Virtual machine name length limits: vm_create: Invalid argument

[io7m]

waxmill will generate a bhyve command execution such as the following:

/usr/sbin/bhyve \
-P \
-A \
-H \
-c cpus=1,sockets=1,cores=1,threads=1 \
-m 1000M \
-s 0:0:0,hostbridge \
-s 0:1:0,virtio-blk,/dev/zvol/storage/vm/50613ad2-3c93-478b-afa0-d8ac613723da/disk-0_1_0 \
-s 0:2:0,virtio-net,tap1,mac=aa:11:cc:a0:b6:66 \
-s 0:3:0,ahci-cd,/storage/images/openbsd-6.7-amd64-install.iso \
-s 0:4:0,lpc \
-l com1,/dev/nmdm_50613ad2-3c93-478b-afa0-d8ac613723da_B \
-l bootrom,/usr/local/share/uefi-firmware/BHYVE_UEFI.fd \
50613ad2-3c93-478b-afa0-d8ac613723da

This will result in:

vm_create: Invalid argument

Why? Because it's still 1970 and we're still counting bytes, apparently:

/*
 * The VM name has to fit into the pathname length constraints of devfs,
 * governed primarily by SPECNAMELEN.  The length is the total number of
 * characters in the full path, relative to the mount point and not 
 * including any leading '/' characters.
 * A prefix and a suffix are added to the name specified by the user.
 * The prefix is usually "vmm/" or "vmm.io/", but can be a few characters
 * longer for future use.
 * The suffix is a string that identifies a bootrom image or some similar
 * image that is attached to the VM. A separator character gets added to
 * the suffix automatically when generating the full path, so it must be
 * accounted for, reducing the effective length by 1.
 * The effective length of a VM name is 229 bytes for FreeBSD 13 and 37
 * bytes for FreeBSD 12.  A minimum length is set for safety and supports
 * a SPECNAMELEN as small as 32 on old systems.
 */
#define VM_MAX_PREFIXLEN 10
#define VM_MAX_SUFFIXLEN 15
#define VM_MIN_NAMELEN   6
#define VM_MAX_NAMELEN \
    (SPECNAMELEN - VM_MAX_PREFIXLEN - VM_MAX_SUFFIXLEN - 1)

Essentially, VM names can be at most 31 characters, but UUIDs are 37 characters long given the standard xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx encoding.

[io7m]

We just need a smaller encoding for UUIDs that only uses filename-safe characters.

[io7m]

Note that the header file says 37, but bhyve will reject anything over 31 characters...