io7m / nfswrap-bsd

Process supervision wrapper for the BSD nfsd

nfswrap-bsd

A wrapper for the BSD nfsd that allows for process supervision.

Requirements

• FreeBSD 11+
• OpenBSD 6.3+ (Untested)

Compiling

make && sudo cp nfsd-wrap /usr/local/bin/nfsd-wrap

Usage

nfsd-wrap: usage: nfsd.sh

nfsd.sh must be a program that starts nfsd and waits for it to exit. An example of a correct nfsd.sh program might be:

#!/bin/sh
exec /usr/sbin/nfsd --debug -h 127.0.0.1 -t -n 4

The --debug flag is an undocumented (and dangerous) flag to the BSD nfsd. When nfsd is run with the --debug flag, it will not fork into the background. Unfortunately, it will also do something rather dangerous as will now be explained.

In normal operation nfsd will declare some signal handlers that ignore signals such as SIGTERM, SIGINT, SIGHUP. During operation, nfsd allocates some kernel resources that are only cleaned up if the process exits by receiving a SIGUSR1 signal. If the kernel resources that nfsd allocates are not cleaned up, the nfsd process will simply crash the next time it is run. There appears to be no way to fix this beyond rebooting the entire system. Unfortunately, when the --debug flag is used, the nfsd process does not set up any signal handlers that ignore signals other than SIGUSR1, meaning that it is very easy to shut down the nfsd process incorrectly and effectively prevent the process from running again until the next reboot.

The nfsd-wrap program acts as a wall between the real nfsd process and the process supervision system (such as runit). If the nfsd-wrap process is sent SIGINT, SIGHUP, SIGTERM, or SIGUSR1, it sends SIGUSR1 to the nfsd process. This ensures that the correct cleanup occurs.