BUG in tty_open when using containers and ptrace

Sukadev Bhattiprolu sukadev at linux.vnet.ibm.com
Tue Jul 14 21:47:44 PDT 2009


Grzegorz Nosek [root at localdomain.pl] wrote:
| On pon, lip 13, 2009 at 11:49:05 -0700, Sukadev Bhattiprolu wrote:
| > Grzegorz Nosek [root at localdomain.pl] wrote:
| > | Simply run it as container init. Sometimes it oopses immediately,
| > 
| > I am trying to reproduce this too and just trying to make sure I get
| > your environment correctly.  I have just built/installed libvirt 0.6.5
| > on Ubuntu 9.04. 
| 
| 0.6.4 or 0.6.5, no difference really (tested both). Host distro is
| Debian Lenny but Ubuntu is probably close enough. Didn't test distro
| kernel, only used hand-built 2.6.30, 2.6.30.1 and 2.6.31-rc2

Ok.  Will try 2.6.31-rc2.

| 
| > The bug does require libvirt+kvm right ?  IOW, does this simple test repro
| > if it is run as a container-init with say lxc-start (i.e from liblxc - see
| > http://lxc.sourceforge.net/) ? Another simpler way to create container-init
| > is the ns_exec program - I assume that won't create the problem ?
| 
| Probably, as it requires a "cross-namespace" pty.  I didn't yet set up a
| lxc config file. Maybe you can help off the top of your head with the
| right incantations?

I don't have any beyond what is in the lxc-source examples. Maybe
Daniel Lezcano has some.

|BTW, where's the canonical source for ns_exec?

It is here: git://git.sr71.net/~hallyn/cr_tests.git
| 
| > Also in an earlier mail you mentioned that disabling the 'newinstance'
| > mount option to devpts did not help. So does your setup work with older
| > kernels like 2.6.28 ?
| 
| Well, I failed to build 2.6.28 with NET_NS (even without SYSFS it wasn't
| available in menuconfig) but 2.6.29 exhibits very similar behaviour,
| i.e. crashes instantly. The difference is that when the pty master
| exits, container init receives a SIGHUP. If it chooses to ignore it and
| open /dev/console anyway, *boom*. Looking roughly at %eip and the
| faulting address, it's the same bug.

So I guess this is the first time we are trying this scenario out (i.e.
it may not be a regression so we can't necessarily use git-bisect).

I am not too familiar with libvirt implementation, but do you think it
is possible to repro this using ns_exec and fewer/minimal namespaces ?

We should be able to get a cross-namespace pty using ns_exec, but am not
sure the set of namespaces need to be cloned -  would it be sufficient if
we clone pid and mount namespaces and remount /dev/pts in child container ?

Thanks,

Sukadev


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