DF-2918 guest environment (all runs)

DragonFly dfbsd 6.5-DEVELOPMENT DragonFly 6.5-DEVELOPMENT #0: Thu Jul  2 06:02:54 UTC 2026
    root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC  x86_64
kernel: stock INVARIANTS kernel #0 (per orchestrator), X86_64_GENERIC
qemu/kvm single-tenant guest, dfbsd-qemu/vm.sh lifecycle

vfs.usermount: 1 (set for the runs; 0 at boot)
kern.securelevel: < 0 (module unload permitted)
Filesystem modules available in /boot/kernel: fuse.ko (loadable),
ext2fs.ko, tmpfs.ko, null.ko, procfs.ko (all latter built into kernel —
kldload fails with "interface X.1 already present").

Filesystem type capability mapping (sys/kern/vfs_syscalls.c get_fscap):
user-mountable types (with vfs.usermount=1) = null, devfs, procfs, tmpfs,
fusefs. NOTE: the fuse module registers vfc_name "fuse"
(sys/vfs/fuse/fuse_vfsops.c:490) while get_fscap() whitelists "fusefs",
so on this stock kernel an unprivileged mounter cannot reach the
sys_mount() vfsconf window for ANY modular filesystem type — the write
side of the race was exercised by a root mounter (identical sys_mount
code path; only the pre-window caps_priv_check at vfs_syscalls.c:152-159
differs).