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SubjectRe: cgroup: BUG: unable to handle kernel NULL pointer dereference
On Sat, Jan 02, 2016 at 01:50:49PM -0800, Jeremiah Mahler wrote:
> Serge,
>
> On Sat, Jan 02, 2016 at 12:24:16PM -0600, Serge E. Hallyn wrote:
> [...]
> >
> > Tried to reproduce with setting CONFIG_CFQ_GROUP_IOSCHED=y, but did not
> > succeed. Could you send me the .config? Also, if someone could send
> > the objdump -d output that might help. Though really, it seems clear
> > that current->nsproxy must be NULL. Hm, that's right - we used to have
> > that issue in pidns (or was it netns) during process exit. I don't know
> > that I'll get time this afternoon, but I'll look into it asap.
> >
> > thanks.
>
> Attached is the .config I used. I can send an objdump, but do you want
> a dump of the kernel, where the cgroup code is?
>
> --
> - Jeremiah Mahler

Thanks - Shoulda looked at the exit path before - exit_io_context is called
after exit_task_namespaces().

I'll have to figure out the best way to handle this. In the past we've
restructured exit code to ensure that anything wanting to dereference
nsproxy happened before exit_task_namespaces(). However, this is only
happening in a debug stmt at blkg_path() in
http://lxr.free-electrons.com/source/include/linux/blk-cgroup.h#L344
so simply returning the init_cgroup_namespace is actually the right thing
to do. I'm tempted to add a init_cgroup_path() which ignores namespaces,
for use in debugging statements.


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