Re: [PATCH 1/2] seccomp: notify user trap about unused filter

[Christian Brauner]

On Wed, May 27, 2020 at 09:25:59AM -0600, Tycho Andersen wrote:
> On Wed, May 27, 2020 at 01:19:01PM +0200, Christian Brauner wrote:
> > +void seccomp_filter_notify(const struct task_struct *tsk)
> > +{
> > +	struct seccomp_filter *orig = tsk->seccomp.filter;
> > +
> > +	while (orig && refcount_dec_and_test(&orig->live)) {
> > +		if (waitqueue_active(&orig->wqh))
> > +			wake_up_poll(&orig->wqh, EPOLLHUP);
> > +		orig = orig->prev;
> > +	}
> > +}
> 
> Is there a reason this can't live in put_seccomp_filter()?

put_seccomp_filter() is called from free_task() which in turn gets
called via delayed_put_task_struct through call_rcu() so the
notification will happen at some point whereas you'd often want the
notification to happen at reliable point. This is why most of that stuff
happens in release_task() which is called in the exit path when the task
is finally marked as dead. This is similar to how cgroup_release() is
called from release_task() whereas cgroup_free() is called from
free_task() or other stuff.