strncpy out of bounds fix (#1785)

* Fix a bug in strncpy and add some tests that verify the behavior

* null-terminate destination buffer in emb_strncpy

embrace-android-sdk/src/main/cpp/utils/string_utils.c

@@ -1,19 +1,24 @@
 #include "string_utils.h"
 #include "../schema/stack_frames.h"
 
-void emb_strncpy(char *dst, const char *src, size_t len) {
-    if (dst == NULL || src == NULL) {
+/**
+ * Copy a string from source to destination. If the source is larger than the destination, it will be truncated.
+ * The destination buffer will always be null-terminated.
+ * @param destination The destination buffer.
+ * @param source The source buffer.
+ * @param destination_len The length of the destination buffer.
+ */
+void emb_strncpy(char *destination, const char *source, size_t destination_len) {
+    if (destination == NULL || source == NULL || destination_len == 0) {
         return;
     }
-    int i = 0;
-    while (i <= len) {
-        char current = src[i];
-        dst[i] = current;
-        if (current == '\0') {
-            break;
-        }
-        i++;
+
+    size_t i;
+    for (i = 0; i < destination_len - 1 && source[i] != '\0'; i++) {
+        destination[i] = source[i];
     }
+
+    destination[i] = '\0'; // Null-terminate the destination buffer.
 }
 
 void emb_convert_to_hex_addr(uint64_t addr, char *buffer) {

embrace-android-sdk/src/main/cpp/utils/string_utils.h

@@ -5,7 +5,7 @@
 extern "C" {
 #endif
 
-void emb_strncpy(char *dst, const char *src, size_t len);
+void emb_strncpy(char *destination, const char *source, size_t destination_len);
 void emb_convert_to_hex_addr(uint64_t addr, char *buffer);
 
 #ifdef __cplusplus

embrace-android-sdk/src/test/cpp/testcases/utilities/test_string_utils.c

@@ -29,9 +29,27 @@ TEST convert_to_hex(void) {
     PASS();
 }
 
+TEST test_string_copy_larger_source(void) {
+    char dst[2];
+    char src[] = "123456";
+    emb_strncpy(dst, src, sizeof(dst));
+    ASSERT_STR_EQ("1", dst);    // the rest of the source is truncated to size 2 (1 and a null terminator)
+    PASS();
+}
+
+TEST test_string_copy_larger_destination(void) {
+    char dst[6] = {0};
+    char src[] = "12";
+    emb_strncpy(dst, src, sizeof(dst));
+    ASSERT_STR_EQ("12", dst);
+    PASS();
+}
+
 SUITE(suite_utilities) {
     RUN_TEST(string_copy_success);
     RUN_TEST(string_null_src);
     RUN_TEST(string_null_dst);
     RUN_TEST(convert_to_hex);
+    RUN_TEST(test_string_copy_larger_source);
+    RUN_TEST(test_string_copy_larger_destination);
 }