Subject: Re: [PATCH RFC bpf-next 1/4] bpf: Handle 8-byte values in DEVMAP and DEVMAP_HASH

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 22 May 2020 14:08:05 +0200

On Thu, 21 May 2020 19:05:23 -0600
David Ahern <[email-masked]> wrote:

> Add support to DEVMAP and DEVMAP_HASH to support 8-byte values as a
> <device index, program id> pair. To do this, a new struct is needed in
> bpf_dtab_netdev to hold the values to return on lookup.

I would like to see us leverage BTF instead of checking on the size
attr->value_size. E.g do the sanity check based on BTF.
Given I don't know the exact details on how this should be done, I will
look into it... I already promised Lorenzo, as we have already
discussed this on IRC.

So, you can Lorenzo can go ahead with this approach, and test the
use-case. And I'll try to figure out if-and-how we can leverage BTF
here.  Input from BTF experts will be much appreciated.

> Signed-off-by: David Ahern <[email-masked]>
> ---
>  kernel/bpf/devmap.c | 53 ++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 40 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index a51d9fb7a359..2c01ce434306 100644
> --- a/kernel/bpf/devmap.c
> +++ b/kernel/bpf/devmap.c
> @@ -60,12 +60,19 @@ struct xdp_dev_bulk_queue {
>  	unsigned int count;
>  };
>  
> +/* devmap value can be dev index or dev index + prog id */
> +struct dev_map_ext_val {
> +	u32 ifindex;	/* must be first for compat with 4-byte values */
> +	u32 prog_id;
> +};
> +
>  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 rcu_head rcu;
>  	unsigned int idx;
> +	struct dev_map_ext_val val;
>  };
>  
>  struct bpf_dtab {
> @@ -108,9 +115,13 @@ static int dev_map_init_map(struct bpf_dtab *dtab, union bpf_attr *attr)
>  	u64 cost = 0;
>  	int err;
>  
> -	/* check sanity of attributes */
> +	/* check sanity of attributes. 2 value sizes supported:
> +	 * 4 bytes: ifindex
> +	 * 8 bytes: ifindex + prog id
> +	 */
>  	if (attr->max_entries == 0 || attr->key_size != 4 ||
> -	    attr->value_size != 4 || attr->map_flags & ~DEV_CREATE_FLAG_MASK)
> +	    (attr->value_size != 4 && attr->value_size != 8) ||
> +	    attr->map_flags & ~DEV_CREATE_FLAG_MASK)
>  		return -EINVAL;
>  
>  	/* Lookup returns a pointer straight to dev->ifindex, so make sure the
[...]

>  static int __dev_map_update_elem(struct net *net, struct bpf_map *map,
> @@ -568,8 +579,16 @@ static int __dev_map_update_elem(struct net *net, struct bpf_map *map,
>  {
>  	struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
>  	struct bpf_dtab_netdev *dev, *old_dev;
> -	u32 ifindex = *(u32 *)value;
>  	u32 i = *(u32 *)key;
> +	u32 ifindex;
> +
> +	if (map->value_size == 4) {
> +		ifindex = *(u32 *)value;
> +	} else {
> +		struct dev_map_ext_val *val = value;
> +
> +		ifindex = val->ifindex;
> +	}
>  
>  	if (unlikely(map_flags > BPF_EXIST))
>  		return -EINVAL;
> @@ -609,10 +628,18 @@ static int __dev_map_hash_update_elem(struct net *net, struct bpf_map *map,
>  {
>  	struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
>  	struct bpf_dtab_netdev *dev, *old_dev;
> -	u32 ifindex = *(u32 *)value;
>  	u32 idx = *(u32 *)key;
>  	unsigned long flags;
>  	int err = -EEXIST;
> +	u32 ifindex;
> +
> +	if (map->value_size == 4) {
> +		ifindex = *(u32 *)value;
> +	} else {
> +		struct dev_map_ext_val *val = value;
> +
> +		ifindex = val->ifindex;
> +	}
>  
>  	if (unlikely(map_flags > BPF_EXIST || !ifindex))
>  		return -EINVAL;

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer