kasan: clean up kasan_requires_meta

Currently, for Generic KASAN mode, kasan_requires_meta is defined to
return kasan_stack_collection_enabled.

Even though the Generic mode does not support disabling stack trace
collection, kasan_requires_meta was implemented in this way to make it
easier to implement the disabling for the Generic mode in the future.

However, for the Generic mode, the per-object metadata also stores the
quarantine link.  So even if disabling stack collection is implemented,
the per-object metadata will still be required.

Fix kasan_requires_meta to return true for the Generic mode and update the
related comments.

This change does not fix any observable bugs but rather just brings the
code to a cleaner state.

Link: https://lkml.kernel.org/r/8086623407095ac1c82377a2107dcc5845f99cfa.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/kasan.h

@@ -101,21 +101,21 @@ static inline bool kasan_sample_page_alloc(unsigned int order)
 
 #ifdef CONFIG_KASAN_GENERIC
 
-/* Generic KASAN uses per-object metadata to store stack traces. */
+/*
+ * Generic KASAN uses per-object metadata to store alloc and free stack traces
+ * and the quarantine link.
+ */
 static inline bool kasan_requires_meta(void)
 {
-	/*
-	 * Technically, Generic KASAN always collects stack traces right now.
-	 * However, let's use kasan_stack_collection_enabled() in case the
-	 * kasan.stacktrace command-line argument is changed to affect
-	 * Generic KASAN.
-	 */
-	return kasan_stack_collection_enabled();
+	return true;
 }
 
 #else /* CONFIG_KASAN_GENERIC */
 
-/* Tag-based KASAN modes do not use per-object metadata. */
+/*
+ * Tag-based KASAN modes do not use per-object metadata: they use the stack
+ * ring to store alloc and free stack traces and do not use qurantine.
+ */
 static inline bool kasan_requires_meta(void)
 {
 	return false;