ZFS does not validate the sharenfs parameter

[asomers]

System information

Type	Version/Name
Distribution Name	FreeBSD
Distribution Version	FreeBSD
Kernel Version	15.0-CURRENT
Architecture	amd64
OpenZFS Version	zfs-2.3.99-114-FreeBSD_ge0039c705 zfs-kmod-2.3.99-114-FreeBSD_ge0039c705

Describe the problem you're observing

On Illumos, ZFS will validate that the "sharenfs" parameter is valid. It will refuse to set it to an invalid value. But FreeBSD does not validate it. This could cause mountd to crash or misbehave. I haven't checked Linux.

Note that this is not a security vulnerability, even in combination with ZFS delegations, because /etc/zfs/exports may only be updated by root, even when the sharenfs property is delegated

Describe how to reproduce the problem

Using bash:

$ sudo zpool create foo da0
$ sudo zfs set sharenfs=$'XXX\n/usr/home' foo
$ sudo cat /etc/zfs/exports 
# !!! DO NOT EDIT THIS FILE MANUALLY !!!

/foo    XXX
/usr/home

Downstream bug reports

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221987