Subject: Re: [PATCH bpf-next 2/5] bpf: Add support to attach bpf program to a devmap entry

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Wed, 27 May 2020 12:01:21 +0200

David Ahern <[email-masked]> writes:

> Add BPF_XDP_DEVMAP attach type for use with programs associated with a
> DEVMAP entry.
>
> DEVMAPs can associate a program with a device entry by setting the
> value to <index, fd> pair. The program associated with the fd must have
> type XDP with expected attach type BPF_XDP_DEVMAP. When a program is
> associated with a device index, the program is run on an XDP_REDIRECT
> and before the buffer is added to the per-cpu queue. At this point
> rxq data is still valid; the next patch adds tx device information
> allowing the prorgam to see both ingress and egress device indices.
>
> XDP generic is skb based and XDP programs do not work with skb's. Block
> the use case by walking maps used by a program that is to be attached
> via xdpgeneric and fail if any of them are DEVMAP / DEVMAP_HASH with
> 8-bytes values.
>
> Block attach of BPF_XDP_DEVMAP programs to devices.
>
> Signed-off-by: David Ahern <[email-masked]>
> ---
>  include/linux/bpf.h            |  5 ++
>  include/uapi/linux/bpf.h       |  1 +
>  kernel/bpf/devmap.c            | 92 ++++++++++++++++++++++++++++++----
>  net/core/dev.c                 | 18 +++++++
>  tools/include/uapi/linux/bpf.h |  1 +
>  5 files changed, 108 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index efe8836b5c48..088751bc09aa 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1242,6 +1242,7 @@ int dev_map_enqueue(struct bpf_dtab_netdev *dst, struct xdp_buff *xdp,
>  		    struct net_device *dev_rx);
>  int dev_map_generic_redirect(struct bpf_dtab_netdev *dst, struct sk_buff *skb,
>  			     struct bpf_prog *xdp_prog);
> +bool dev_map_can_have_prog(struct bpf_map *map);
>  
>  struct bpf_cpu_map_entry *__cpu_map_lookup_elem(struct bpf_map *map, u32 key);
>  void __cpu_map_flush(void);
> @@ -1355,6 +1356,10 @@ static inline struct net_device  *__dev_map_hash_lookup_elem(struct bpf_map *map
>  {
>  	return NULL;
>  }
> +static inline bool dev_map_can_have_prog(struct bpf_map *map)
> +{
> +	return false;
> +}
>  
>  static inline void __dev_flush(void)
>  {
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 97e1fd19ff58..8c2c0d0c9a0e 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -224,6 +224,7 @@ enum bpf_attach_type {
>  	BPF_CGROUP_INET6_GETPEERNAME,
>  	BPF_CGROUP_INET4_GETSOCKNAME,
>  	BPF_CGROUP_INET6_GETSOCKNAME,
> +	BPF_XDP_DEVMAP,
>  	__MAX_BPF_ATTACH_TYPE
>  };
>  
> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index 95db6d8beebc..7658b3e2e7fc 100644
> --- a/kernel/bpf/devmap.c
> +++ b/kernel/bpf/devmap.c
> @@ -73,6 +73,7 @@ struct bpf_dtab_netdev {
>  	struct net_device *dev; /* must be first member, due to tracepoint */
>  	struct hlist_node index_hlist;
>  	struct bpf_dtab *dtab;
> +	struct bpf_prog *xdp_prog;
>  	struct rcu_head rcu;
>  	unsigned int idx;
>  	struct dev_map_ext_val val;
> @@ -231,6 +232,8 @@ static void dev_map_free(struct bpf_map *map)
>  
>  			hlist_for_each_entry_safe(dev, next, head, index_hlist) {
>  				hlist_del_rcu(&dev->index_hlist);
> +				if (dev->xdp_prog)
> +					bpf_prog_put(dev->xdp_prog);
>  				dev_put(dev->dev);
>  				kfree(dev);
>  			}
> @@ -245,6 +248,8 @@ static void dev_map_free(struct bpf_map *map)
>  			if (!dev)
>  				continue;
>  
> +			if (dev->xdp_prog)
> +				bpf_prog_put(dev->xdp_prog);
>  			dev_put(dev->dev);
>  			kfree(dev);
>  		}
> @@ -331,6 +336,16 @@ static int dev_map_hash_get_next_key(struct bpf_map *map, void *key,
>  	return -ENOENT;
>  }
>  
> +bool dev_map_can_have_prog(struct bpf_map *map)
> +{
> +	if ((map->map_type == BPF_MAP_TYPE_DEVMAP ||
> +	     map->map_type == BPF_MAP_TYPE_DEVMAP_HASH) &&
> +	    map->value_size != 4)
> +		return true;
> +
> +	return false;
> +}
> +
>  static int bq_xmit_all(struct xdp_dev_bulk_queue *bq, u32 flags)
>  {
>  	struct net_device *dev = bq->dev;
> @@ -455,6 +470,35 @@ static inline int __xdp_enqueue(struct net_device *dev, struct xdp_buff *xdp,
>  	return bq_enqueue(dev, xdpf, dev_rx);
>  }
>  
> +static struct xdp_buff *dev_map_run_prog(struct net_device *dev,
> +					 struct xdp_buff *xdp,
> +					 struct bpf_prog *xdp_prog)
> +{
> +	u32 act;
> +
> +	act = bpf_prog_run_xdp(xdp_prog, xdp);
> +	switch (act) {
> +	case XDP_DROP:
> +		fallthrough;
> +	case XDP_PASS:
> +		break;
> +	default:
> +		bpf_warn_invalid_xdp_action(act);
> +		fallthrough;
> +	case XDP_ABORTED:
> +		trace_xdp_exception(dev, xdp_prog, act);
> +		act = XDP_DROP;
> +		break;
> +	}
> +
> +	if (act == XDP_DROP) {
> +		xdp_return_buff(xdp);
> +		xdp = NULL;
> +	}
> +
> +	return xdp;
> +}
> +
>  int dev_xdp_enqueue(struct net_device *dev, struct xdp_buff *xdp,
>  		    struct net_device *dev_rx)
>  {
> @@ -466,6 +510,11 @@ int dev_map_enqueue(struct bpf_dtab_netdev *dst, struct xdp_buff *xdp,
>  {
>  	struct net_device *dev = dst->dev;
>  
> +	if (dst->xdp_prog) {
> +		xdp = dev_map_run_prog(dev, xdp, dst->xdp_prog);
> +		if (!xdp)
> +			return 0;
> +	}
>  	return __xdp_enqueue(dev, xdp, dev_rx);
>  }

Did you give any special consideration to where the hook should be? I'm
asking because my immediate thought was that it should be on flush
(i.e., in bq_xmit_all()), but now that I see this I'm so sure anymore.
What were your thoughts around this?

-Toke