keys: Provide a key_try_get() function and use it

Add a key_try_get() function to try to get a ref on a key and switch code
that's manipulating the key refcount directly to use it.  This will allow a
tracepoint to be emplaced there later.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jarkko Sakkinen <jarkko@kernel.org>
cc: keyrings@vger.kernel.org
cc: linux-security-module@vger.kernel.org

include/linux/key.h

@@ -300,6 +300,7 @@ extern struct key *key_alloc(struct key_type *type,
 extern void key_revoke(struct key *key);
 extern void key_invalidate(struct key *key);
 struct key *key_get(struct key *key);
+struct key *key_try_get(struct key *key);
 extern void key_put(struct key *key);
 extern bool key_put_tag(struct key_tag *tag);
 extern void key_remove_domain(struct key_tag *domain_tag);

security/keys/key.c

@@ -649,6 +649,20 @@ struct key *key_get(struct key *key)
 }
 EXPORT_SYMBOL(key_get);
 
+/**
+ * key_try_get - Get a ref on a key if its refcount is not non-zero.
+ * @key: The key to get a reference on.
+ *
+ * Get a reference on a key unless it has no references and return true if
+ * successful.  @key must not be NULL.
+ */
+struct key *key_try_get(struct key *key)
+{
+	if (!refcount_inc_not_zero(&key->usage))
+		return NULL;
+	return key;
+}
+
 /**
  * key_put - Discard a reference to a key.
  * @key: The key to discard a reference from.
@@ -709,7 +723,7 @@ struct key *key_lookup(key_serial_t id)
 	/* A key is allowed to be looked up only if someone still owns a
 	 * reference to it - otherwise it's awaiting the gc.
 	 */
-	if (!refcount_inc_not_zero(&key->usage))
+	if (!key_try_get(key))
 		goto not_found;
 
 error:

security/keys/keyring.c

@@ -1174,7 +1174,7 @@ struct key *find_keyring_by_name(const char *name, bool uid_keyring)
 		/* we've got a match but we might end up racing with
 		 * key_cleanup() if the keyring is currently 'dead'
 		 * (ie. it has a zero usage count) */
-		if (!refcount_inc_not_zero(&keyring->usage))
+		if (!key_try_get(keyring))
 			continue;
 		keyring->last_used_at = ktime_get_real_seconds();
 		goto out;