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diff --git a/Documentation/trace/rv/da_monitor_synthesis.rst b/Documentation/trace/rv/da_monitor_synthesis.rst new file mode 100644 index 000000000000..0dbdcd1e62b9 --- /dev/null +++ b/Documentation/trace/rv/da_monitor_synthesis.rst @@ -0,0 +1,147 @@ +Deterministic Automata Monitor Synthesis +======================================== + +The starting point for the application of runtime verification (RV) technics +is the *specification* or *modeling* of the desired (or undesired) behavior +of the system under scrutiny. + +The formal representation needs to be then *synthesized* into a *monitor* +that can then be used in the analysis of the trace of the system. The +*monitor* connects to the system via an *instrumentation* that converts +the events from the *system* to the events of the *specification*. + + +In Linux terms, the runtime verification monitors are encapsulated inside +the *RV monitor* abstraction. The RV monitor includes a set of instances +of the monitor (per-cpu monitor, per-task monitor, and so on), the helper +functions that glue the monitor to the system reference model, and the +trace output as a reaction to event parsing and exceptions, 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> + +DA monitor synthesis +-------------------- + +The synthesis of automata-based models into the Linux *RV monitor* abstraction +is automated by the dot2k tool and the rv/da_monitor.h header file that +contains a set of macros that automatically generate the monitor's code. + +dot2k +----- + +The dot2k utility leverages dot2c by converting an automaton model in +the DOT format into the C representation [1] and creating the skeleton of +a kernel monitor in C. + +For example, it is possible to transform the wip.dot model present in +[1] into a per-cpu monitor with the following command:: + + $ dot2k -d wip.dot -t per_cpu + +This will create a directory named wip/ with the following files: + +- wip.h: the wip model in C +- wip.c: the RV monitor + +The wip.c file contains the monitor declaration and the starting point for +the system instrumentation. + +Monitor macros +-------------- + +The rv/da_monitor.h enables automatic code generation for the *Monitor +Instance(s)* using C macros. + +The benefits of the usage of macro for monitor synthesis are 3-fold as it: + +- Reduces the code duplication; +- Facilitates the bug fix/improvement; +- Avoids the case of developers changing the core of the monitor code + to manipulate the model in a (let's say) non-standard way. + +This initial implementation presents three different types of monitor instances: + +- ``#define DECLARE_DA_MON_GLOBAL(name, type)`` +- ``#define DECLARE_DA_MON_PER_CPU(name, type)`` +- ``#define DECLARE_DA_MON_PER_TASK(name, type)`` + +The first declares the functions for a global deterministic automata monitor, +the second for monitors with per-cpu instances, and the third with per-task +instances. + +In all cases, the 'name' argument is a string that identifies the monitor, and +the 'type' argument is the data type used by dot2k on the representation of +the model in C. + +For example, the wip model with two states and three events can be +stored in an 'unsigned char' type. Considering that the preemption control +is a per-cpu behavior, the monitor declaration in the 'wip.c' file is:: + + DECLARE_DA_MON_PER_CPU(wip, unsigned char); + +The monitor is executed by sending events to be processed via the functions +presented below:: + + da_handle_event_$(MONITOR_NAME)($(event from event enum)); + da_handle_start_event_$(MONITOR_NAME)($(event from event enum)); + da_handle_start_run_event_$(MONITOR_NAME)($(event from event enum)); + +The function ``da_handle_event_$(MONITOR_NAME)()`` is the regular case where +the event will be processed if the monitor is processing events. + +When a monitor is enabled, it is placed in the initial state of the automata. +However, the monitor does not know if the system is in the *initial state*. + +The ``da_handle_start_event_$(MONITOR_NAME)()`` function is used to notify the +monitor that the system is returning to the initial state, so the monitor can +start monitoring the next event. + +The ``da_handle_start_run_event_$(MONITOR_NAME)()`` function is used to notify +the monitor that the system is known to be in the initial state, so the +monitor can start monitoring and monitor the current event. + +Using the wip model as example, the events "preempt_disable" and +"sched_waking" should be sent to monitor, respectively, via [2]:: + + da_handle_event_wip(preempt_disable_wip); + da_handle_event_wip(sched_waking_wip); + +While the event "preempt_enabled" will use:: + + da_handle_start_event_wip(preempt_enable_wip); + +To notify the monitor that the system will be returning to the initial state, +so the system and the monitor should be in sync. + +Final remarks +------------- + +With the monitor synthesis in place using the rv/da_monitor.h and +dot2k, the developer's work should be limited to the instrumentation +of the system, increasing the confidence in the overall approach. + +[1] For details about deterministic automata format and the translation +from one representation to another, see:: + + Documentation/trace/rv/deterministic_automata.rst + +[2] dot2k appends the monitor's name suffix to the events enums to +avoid conflicting variables when exporting the global vmlinux.h +use by BPF programs. |