[FS] Add app data folder, add path safety checks

include/fs/archive_base.hpp

@@ -1,6 +1,8 @@
 #pragma once
+#include <cassert>
 #include <optional>
 #include <string>
+#include <type_traits>
 #include <vector>
 #include "helpers.hpp"
 #include "memory.hpp"
@@ -93,6 +95,51 @@ protected:
     using Handle = u32;
     Memory& mem;
 
+    // Returns if a specified 3DS path in UTF16 or ASCII format is safe or not
+    // A 3DS path is considered safe if its first character is '/' which means we're not trying to access anything outside the root of the fs
+    // And if it doesn't contain enough instances of ".." (Indicating "climb up a folder" in filesystems) to let the software climb up the directory tree
+    // And access files outside of the emulator's app data folder
+    template <u32 format>
+    bool isPathSafe(const FSPath& path) {
+        static_assert(format == PathType::ASCII || format == PathType::UTF16);
+        using String = std::conditional<format == PathType::UTF16, std::u16string, std::string>::type; // String type for the path
+        using Char = String::value_type; // Char type for the path
+
+        String pathString, dots;
+        if constexpr (std::is_same<String, std::u16string>()) {
+            pathString = path.utf16_string;
+            dots = u"..";
+        } else {
+            pathString = path.string;
+            dots = "..";
+        }
+
+        // If the path string doesn't begin with / then that means it's accessing outside the FS root, which is invalid & unsafe
+        if (pathString[0] != Char('/')) return false;
+
+        // Counts how many folders sans the root our file is nested under. 
+        // If it's < 0 at any point of parsing, then the path is unsafe and tries to crawl outside our file sandbox.
+        // If it's 0 then this is the FS root.
+        // If it's > 0 then we're in a subdirectory of the root.
+        int level = 0;
+        
+        // Split the string on / characters and see how many of the substrings are ".."
+        size_t pos = 0;
+        while ((pos = pathString.find(Char('/'))) != String::npos) {
+            String token = pathString.substr(0, pos);
+            pathString.erase(0, pos + 1);
+
+            if (token == dots) {
+                level--;
+                if (level < 0) return false;
+            } else {
+                level++;
+            }
+        }
+
+        return true;
+    }
+
 public:
     virtual std::string name() = 0;
     virtual u64 getFreeBytes() = 0;