Bluetooth: Host: Define bt_smp_err_to_str()

This API converts a SMP error code to a string.
This can be useful if application developers want
to print them in the applications.

BT_SMP_ERR_SUCCESS was added for completeness.

Later we can also use them in the host to improve debuggability.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>

subsys/bluetooth/host/smp.c

@@ -500,6 +500,38 @@ static uint8_t smp_err_get(enum bt_security_err auth_err)
 	}
 }
 
+const char *bt_smp_err_to_str(uint8_t smp_err)
+{
+	#define SMP_ERR(err) [err] = #err
+
+	const char * const mapping_table[] = {
+		SMP_ERR(BT_SMP_ERR_SUCCESS),
+		SMP_ERR(BT_SMP_ERR_PASSKEY_ENTRY_FAILED),
+		SMP_ERR(BT_SMP_ERR_OOB_NOT_AVAIL),
+		SMP_ERR(BT_SMP_ERR_AUTH_REQUIREMENTS),
+		SMP_ERR(BT_SMP_ERR_CONFIRM_FAILED),
+		SMP_ERR(BT_SMP_ERR_PAIRING_NOTSUPP),
+		SMP_ERR(BT_SMP_ERR_ENC_KEY_SIZE),
+		SMP_ERR(BT_SMP_ERR_CMD_NOTSUPP),
+		SMP_ERR(BT_SMP_ERR_UNSPECIFIED),
+		SMP_ERR(BT_SMP_ERR_REPEATED_ATTEMPTS),
+		SMP_ERR(BT_SMP_ERR_INVALID_PARAMS),
+		SMP_ERR(BT_SMP_ERR_DHKEY_CHECK_FAILED),
+		SMP_ERR(BT_SMP_ERR_NUMERIC_COMP_FAILED),
+		SMP_ERR(BT_SMP_ERR_BREDR_PAIRING_IN_PROGRESS),
+		SMP_ERR(BT_SMP_ERR_CROSS_TRANSP_NOT_ALLOWED),
+		SMP_ERR(BT_SMP_ERR_KEY_REJECTED),
+	};
+
+	if (smp_err < ARRAY_SIZE(mapping_table) && mapping_table[smp_err]) {
+		return mapping_table[smp_err];
+	} else {
+		return "(unknown)";
+	}
+
+	#undef SMP_ERR
+}
+
 static struct net_buf *smp_create_pdu(struct bt_smp *smp, uint8_t op, size_t len)
 {
 	struct bt_smp_hdr *hdr;

subsys/bluetooth/host/smp.h

@@ -13,6 +13,7 @@ struct bt_smp_hdr {
 	uint8_t  code;
 } __packed;
 
+#define BT_SMP_ERR_SUCCESS                      0x00
 #define BT_SMP_ERR_PASSKEY_ENTRY_FAILED		0x01
 #define BT_SMP_ERR_OOB_NOT_AVAIL		0x02
 #define BT_SMP_ERR_AUTH_REQUIREMENTS		0x03
@@ -178,3 +179,17 @@ int bt_smp_sign(struct bt_conn *conn, struct net_buf *buf);
 
 /** Generate IRK from Identity Root (IR) */
 int bt_smp_irk_get(uint8_t *ir, uint8_t *irk);
+
+/** Converts a SMP error to string.
+ *
+ * The error codes are described in the Bluetooth Core specification,
+ * Vol 3, Part H, Section 3.5.5.
+ *
+ * The Security Manager Protocol documentation found in Vol 4, Part H,
+ * describes when the different error codes are used.
+ *
+ * See also the defined BT_SMP_ERR_* macros.
+ *
+ * @return The string representation of the SMP error code.
+ */
+const char *bt_smp_err_to_str(uint8_t smp_err);

tests/bluetooth/smp/CMakeLists.txt

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(bluetooth_gatt)
+
+FILE(GLOB app_sources src/*.c)
+target_sources(app PRIVATE ${app_sources})

tests/bluetooth/smp/prj.conf

@@ -0,0 +1,10 @@
+CONFIG_TEST=y
+CONFIG_ZTEST=y
+
+CONFIG_BT=y
+CONFIG_BT_SMP=y
+CONFIG_BT_CTLR=n
+CONFIG_BT_H4=n
+
+CONFIG_LOG=y
+CONFIG_BT_PERIPHERAL=y

tests/bluetooth/smp/src/main.c

@@ -0,0 +1,38 @@
+/* main.c - Application main entry point */
+
+/*
+ * Copyright (c) 2024 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/kernel.h>
+#include <stddef.h>
+#include <zephyr/ztest.h>
+
+#include <zephyr/bluetooth/bluetooth.h>
+#include <zephyr/bluetooth/conn.h>
+#include <../subsys/bluetooth/host/smp.h>
+
+ZTEST_SUITE(test_smp, NULL, NULL, NULL, NULL, NULL);
+
+ZTEST(test_smp, test_bt_smp_err_to_str)
+{
+	/* Test a couple of entries */
+	zassert_str_equal(bt_smp_err_to_str(0x00),
+			  "BT_SMP_ERR_SUCCESS");
+	zassert_str_equal(bt_smp_err_to_str(0x0a),
+			  "BT_SMP_ERR_INVALID_PARAMS");
+	zassert_str_equal(bt_smp_err_to_str(0x0F),
+			  "BT_SMP_ERR_KEY_REJECTED");
+
+	/* Test entries that are not used */
+	zassert_mem_equal(bt_smp_err_to_str(0x10),
+			  "(unknown)", strlen("(unknown)"));
+	zassert_mem_equal(bt_smp_err_to_str(0xFF),
+			  "(unknown)", strlen("(unknown)"));
+
+	for (uint16_t i = 0; i <= UINT8_MAX; i++) {
+		zassert_not_null(bt_smp_err_to_str(i), ": %d", i);
+	}
+}

tests/bluetooth/smp/test.overlay

@@ -0,0 +1,5 @@
+/ {
+	chosen {
+		/delete-property/ zephyr,bt-hci;
+	};
+};

tests/bluetooth/smp/testcase.yaml

@@ -0,0 +1,16 @@
+tests:
+  bluetooth.smp:
+    extra_args:
+      - EXTRA_DTC_OVERLAY_FILE="test.overlay"
+    platform_allow:
+      - native_posix
+      - native_posix/native/64
+      - native_sim
+      - native_sim/native/64
+      - qemu_x86
+      - qemu_cortex_m3
+    integration_platforms:
+      - native_sim
+    tags:
+      - bluetooth
+      - smp