bhyveload: validate character disk devices

[novel]

Summary

Currently, bhyveload(8) does not validate the supplied disk
image path. For example, it allows passing the /dev/null
device, which later fails in userboot because it does not
support DIOCGSECTORSIZE and DIOCGMEDIASIZE ioctls (see
userdisk_init() in stand/userboot/userboot/userboot_disk.c).

Fix that by checking DIOCGSECTORSIZE and DIOCGMEDIASIZE ioctls early.
A similar check already exists in bhyve(8). While here, make
cb_diskioctl() report the obtained sector size instead of
hard-coding 512.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation

Test Plan

Unpatched:

# bhyveload -m 2048 -d /dev/null test
Consoles: userboot  

FreeBSD/amd64 User boot lua, Revision 3.0
(Thu Aug 27 10:40:55 UTC 2026 root@freebsd14)
Bus error
#

Patched:

# bhyveload -m 2048 -d /dev/null test
bhyveload: Could not open '/dev/null': Inappropriate ioctl for device
# echo $?
64
#

[novel]

ping?

[markj]

usr.sbin/bhyveload/bhyveload.c

763

Note that cb_diskioctl() hard-codes a sector size of 512, so here we're checking what the disk reports and then throwing away the result.

[novel]

cb_diskioctl: report the actual sector size

[novel]

cb_diskioctl: report the actual sector size

[markj]

usr.sbin/bhyveload/bhyveload.c

368–375

I would just fetch it again here rather than keeping around extra state, just like we issue DIOCGMEDIASIZE below.

[novel]

Do not cache sector size, fetch it directly in cb_diskioctl()