Re: [PATCH 3/7] seccomp_filter: Enable ftrace-based system call filtering

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Will Drewry (wad@chromium.org):
> My intent was to make them available for use by seccomp.c during state
> teardown/dup.  I don't think there's benefit to exposing them outside
> of that.  Would dropping the export, and adding an local seccomp.h
> with the shared functions in them resolve that more cleanly?

And add a clear comment explaining :)

> > Your code would have been correct if you could call kzalloc under
> > rcu_read_lock() (which you can on some kernel configurations but not
> > all). The issue is that you need to pull out that allocation from the
> > rcu_read_lock() because rcu_read_lock assumes you can't preempt, and
> > that allocation can schedule out. The access to the filters must be done
> > under rcu_read_lock(), other than that, you're fine.
> 
> That makes sense.  I think I'd prefer to not share those functions
> rather than guard the list just in case a future consumer of the
> interface comes along.  Would that make sense to you?  Since I don't
> see any other users right now other than seccomp.c, it might make
> sense to tackle the impact when an actual need arises.
> 
> I'll go whichever way pointed on this, though.

Complicating the locking for nonexistent users doesn't seem like the
right way.

-serge