MemoryManager: Add validate_userspace_string()

A bit crude, should be replaced by a strdup_from_user() helper to avoid touching userspace memory directly.
But it'll do for now.

kernel/src/memory/MemoryManager.cpp

@@ -347,6 +347,23 @@ namespace MemoryManager
         return false;
     }
 
+    // FIXME: Replace this with some kind of strdup_from_user() function.
+    bool validate_userspace_string(u64 address)
+    {
+        if (!validate_readable_page(address)) return false;
+
+        while (*(char*)address != 0)
+        {
+            address++;
+            if (address % ARCH_PAGE_SIZE)
+            {
+                if (!validate_readable_page(address)) return false;
+            }
+        }
+
+        return true;
+    }
+
     usize free()
     {
         return free_mem;

kernel/src/memory/MemoryManager.h

@@ -20,6 +20,8 @@ namespace MemoryManager
     bool validate_readable_page(u64 address);
     bool validate_writable_page(u64 address);
 
+    bool validate_userspace_string(u64 address);
+
     Result<void> map_frames_at(u64 virt, u64 phys, usize count, int flags);
 
     Result<u64> alloc_at(u64 virt, usize count, int flags);