Re: [PATCH net-next 3/3] tap: XDP support

[Daniel Borkmann]

On 07/28/2017 05:46 AM, Michael S. Tsirkin wrote:
> On Fri, Jul 28, 2017 at 11:28:54AM +0800, Jason Wang wrote:
> > > > +	old_prog = rtnl_dereference(tun->xdp_prog);
> > > > +	if (old_prog)
> > > > +		bpf_prog_put(old_prog);
> > > > +	rcu_assign_pointer(tun->xdp_prog, prog);
> > > Is this OK?  Could this lead to the program getting freed and then
> > > datapath accessing a stale pointer?  I mean in the scenario where the
> > > process gets pre-empted between the bpf_prog_put() and
> > > rcu_assign_pointer()?
> >
> > Will call bpf_prog_put() after rcu_assign_pointer().

+1, good catch Jakub.

> I suspect you need to sync RCU or something before that.

No, see for example generic_xdp_install().