Re: [PATCH] net/mlx4_en: Fix bpf_prog_add ref_cnt in mlx4

[Zhiyi Sun <zhiyisun@gmail.com>]

On Wed, Nov 09, 2016 at 10:05:31AM +0100, Daniel Borkmann wrote:
> On 11/09/2016 08:35 AM, Zhiyi Sun wrote:
> > There are rx_ring_num queues. Each queue will load xdp prog. So
> > bpf_prog_add() should add rx_ring_num to ref_cnt.
> > 
> > Signed-off-by: Zhiyi Sun <zhiyisun@gmail.com>
> 
> Your analysis looks incorrect to me. Please elaborate in more detail why
> you think current code is buggy ...
> 

Yes, you are correct. My patch is incorrect. It is not a bug.

> Call path is dev_change_xdp_fd(), which does bpf_prog_get_type() on the
> fd. This already takes a ref and only drops it in case of error. Thus
> in mlx4_xdp_set(), you only need priv->rx_ring_num - 1 refs for the rest
> of the rings, so that dropping refs from old_prog makes sure we release
> it again. Looks correct to me (maybe a comment would have helped there).
> 

I thought mlx4's code is incorrect because in mlx5's driver, function
mlx5e_xdp_set() calls a pair of bpf_prog_add/put, the number of add and
put to the refs are same. I didn't notice that one "add" has been called in its
calller. So, it seems that mlx5's code is incorrect, right?

> >   drivers/net/ethernet/mellanox/mlx4/en_netdev.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/mellanox/mlx4/en_netdev.c b/drivers/net/ethernet/mellanox/mlx4/en_netdev.c
> > index 12c99a2..d25e150 100644
> > --- a/drivers/net/ethernet/mellanox/mlx4/en_netdev.c
> > +++ b/drivers/net/ethernet/mellanox/mlx4/en_netdev.c
> > @@ -2650,7 +2650,7 @@ static int mlx4_xdp_set(struct net_device *dev, struct bpf_prog *prog)
> >   	 */
> >   	if (priv->xdp_ring_num == xdp_ring_num) {
> >   		if (prog) {
> > -			prog = bpf_prog_add(prog, priv->rx_ring_num - 1);
> > +			prog = bpf_prog_add(prog, priv->rx_ring_num);
> >   			if (IS_ERR(prog))
> >   				return PTR_ERR(prog);
> >   		}
> > @@ -2680,7 +2680,7 @@ static int mlx4_xdp_set(struct net_device *dev, struct bpf_prog *prog)
> >   	}
> > 
> >   	if (prog) {
> > -		prog = bpf_prog_add(prog, priv->rx_ring_num - 1);
> > +		prog = bpf_prog_add(prog, priv->rx_ring_num);
> >   		if (IS_ERR(prog))
> >   			return PTR_ERR(prog);
> >   	}
> > 
>