Parameter Expansion

The GRR configuration file format allows for expansion of configuration parameters inside other parameters. For example consider the following configuration file: GRR

Nanny.service_name: "%("
Nanny.service_key_hive: HKEY_LOCAL_MACHINE
Nanny.service_key: Software\\%(
Nanny.child_command_line: |
  %(child_binary) --config "%(Client.install_path)\\%(Client.binary_name).yaml"

The expansion sequence %(parameter_name) substitutes or expands the parameter into the string. This allows us to build values automatically based on other values. For example above, the Nanny.service_name will be GRRservice.exe and the service_key will be set to Software\GRR

Expansion is very useful, but sometimes it gets in the way. For example, if we need to pass literal % escape sequences. Consider the Logging.format parameter which is actually a python format string:

Logging.format: \%(levelname\)s \%(module\)s:\%(lineno\)s] \%(message\)s
Logging.format: %{%(levelname)s %(module)s:%(lineno)s] %(message)s}
  • This form escapes GRR’s special escaping sequence by preceding both opening and closing sequences with the backslash character.

  • This variation uses the literal expansion sequence %{} to declare the entire line as a literal string and prevent expansion.