Firewall

May 30, 1996
Brian Bershad

Overview

Firewall is a core kernel module that is responsible for keeping the machine up in the most difficult of times. Kernel runtime protection faults and failures in m3core runtime services are reflected back through to firewall, where they are converted into SpinExceptions and then propogated to the offending thread.

Design and Implementation

From Firewall.m3:


(*
 *
 * We take over the RTMisc.Fatal* facilities from the runtime.  This module
 * does several things. First, we map all checked runtime exceptions out of
 * process failures and back into exceptions.  CheckedRuntimeError
 * raises a SpinException.CheckedRuntimeError which can be caught by the
 * offending thread.
 * Consequently, any and all procedures can RAISE a
 * SpinException.CheckedRuntimeError.  To ensure this, we inform the m3 runtime
 * of a set of implicit exceptions that, when raised, should not cause an
 * runtime error if they are not included in a procedure's RAISES clause.
 *
 * If an exception is raised but not caught by the raising thread, then
 * the runtime will notify us of an "Unhandled Exception." Clearly, we can't
 * reraise in the context of the offending thread, so we stop the offending
 * thread in an 'exceptional' state that reflects the unhandled exception.
 *
 * NOTE: If the offending thread is a privileged kernel thread (TAZ), then
 * we go through RTProcess.Crash, which will halt the system.
 *
 * We rely on the dispatcher to hijack events having to do with process and
 * system failure. We route these events into cleaner handlers.
 *)

bershad@cs.washington.edu