netdev - Re: [RFC PATCH 3/5] bpf: add BPF_PROG_ATTACH and BPF_PROG

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Date:	Wed, 17 Aug 2016 11:10:20 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Daniel Mack <daniel@...que.org>, htejun@...com,
	daniel@...earbox.net, ast@...com, davem@...emloft.net,
	kafai@...com, fw@...len.de, pablo@...filter.org, harald@...hat.com,
	netdev@...r.kernel.org
Subject: Re: [RFC PATCH 3/5] bpf: add BPF_PROG_ATTACH and BPF_PROG_DETACH
 commands

On Wed, Aug 17, 2016 at 09:16:02AM -0700, Eric Dumazet wrote:
> 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.

yep. this rcu_lock/unlock won't solve the race between parallel
bpf_prog_attach calls.
please use xchg() similar to bpf_fd_array_map_update_elem()
Then prog pointer will be swapped automically and bpf_prog_put()
will free it via call_rcu.
The reader side in sk_filter_cgroup_bpf() looks correct.