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Date: Mon, 14 Nov 2016 20:27:47 +0200
From: Saeed Mahameed <saeedm@...lanox.com>
To: Daniel Borkmann <daniel@...earbox.net>, <davem@...emloft.net>
CC: <alexei.starovoitov@...il.com>, <bblanco@...mgrid.com>,
<tariqt@...lanox.com>, <zhiyisun@...il.com>, <ranas@...lanox.com>,
<netdev@...r.kernel.org>
Subject: Re: [PATCH net 2/3] bpf, mlx5: fix various refcount/prog issues in
mlx5e_xdp_set
On 11/14/2016 02:43 AM, Daniel Borkmann wrote:
> There are multiple issues in mlx5e_xdp_set():
>
> 1) prog can be NULL, so calling unconditionally into bpf_prog_add(prog,
> priv->params.num_channels) can end badly.
not correct, if prog is null we will never get to bpf_prog_add:
reset = (!priv->xdp_prog || !prog);
[...]
if (!test_bit(MLX5E_STATE_OPENED, &priv->state) || reset)
goto unlock;
bpf_prog_add...
>
> 2) The batched bpf_prog_add() should be done at an earlier point in
> time. This makes sure that we cannot fail anymore at the time we
> want to set the program for each channel. This only means that we
> have to undo the bpf_prog_add() in case we return early due to
> reset or device not in MLX5E_STATE_OPENED yet. Note, err is 0 here.
>
It is delayed for a reason, we do delayed batched bpf_prog_add()
only when reset is not required (exchanging prog/old_prg) when both prog and old_prog are not null,
which means the only thing that could fail in this case is bpf_prog_add.
so i don't see any reason for changing the logic, checking for bpf_prog_add return value would be sufficient.
Sorry I need to go for now, I will continue reviewing this patch later. but this patch looks a little bit exaggerated.
> 3) When swapping the priv->xdp_prog, then no extra reference count must
> be taken since we got that from call path via dev_change_xdp_fd()
> already. Otherwise, we'd never be able to free the program. Also,
> bpf_prog_add() without checking the return code could fail.
>
> Fixes: 86994156c736 ("net/mlx5e: XDP fast RX drop bpf programs support")
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> ---
> drivers/net/ethernet/mellanox/mlx5/core/en_main.c | 25 ++++++++++++++++++-----
> include/linux/bpf.h | 5 +++++
> kernel/bpf/syscall.c | 11 ++++++++++
> 3 files changed, 36 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> index 2b83667..c90610a 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> @@ -3125,6 +3125,17 @@ static int mlx5e_xdp_set(struct net_device *netdev, struct bpf_prog *prog)
> goto unlock;
> }
>
> + if (prog) {
> + /* num_channels is invariant here, so we can take the
> + * batched reference right upfront.
> + */
> + prog = bpf_prog_add(prog, priv->params.num_channels);
> + if (IS_ERR(prog)) {
> + err = PTR_ERR(prog);
> + goto unlock;
> + }
> + }
> +
> was_opened = test_bit(MLX5E_STATE_OPENED, &priv->state);
> /* no need for full reset when exchanging programs */
> reset = (!priv->xdp_prog || !prog);
> @@ -3132,10 +3143,10 @@ static int mlx5e_xdp_set(struct net_device *netdev, struct bpf_prog *prog)
> if (was_opened && reset)
> mlx5e_close_locked(netdev);
>
> - /* exchange programs */
> + /* exchange programs, extra prog reference we got from caller
> + * as long as we don't fail from this point onwards.
> + */
> old_prog = xchg(&priv->xdp_prog, prog);
> - if (prog)
> - bpf_prog_add(prog, 1);
> if (old_prog)
> bpf_prog_put(old_prog);
>
> @@ -3146,12 +3157,11 @@ static int mlx5e_xdp_set(struct net_device *netdev, struct bpf_prog *prog)
> mlx5e_open_locked(netdev);
>
> if (!test_bit(MLX5E_STATE_OPENED, &priv->state) || reset)
> - goto unlock;
> + goto unlock_put;
>
> /* exchanging programs w/o reset, we update ref counts on behalf
> * of the channels RQs here.
> */
> - bpf_prog_add(prog, priv->params.num_channels);
> for (i = 0; i < priv->params.num_channels; i++) {
> struct mlx5e_channel *c = priv->channel[i];
>
> @@ -3173,6 +3183,11 @@ static int mlx5e_xdp_set(struct net_device *netdev, struct bpf_prog *prog)
> unlock:
> mutex_unlock(&priv->state_lock);
> return err;
> +unlock_put:
> + /* reference on priv->xdp_prog is still held at this point */
> + if (prog)
> + bpf_prog_sub(prog, priv->params.num_channels);
> + goto unlock;
> }
>
> static bool mlx5e_xdp_attached(struct net_device *dev)
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index c201017..ca495fd 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -234,6 +234,7 @@ u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
> struct bpf_prog *bpf_prog_get(u32 ufd);
> struct bpf_prog *bpf_prog_get_type(u32 ufd, enum bpf_prog_type type);
> struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i);
> +void bpf_prog_sub(struct bpf_prog *prog, int i);
> struct bpf_prog *bpf_prog_inc(struct bpf_prog *prog);
> void bpf_prog_put(struct bpf_prog *prog);
>
> @@ -303,6 +304,10 @@ static inline struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i)
> return ERR_PTR(-EOPNOTSUPP);
> }
>
> +static inline void bpf_prog_sub(struct bpf_prog *prog, int i)
> +{
> +}
> +
> static inline void bpf_prog_put(struct bpf_prog *prog)
> {
> }
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 751e806..a0fca9f 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -682,6 +682,17 @@ struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i)
> }
> EXPORT_SYMBOL_GPL(bpf_prog_add);
>
> +void bpf_prog_sub(struct bpf_prog *prog, int i)
> +{
> + /* Only to be used for undoing previous bpf_prog_add() in some
> + * error path. We still know that another entity in our call
> + * path holds a reference to the program, thus atomic_sub() can
> + * be safely used in such cases!
> + */
> + WARN_ON(atomic_sub_return(i, &prog->aux->refcnt) == 0);
> +}
> +EXPORT_SYMBOL_GPL(bpf_prog_sub);
> +
> struct bpf_prog *bpf_prog_inc(struct bpf_prog *prog)
> {
> return bpf_prog_add(prog, 1);
>
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