using /dev/null in chrooted enviroment

[Giorgos Keramidas]

On 2005-12-28 01:16, Tzachi Tager <tzachi_tager at yahoo.com> wrote:
> Hi,
>
> Giorgos Keramidas <keramida at freebsd.org> wrote:  On 2005-12-27 06:49, Tzachi Tager  wrote:
> > Hi,
> > I'm trying to create chrooted enviroment, i did the folllowing
> > in the chrooted enviroment:
> >
> > mknode c null 0 24
> > mknode c zero 0 25
> >
> > And then i did chroot and those files are under /dev, but when
> > i wrote a short program in C that tries to open /dev/null
> > (within the chrooted env.) i got:
> >
> > "/dev/null-operation is not supported"
>
> > You forgot to mention what version of FreeBSD you are using.  It
> > may also help to see the contents of the chrooted /dev directory.
>
> We are using:
>
> FreeBSD freebsd6.0 6.0-RELEASE FreeBSD 6.0-RELEASE #0:
> Thu Nov  3  09:36:13 UTC 2005
> root at x64.samsco.home:/usr/obj/usr/src/sys/GENERIC  i386
>
> We tried:
>
> 1) To create only /dev/null and /dev/zero (that configuration worked in FreeBSD 5.4)
> 2) To create the same /dev contents via: tar -cf - /dev | tar -xf -
>
> The original system has /dev mounted as: devfs on /dev (devfs, local)

That's not the correct way to populate a /dev directory in a chroot
anymore.  None of the above will work as expected, AFAIK.  The best way
to create a second devfs mount-point that only contains /dev/zero and
/dev/null is to remount devfs and 'hide' all teh rest of the device
nodes:

> PS: It seems like if we mount devfs to the chrooted env - everything
> works. The problem is that chroot directory can be placed everywhere,
> but mount doesn't accept very long path:
>
> # mount -t devfs none dev
> mount_devfs: File name too long
>
> # pwd
> /tmp/build.debugger-php-4.0.6.5269/debugger-php-4.0.6-DEBUG_SERVER_5_1_0_RC3_20051215

This is where you hit an internal limit of the kernel.  The limit is
defined as 88 bytes at /usr/src/sys/sys/mount.h:

# root at flame:/usr/src/sys/sys# grep MNAMELEN mount*
# #define MNAMELEN        88              /* size of on/from name bufs */
#         char      f_mntfromname[MNAMELEN];  /* mounted filesystem */
#         char      f_mntonname[MNAMELEN];    /* directory on which mounted */

I'm not sure if this is easy to bump.  The best person to answer is
someone with more experience with the kernel internals.

A workaround that you can use in the mean time is to use shorter names
for the chroot directories, i.e.:

    # mkdir -p /tmp/chroot/100/dev
    # mount -t devfs devfs /tmp/chroot/100/dev

Regards,
Giorgos