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1Deterministic Automata Monitor Synthesis2========================================3 4The starting point for the application of runtime verification (RV) techniques5is the *specification* or *modeling* of the desired (or undesired) behavior6of the system under scrutiny.7 8The formal representation needs to be then *synthesized* into a *monitor*9that can then be used in the analysis of the trace of the system. The10*monitor* connects to the system via an *instrumentation* that converts11the events from the *system* to the events of the *specification*.12 13 14In Linux terms, the runtime verification monitors are encapsulated inside15the *RV monitor* abstraction. The RV monitor includes a set of instances16of the monitor (per-cpu monitor, per-task monitor, and so on), the helper17functions that glue the monitor to the system reference model, and the18trace output as a reaction to event parsing and exceptions, as depicted19below::20 21 Linux +----- RV Monitor ----------------------------------+ Formal22 Realm | | Realm23 +-------------------+ +----------------+ +-----------------+24 | Linux kernel | | Monitor | | Reference |25 | Tracing | -> | Instance(s) | <- | Model |26 | (instrumentation) | | (verification) | | (specification) |27 +-------------------+ +----------------+ +-----------------+28 | | |29 | V |30 | +----------+ |31 | | Reaction | |32 | +--+--+--+-+ |33 | | | | |34 | | | +-> trace output ? |35 +------------------------|--|----------------------+36 | +----> panic ?37 +-------> <user-specified>38 39DA monitor synthesis40--------------------41 42The synthesis of automata-based models into the Linux *RV monitor* abstraction43is automated by the dot2k tool and the rv/da_monitor.h header file that44contains a set of macros that automatically generate the monitor's code.45 46dot2k47-----48 49The dot2k utility leverages dot2c by converting an automaton model in50the DOT format into the C representation [1] and creating the skeleton of51a kernel monitor in C.52 53For example, it is possible to transform the wip.dot model present in54[1] into a per-cpu monitor with the following command::55 56 $ dot2k -d wip.dot -t per_cpu57 58This will create a directory named wip/ with the following files:59 60- wip.h: the wip model in C61- wip.c: the RV monitor62 63The wip.c file contains the monitor declaration and the starting point for64the system instrumentation.65 66Monitor macros67--------------68 69The rv/da_monitor.h enables automatic code generation for the *Monitor70Instance(s)* using C macros.71 72The benefits of the usage of macro for monitor synthesis are 3-fold as it:73 74- Reduces the code duplication;75- Facilitates the bug fix/improvement;76- Avoids the case of developers changing the core of the monitor code77 to manipulate the model in a (let's say) non-standard way.78 79This initial implementation presents three different types of monitor instances:80 81- ``#define DECLARE_DA_MON_GLOBAL(name, type)``82- ``#define DECLARE_DA_MON_PER_CPU(name, type)``83- ``#define DECLARE_DA_MON_PER_TASK(name, type)``84 85The first declares the functions for a global deterministic automata monitor,86the second for monitors with per-cpu instances, and the third with per-task87instances.88 89In all cases, the 'name' argument is a string that identifies the monitor, and90the 'type' argument is the data type used by dot2k on the representation of91the model in C.92 93For example, the wip model with two states and three events can be94stored in an 'unsigned char' type. Considering that the preemption control95is a per-cpu behavior, the monitor declaration in the 'wip.c' file is::96 97 DECLARE_DA_MON_PER_CPU(wip, unsigned char);98 99The monitor is executed by sending events to be processed via the functions100presented below::101 102 da_handle_event_$(MONITOR_NAME)($(event from event enum));103 da_handle_start_event_$(MONITOR_NAME)($(event from event enum));104 da_handle_start_run_event_$(MONITOR_NAME)($(event from event enum));105 106The function ``da_handle_event_$(MONITOR_NAME)()`` is the regular case where107the event will be processed if the monitor is processing events.108 109When a monitor is enabled, it is placed in the initial state of the automata.110However, the monitor does not know if the system is in the *initial state*.111 112The ``da_handle_start_event_$(MONITOR_NAME)()`` function is used to notify the113monitor that the system is returning to the initial state, so the monitor can114start monitoring the next event.115 116The ``da_handle_start_run_event_$(MONITOR_NAME)()`` function is used to notify117the monitor that the system is known to be in the initial state, so the118monitor can start monitoring and monitor the current event.119 120Using the wip model as example, the events "preempt_disable" and121"sched_waking" should be sent to monitor, respectively, via [2]::122 123 da_handle_event_wip(preempt_disable_wip);124 da_handle_event_wip(sched_waking_wip);125 126While the event "preempt_enabled" will use::127 128 da_handle_start_event_wip(preempt_enable_wip);129 130To notify the monitor that the system will be returning to the initial state,131so the system and the monitor should be in sync.132 133Final remarks134-------------135 136With the monitor synthesis in place using the rv/da_monitor.h and137dot2k, the developer's work should be limited to the instrumentation138of the system, increasing the confidence in the overall approach.139 140[1] For details about deterministic automata format and the translation141from one representation to another, see::142 143 Documentation/trace/rv/deterministic_automata.rst144 145[2] dot2k appends the monitor's name suffix to the events enums to146avoid conflicting variables when exporting the global vmlinux.h147use by BPF programs.148