Re: [RFC PATCH 3/5] bpf: add BPF_PROG_ATTACH and BPF_PROG_DETACH commands

[Eric Dumazet]

On Wed, 2016-08-17 at 16:00 +0200, Daniel Mack wrote:

> +             progp = is_ingress ? &cgrp->bpf_ingress : &cgrp->bpf_egress;
> +
> +             rcu_read_lock();
> +             old_prog = rcu_dereference(*progp);
> +             rcu_assign_pointer(*progp, prog);
> +
> +             if (old_prog)
> +                     bpf_prog_put(old_prog);
> +
> +             rcu_read_unlock();


This is a bogus locking strategy.

You do not want to use rcu_read_lock()/rcu_read_unlock() here, but
appropriate writer exclusion (a mutex probably, or a spinlock)

Then use rcu_dereference_protected() instead of rcu_dereference(*progp);