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

[Daniel Borkmann]

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 <zhiyi...@gmail.com>

Your analysis looks incorrect to me. Please elaborate in more detail why
you think current code is buggy ...

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).

  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);
        }