Documentation/trace/rv/runtime-verification.rst

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====================
Runtime Verification
====================

Runtime Verification (RV) is a lightweight (yet rigorous) method that
complements classical exhaustive verification techniques (such as *model
checking* and *theorem proving*) with a more practical approach for complex
systems.

Instead of relying on a fine-grained model of a system (e.g., a
re-implementation at instruction level), RV works by analyzing the trace of the
system's actual execution, comparing it against a formal specification of
the system behavior.

The main advantage is that RV can give precise information on the runtime
behavior of the monitored system, without the pitfalls of developing models
that require a re-implementation of the entire system in a modeling language.
Moreover, given an efficient monitoring method, it is possible to execute an
*online* verification of a system, enabling the *reaction* for unexpected
events, avoiding, for example, the propagation of a failure on safety-critical
systems.

Runtime Monitors and Reactors
=============================

A monitor is the central part of the runtime verification of a system. The
monitor stands in between the formal specification of the desired (or
undesired) behavior, and the trace of the actual system.

In Linux terms, the runtime verification monitors are encapsulated inside the
*RV monitor* abstraction. A *RV monitor* includes a reference model of the
system, a set of instances of the monitor (per-cpu monitor, per-task monitor,
and so on), and the helper functions that glue the monitor to the system via
trace, as depicted below::

 Linux   +---- RV Monitor ----------------------------------+ Formal
  Realm  |                                                  |  Realm
  +-------------------+     +----------------+     +-----------------+
  |   Linux kernel    |     |     Monitor    |     |     Reference   |
  |     Tracing       |  -> |   Instance(s)  | <-  |       Model     |
  | (instrumentation) |     | (verification) |     | (specification) |
  +-------------------+     +----------------+     +-----------------+
         |                          |                       |
         |                          V                       |
         |                     +----------+                 |
         |                     | Reaction |                 |
         |                     +--+--+--+-+                 |
         |                        |  |  |                   |
         |                        |  |  +-> trace output ?  |
         +------------------------|--|----------------------+
                                  |  +----> panic ?
                                  +-------> <user-specified>

In addition to the verification and monitoring of the system, a monitor can
react to an unexpected event. The forms of reaction can vary from logging the
event occurrence to the enforcement of the correct behavior to the extreme
action of taking a system down to avoid the propagation of a failure.

In Linux terms, a *reactor* is an reaction method available for *RV monitors*.
By default, all monitors should provide a trace output of their actions,
which is already a reaction. In addition, other reactions will be available
so the user can enable them as needed.

For further information about the principles of runtime verification and
RV applied to Linux:

  Bartocci, Ezio, et al. *Introduction to runtime verification.* In: Lectures on
  Runtime Verification. Springer, Cham, 2018. p. 1-33.

  Falcone, Ylies, et al. *A taxonomy for classifying runtime verification tools.*

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