lib/stackdepot.c: use a non-instrumented version of memcmp()

stackdepot used to call memcmp(), which compiler tools normally
instrument, therefore every lookup used to unnecessarily call instrumented
code.  This is somewhat ok in the case of KASAN, but under KMSAN a lot of
time was spent in the instrumentation.

Link: http://lkml.kernel.org/r/20171117172149.69562-1-glider@google.com
Signed-off-by: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/stackdepot.c

@@ -163,6 +163,21 @@ static inline u32 hash_stack(unsigned long *entries, unsigned int size)
 			       STACK_HASH_SEED);
 }
 
+/* Use our own, non-instrumented version of memcmp().
+ *
+ * We actually don't care about the order, just the equality.
+ */
+static inline
+int stackdepot_memcmp(const unsigned long *u1, const unsigned long *u2,
+			unsigned int n)
+{
+	for ( ; n-- ; u1++, u2++) {
+		if (*u1 != *u2)
+			return 1;
+	}
+	return 0;
+}
+
 /* Find a stack that is equal to the one stored in entries in the hash */
 static inline struct stack_record *find_stack(struct stack_record *bucket,
 					     unsigned long *entries, int size,
@@ -173,10 +188,8 @@ static inline struct stack_record *find_stack(struct stack_record *bucket,
 	for (found = bucket; found; found = found->next) {
 		if (found->hash == hash &&
 		    found->size == size &&
-		    !memcmp(entries, found->entries,
-			    size * sizeof(unsigned long))) {
+		    !stackdepot_memcmp(entries, found->entries, size))
 			return found;
-		}
 	}
 	return NULL;
 }