Function kern_reroot

freebsd/kern/kern_shutdown.c:498–589

* The system call that results in changing the rootfs. */

 * The system call that results in changing the rootfs.
 */
static int
kern_reroot(void)
{
	struct vnode *oldrootvnode, *vp;
	struct mount *mp, *devmp;
	int error;

	if (curproc != initproc)
		return (EPERM);

	/*
	 * Mark the filesystem containing currently-running executable
	 * (the temporary copy of init(8)) busy.
	 */
	vp = curproc->p_textvp;
	error = vn_lock(vp, LK_SHARED);
	if (error != 0)
		return (error);
	mp = vp->v_mount;
	error = vfs_busy(mp, MBF_NOWAIT);
	if (error != 0) {
		vfs_ref(mp);
		VOP_UNLOCK(vp);
		error = vfs_busy(mp, 0);
		vn_lock(vp, LK_SHARED | LK_RETRY);
		vfs_rel(mp);
		if (error != 0) {
			VOP_UNLOCK(vp);
			return (ENOENT);
		}
		if (VN_IS_DOOMED(vp)) {
			VOP_UNLOCK(vp);
			vfs_unbusy(mp);
			return (ENOENT);
		}
	}
	VOP_UNLOCK(vp);

	/*
	 * Remove the filesystem containing currently-running executable
	 * from the mount list, to prevent it from being unmounted
	 * by vfs_unmountall(), and to avoid confusing vfs_mountroot().
	 *
	 * Also preserve /dev - forcibly unmounting it could cause driver
	 * reinitialization.
	 */

	vfs_ref(rootdevmp);
	devmp = rootdevmp;
	rootdevmp = NULL;

	mtx_lock(&mountlist_mtx);
	TAILQ_REMOVE(&mountlist, mp, mnt_list);
	TAILQ_REMOVE(&mountlist, devmp, mnt_list);
	mtx_unlock(&mountlist_mtx);

	oldrootvnode = rootvnode;