The changes to jail and the jail rc script between 9 and 10 break ezjail image jails. Ezjail specifies the jailroot in an fstab file under /etc/fstab.jailname. The issue is that when the rc script autocreates /var/run/jail.jailname.conf it adds an entry to the mount parameter for procfs. The order that the jail command processes mounts is to mount things in the mounts config parameter first and them mount from fstab. When the root is in fstab this causes jail to try to mount proc before the root has been mounted, so mount returns an error because the directory ${jailroot}/proc does not exist.

ezjail should probably be updated to create jail config files on 10+, however there is still an ordering issue if a user puts a mount in the config that is under a directory that is mounted in fstab.

The ordering issue occurs in /usr/src/usr.sbin/jail/jail.c. The issue is that IP_MOUNT comes before IP__MOUNT_FROM_FSTAB. Of course reordering this would just break things in the opposite direction, so there would be an issue if a mount in fstab was under a mount that is in the config.

# /usr/src/usr.sbin/jail/jail.c
static const enum intparam startcommands[] = {
    IP__NULL,
#ifdef INET
    IP__IP4_IFADDR,
#endif
#ifdef INET6
    IP__IP6_IFADDR,
#endif
    IP_MOUNT,
    IP__MOUNT_FROM_FSTAB,
    IP_MOUNT_DEVFS,
    IP_MOUNT_FDESCFS,
    IP_EXEC_PRESTART,
    IP__OP,
    IP_VNET_INTERFACE,
    IP_EXEC_START,
    IP_COMMAND,
    IP_EXEC_POSTSTART,
    IP__NULL
};

# cat /var/run/jail.test3.conf 
# Generated by rc.d/jail at 2015-02-01 08:14:07
test3 {
        host.hostname = "test3";
        path = "/usr/jails/test3";
        ip4.addr += "10.2.1.152/32";
        allow.raw_sockets = 0;
        exec.clean;
        exec.system_user = "root";
        exec.jail_user = "root";
        exec.start += "/bin/sh /etc/rc";
        exec.stop = "";
        exec.consolelog = "/var/log/jail_test3_console.log";
        mount.fstab = "/etc/fstab.test3";
        mount.devfs;
        mount.fdescfs;
        mount +=  "procfs /usr/jails/test3/proc procfs rw 0 0";
        allow.mount;
        allow.set_hostname = 0;
        allow.sysvipc = 0;
}

# cat /etc/fstab.test3 
/usr/jails/test3.device /usr/jails/test3 ufs rw 0 0
/usr/jails/basejail /usr/jails/test3/basejail nullfs ro 0 0