(string_list_join_concat) Prevent undefined behaviour if input string buffer is unterminated

libretro-common/include/string/stdstring.h

@@ -70,7 +70,18 @@ static INLINE bool string_ends_with(const char *str, const char *suffix)
    return string_ends_with_size(str, suffix, strlen(str), strlen(suffix));
 }
 
-
+/* Returns the length of 'str' (c.f. strlen()), but only
+ * checks the first 'size' characters
+ * - If 'str' is NULL, returns 0
+ * - If 'str' is not NULL and no '\0' character is found
+ *   in the first 'size' characters, returns 'size' */
+static INLINE size_t strlen_size(const char *str, size_t size)
+{
+   size_t i = 0;
+   if (str)
+      for(; (i < size) && str[i]; ++i);
+   return i;
+}
 
 #define STRLEN_CONST(x)                   ((sizeof((x))-1))
 

libretro-common/lists/string_list.c

@@ -245,7 +245,18 @@ void string_list_set(struct string_list *list,
 void string_list_join_concat(char *buffer, size_t size,
       const struct string_list *list, const char *delim)
 {
-   size_t i, len = strlen(buffer);
+   size_t i;
+   size_t len = strlen_size(buffer, size);
+
+   /* If buffer is already 'full', nothing
+    * further can be added
+    * > This condition will also be triggered
+    *   if buffer is not NUL-terminated,
+    *   in which case any attempt to increment
+    *   buffer or decrement size would lead to
+    *   undefined behaviour */
+   if (len >= size)
+      return;
 
    buffer += len;
    size   -= len;