[Bug 215471] Using bsnmpd with the snmp_hostres module on a vmware ESXi guest with a disconnected CD drive uses 100% CPU

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215471

            Bug ID: 215471
           Summary: Using bsnmpd with the snmp_hostres module on a vmware
                    ESXi guest with a disconnected CD drive uses 100% CPU
           Product: Base System
           Version: 11.0-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: misc
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: jimp at pfsense.org
                CC: freebsd-amd64 at FreeBSD.org
                CC: freebsd-amd64 at FreeBSD.org

Created attachment 178177
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=178177&action=edit
A configuration file for bsnmpd which can reproduce the high CPU usage problem
on a vmware guest

When running FreeBSD 11.0-RELEASE (from release until at least -p5), there is a
combination of factors that leads to bsnmpd using 100% CPU.

It seems to require all of the following:

* FreeBSD 11 running as a guest in VMWare ESXi
* The guest has a "CD/DVD Drive" listed in its hardware on ESX
* The CD/DVD Drive is in a disconnected state
* The snmp_hostres module is loaded in the bsnmpd configuration file
* bsnmpd enabled and running

Here is a very small snmpd.config that can replicate the problem in a similar
environment:

==================
%snmpd
sysDescr                        = "FreeBSD doctor.example.com FreeBSD
11.0-RELEASE-p5 amd64"
begemotSnmpdCommunityString.0.1 = "supersecret"
begemotSnmpdPortStatus.0.0.0.0.161 = 1
begemotSnmpdModulePath."mibII"  = "/usr/lib/snmp_mibII.so"
begemotSnmpdModulePath."hostres"     = "/usr/lib/snmp_hostres.so"
==================

If bsnmpd is run on this guest with that configuration, it consumes all
available CPU. 

I'll attach the above snmpd.config, along with top output and a sample of the
truss output taken while bsnmpd was running.

Currently the only workaround is to disable the hostres module, but that means
losing access to any of the information it provides.

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