[PATCH v2] lib/stackdepot: use a non-instrumented version of memcmp()

[Alexander Potapenko <glider@google.com>]

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.

Signed-off-by: Alexander Potapenko <glider@google.com>
---
v2: As requested by Andrey Ryabinin, made the parameters unsigned long *
---
 lib/stackdepot.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index f87d138e9672..e513459a5601 100644
--- a/lib/stackdepot.c
+++ b/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;
 }
-- 
2.15.0.448.gf294e3d99a-goog