sdk/include/fail-simulator-on-error.h

#include "compartment.h"
#include <cheri.hh>
#include <debug.hh>
#include <priv/riscv.h>
#include <simulator.h>

using DebugErrorHandler = ConditionalDebug<
#ifdef NDEBUG
  false
#else
  true
#endif
  ,
  "Error handler">;

/**
 * A error handler that reports unexpected traps and calls the scheduler to
 * request that the simulator exit with exit code 1 (FAILURE).
 *
 * This is useful because the default behaviour in the absence of an error
 * handler is to unwind the trusted stack and exit the thread when the top of
 * the stack is reached. If all threads exit then the scheduler will exit the
 * simulation with SUCCESS, which might mask errors.
 *
 * We attempt to detect the trap that occurs when a thread returns from its
 * entry point function and do not halt the simulation as this is considered
 * normal.
 *
 * Unwinds caused by errors in called compartments are also detected and are not
 * treated as errors -- instead we resume execution at the call point with a
 * return value of -1 as though this handler is not present.
 *
 * On non-simulation platforms this error handler reverts to the default force
 * unwind behaviour.
 *
 * If NDEBUG is not defined then errors and thread exits will be logged.
 *
 * To use this simply include this file in your compartment's c / cc file.
 */
extern "C" ErrorRecoveryBehaviour
compartment_error_handler(ErrorState *frame, size_t mcause, size_t mtval)
{
	if (mcause == priv::MCAUSE_CHERI)
	{
		// An unexpected error -- log it and end the simulation with error.
		// Note: handle CZR differently as `get_register_value` will return a
		// nullptr which we cannot dereference.

		auto [exceptionCode, registerNumber] =
		  CHERI::extract_cheri_mtval(mtval);

		DebugErrorHandler::log(
		  "{} error at {} (return address: {}), with capability register "
		  "{}: {}",
		  exceptionCode,
		  frame->pcc,
		  frame->get_register_value<CHERI::RegisterNumber::CRA>(),
		  registerNumber,
		  registerNumber == CHERI::RegisterNumber::CZR
		    ? nullptr
		    : *frame->get_register_value(registerNumber));
	}
	else
	{
		// other error (e.g. __builtin_trap causes ReservedInstruciton)
		// log and end simulation with error.
		DebugErrorHandler::log("Unhandled error {} at {}", mcause, frame->pcc);
	}

#ifdef SIMULATION
	/*
	 * simulation exit may fail (say, we're not on a simulator or there isn't
	 * enough stack space to invoke the function.  In that case, just fall back
	 * to forcibly unwinding.
	 */
	(void)scheduler_simulation_exit(1);
#endif

	return ErrorRecoveryBehaviour::ForceUnwind;
}