+static inline void kasan_enable(void)
+{
+	static_branch_enable(&kasan_flag_enabled);
+}
+#else
+/* For architectures that can enable KASAN early, use compile-time check. */
+static __always_inline bool kasan_shadow_initialized(void)
 {
 	return kasan_enabled();
 }
 
-#else /* CONFIG_KASAN_HW_TAGS */
+/* No-op for architectures that don't need deferred KASAN. */
+static inline void kasan_enable(void) {}
+#endif /* CONFIG_ARCH_DEFER_KASAN */
 
-static inline bool kasan_enabled(void)
+#ifdef CONFIG_KASAN_HW_TAGS
+static inline bool kasan_hw_tags_enabled(void)
 {
-	return IS_ENABLED(CONFIG_KASAN);
+	return kasan_enabled();
 }
-
+#else
 static inline bool kasan_hw_tags_enabled(void)
 {
 	return false;
 }
-
 #endif /* CONFIG_KASAN_HW_TAGS */
 
 #endif /* LINUX_KASAN_ENABLED_H */
diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index 890011071f2..51a8293d1af 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -543,6 +543,12 @@ void kasan_report_async(void);
 
 #endif /* CONFIG_KASAN_HW_TAGS */
 
+#ifdef CONFIG_KASAN_GENERIC
+void __init kasan_init_generic(void);
+#else
+static inline void kasan_init_generic(void) { }
+#endif
+
 #ifdef CONFIG_KASAN_SW_TAGS
 void __init kasan_init_sw_tags(void);
 #else
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index ed4873e18c7..c3a6446404d 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -32,6 +32,15 @@
 #include "kasan.h"
 #include "../slab.h"
 
+#ifdef CONFIG_ARCH_DEFER_KASAN
+/*
+ * Definition of the unified static key declared in kasan-enabled.h.
+ * This provides consistent runtime enable/disable across KASAN modes.
+ */
+DEFINE_STATIC_KEY_FALSE(kasan_flag_enabled);
+EXPORT_SYMBOL(kasan_flag_enabled);
+#endif
+
 struct slab *kasan_addr_to_slab(const void *addr)
 {
 	if (virt_addr_valid(addr))
diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
index d54e89f8c3e..03b6d322ff6 100644
--- a/mm/kasan/generic.c
+++ b/mm/kasan/generic.c
@@ -36,6 +36,17 @@
 #include "kasan.h"
 #include "../slab.h"
 
+/*
+ * Initialize Generic KASAN and enable runtime checks.
+ * This should be called from arch kasan_init() once shadow memory is ready.
+ */
+void __init kasan_init_generic(void)
+{
+	kasan_enable();
+
+	pr_info("KernelAddressSanitizer initialized (generic)\n");
+}
+
 /*
  * All functions below always inlined so compiler could
  * perform better optimizations in each of __asan_loadX/__assn_storeX
diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
index 9a6927394b5..c8289a3feab 100644
--- a/mm/kasan/hw_tags.c
+++ b/mm/kasan/hw_tags.c
@@ -45,13 +45,6 @@ static enum kasan_arg kasan_arg __ro_after_init;
 static enum kasan_arg_mode kasan_arg_mode __ro_after_init;
 static enum kasan_arg_vmalloc kasan_arg_vmalloc __initdata;
 
-/*
- * 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, or asymmetric.
  * Defaults to KASAN_MODE_SYNC.
@@ -260,7 +253,7 @@ void __init kasan_init_hw_tags(void)
 	kasan_init_tags();
 
 	/* KASAN is now initialized, enable it. */
-	static_branch_enable(&kasan_flag_enabled);
+	kasan_enable();
 
 	pr_info("KernelAddressSanitizer initialized (hw-tags, mode=%s, vmalloc=%s, stacktrace=%s)\n",
 		kasan_mode_info(),
diff --git a/mm/kasan/sw_tags.c b/mm/kasan/sw_tags.c
index b9382b5b6a3..275bcbbf612 100644
--- a/mm/kasan/sw_tags.c
+++ b/mm/kasan/sw_tags.c
@@ -45,6 +45,8 @@ void __init kasan_init_sw_tags(void)
 
 	kasan_init_tags();
 
+	kasan_enable();
+
 	pr_info("KernelAddressSanitizer initialized (sw-tags, stacktrace=%s)\n",
 		str_on_off(kasan_stack_collection_enabled()));
 }
-- 
2.34.1