On Wed, Jun 21, 2006 at 09:41:42AM +1200, Joshua Sandbrook wrote:
> Gidday
>
> Im writing a shell at the moment that chroots into a users home dir and then
> runs only the sftp-server program ( which is in the uses home dir ).
>
> Anyway, it wont work unless /dev/null is present in the chroot...
>
> I am using execve to run sftp-server, and I am wondering if it has something
> to do with stdout / stdin / stderr fd's being closed on execve?
>
> Can anyone help me here?
Well, since nobody else seems to respond...
If you can set it up in a controlled testing environment, you could
ktrace(1) it. This would tell you, at the very least, what program
actually opens /dev/null.
A quick grep through /usr/src/usr.bin/ssh suggests that ssh (and, most
likely, sftp) interacts with /dev/null quite a bit. It might be possible
to change the code to work without, but that would take quite a bit of
work I fear.
An alternative hack would be to change sftp directly; in this case, it
can safely open /dev/null and then call chroot() at the appropriate
time. OTOH, you have a custom patch that you should apply at the
appropriate time and place, which, too, has its disadvantages.
Also, be *very* careful in writing the shell, as it must be suid root
for what you want it to do... this, in fact, suggests that the best
solution might be to write a trivial shell and just have
/home/*/dev/null. Or, for that matter, /home/dev/null and chroot into
/home.
Joachim
