kern/99758: chown/chmod pty slave side in kernel

[Atsuo Ohki]

I wrote:
>  I got stress2.tgz and done `./run.sh pty.cfg' and got the message like
> 
> 	Memory modified after free ...
> 	Most recently used by DEVFS1
> 
>  The reason for this panic is devfs_close() in fs/devfs/devfs_vnops.c.
>  As you see, devfs_close() eventually calls ptcclose()/ptsclose()
>  which calls pty_maybecleanup() destroying devs for ptc&pts, but
>  devfs_close() then calls dev_relthread() which may access just freeed dev.
> 
>  I'm afraid that devfs is not designed to handle destroing dev during
>  close operation.
> 
>  I'm working on this problem with the idea:
>   i) destory_dev() should not free dev, but just mark inactive.
>   ii) devfs_populate() should actually free an inactive dev.
>   iii) modify devfs_find() and other routines to take care of an inactive dev
> .
>  But no success yet ;-<

 I achieved a little success.
 Now (really now!),  pty test of stress2 is running.

 I modified as follow:
 
 i) destroy_dev() in cdevs' close routine is not appropriate.
    I introduced hide_dev() (in kern/kern_conf.c) and
    devfs_hide() (in fs/devfs/devfs_devs.c) to make dev invisible 
    from userland. (via devfs_find(), devfs_readdir() and so on.)

 ii) pty_maybecleanup() no longer calls destroy_dev(), but
     calls hide_dev() to make pts/ptc invisible.
     when both of pts/ptc are closed, link them to pt_free_list
     as usual (structure dev for ptc/pts are not destroyied!).

     pty_new() now takes care of destorying dev for ptc/pts.
     when a new ptc/pts is requested, search pt_free_list to
     find a devs' which are free!(i.e. si_usecount == 0).
     if found, destroy existing devs.
     if not found, things goes by as before.

     I use the name ptsXX, ptcXX instead of pts/XX, ptc/XX.
     (original naming causes system hungup related to vnode operation.
      I must solve this problem.)