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;
}