Re: devlinks: an alternative to devfs

James Mastros (root@jennifer-unix.dyn.ml.org)
Thu, 8 Jan 1998 18:32:24 -0500 (EST)

On Thu, 8 Jan 1998, Richard Gooch wrote:
> > > So? The unlink() will cause a delete entry to be maintained in the
> > > internal database.
> >
> > Oh, really?! So if I go into /dev and unlink a *nonexistent* name,
> > and then an hour later I insmod a driver, that driver is *not* going to
> > appear? That is a nasty bit of state. Also it means the people in
> > /dev can't use it either, so it doesn't even work.
> >
> > I think you are missing the point. I am talking about insmod'ing
> > drivers after the system is running and after the chroot jail is
> > set up and in use, not insmod'ing at boot time.
>
> I mean that if you do:
> # rm /gaol/dev/lp0
> # insmod lp
> # ls -lF /dev/lp0
> crw-rw-rw- 1 root root 6, 0 Aug 30 1992 /dev/lp0
> # ls -lF /gaol/dev/lp0
> /bin/ls: /gaol/dev/lp0: No such file or directory
>
> That's what you want, isn't it? This assumes that you know which
> devices you want to disable in your gaol.

Umm... that may be what he wants (probably is), but it isn't what would
happen. I belive that the VFS code checks for file existance and
permissions (of the directory, except in non sitckybit directories) before
calling sb->s_ops->unlink. I could well be wrong here.

> And, we can be even more clever than that. By using:
> # mount -t devfs -o explicit none /gaol/dev
> # mknod /gaol/dev/null c 0 0
> # mknod /gaol/dev/zero c 0 0
>
> we can ensure that *only* dev/null and dev/zero will exist in the
> chroot environment. In other words, you don't need to know which
> devices *may* be possible. This is even better than pre-emptively
> removing entries.

Exactly, and this shouldn't be difficult.

> Regards,
> Richard....

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