kasan: respect CONFIG_KASAN_VMALLOC for kasan_flag_vmalloc

Never enable the kasan_flag_vmalloc static branch unless
CONFIG_KASAN_VMALLOC is enabled.

This does not fix any observable bugs (vmalloc annotations for the HW_TAGS
mode are no-op with CONFIG_KASAN_VMALLOC disabled) but rather just cleans
up the code.

Link: https://lkml.kernel.org/r/3e5c933c8f6b59bd587efb05c407964be951772c.1703188911.git.andreyknvl@google.comSigned-off-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/kasan/hw_tags.c

@@ -57,7 +57,11 @@ enum kasan_mode kasan_mode __ro_after_init;
 EXPORT_SYMBOL_GPL(kasan_mode);
 
 /* Whether to enable vmalloc tagging. */
+#ifdef CONFIG_KASAN_VMALLOC
 DEFINE_STATIC_KEY_TRUE(kasan_flag_vmalloc);
+#else
+DEFINE_STATIC_KEY_FALSE(kasan_flag_vmalloc);
+#endif
 
 #define PAGE_ALLOC_SAMPLE_DEFAULT	1
 #define PAGE_ALLOC_SAMPLE_ORDER_DEFAULT	3
@@ -119,6 +123,9 @@ static int __init early_kasan_flag_vmalloc(char *arg)
 	if (!arg)
 		return -EINVAL;
 
+	if (!IS_ENABLED(CONFIG_KASAN_VMALLOC))
+		return 0;
+
 	if (!strcmp(arg, "off"))
 		kasan_arg_vmalloc = KASAN_ARG_VMALLOC_OFF;
 	else if (!strcmp(arg, "on"))

mm/kasan/kasan.h

@@ -49,6 +49,7 @@ DECLARE_PER_CPU(long, kasan_page_alloc_skip);
 
 static inline bool kasan_vmalloc_enabled(void)
 {
+	/* Static branch is never enabled with CONFIG_KASAN_VMALLOC disabled. */
 	return static_branch_likely(&kasan_flag_vmalloc);
 }