mac: Revise incorrect comments about EXC_RESOURCE and EXC_GUARD
mac: Revise incorrect comments about EXC_RESOURCE and EXC_GUARD

launchd actually does set the EXC_RESOURCE and EXC_GUARD handlers
exactly the same way that it sets the EXC_CRASH handler. See 10.9.5
launchd-842.92.1/src/core.c job_setup_exception_port().

Cases where an EXC_CRASH handler is set but EXC_RESOURCE and EXC_GUARD
handlers are not set occur when the exception ports are set by
/usr/bin/login instead of launchd. login looks up the
exception-reporting service by name and sets the exception port without
including EXC_MASK_RESOURCE or EXC_MASK_GUARD in the mask. See 10.10.5
system_cmds-643.30.1/login.tproj/login.c main().

login is a setuid executable, so it does not inherit its parent process’
exception handlers. See 10.10.5 xnu-2782.40.9/osfmk/kern/ipc_tt.c
ipc_task_reset().

Terminal.app executes login when establishing its command-line
environment, so the exception handlers set for Terminal.app itself
(including EXC_MASK_CRASH, EXC_MASK_RESOURCE, and EXC_MASK_GUARD) are
discarded, and then login sets an exception handler only for
EXC_MASK_CRASH. The same thing occurs for any other process descended
from login, including SSH sessions, because sshd executes login.

This is a bug in login filed as Apple radar 22978644. This bug led to a
misunderstanding about the use of EXC_RESOURCE and EXC_GUARD. Comments
that discuss this behavior are now reworded to be accurate, and
non-fatal EXC_RESOURCE exceptions are made eligible for forwarding to
the user ReportCrash (because it would normally handle them in the
absence of Crashpad) while Crashpad itself will still skip processing
them.

R=rsesek@chromium.org

Review URL: https://codereview.chromium.org/1391453002 .

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