Move the length check, and require a non-zero length to protect is_valid.

volatility/framework/layers/physical.py

@@ -80,6 +80,8 @@ class FileLayer(interfaces.layers.DataLayerInterface):
 
     def is_valid(self, offset, length = 1):
         """Returns whether the offset is valid or not"""
+        if length <= 0:
+            raise TypeError("Length must be positive")
         return (self.minimum_address <= offset <= self.maximum_address and
                 self.minimum_address <= offset + length - 1 <= self.maximum_address)
 
@@ -87,8 +89,6 @@ class FileLayer(interfaces.layers.DataLayerInterface):
         """Reads from the file at offset for length"""
         if not self.is_valid(offset, length):
             raise exceptions.InvalidAddressException("Offset outside of the " + self.name + " file boundaries")
-        if length < 0:
-            raise TypeError("Length must be positive")
         self._file.seek(offset)
         data = self._file.read(length)
         if len(data) < length: