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

On Wed, Nov 09, 2016 at 10:05:31AM +0100, Daniel Borkmann wrote:

On 11/09/2016 08:35 AM, Zhiyi Sun wrote:

quoted

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 <redacted>

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?

quoted

  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);
  	}

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