Subject: Re: [PATCH bpf 2/3] bpf: devmap adjust uapi for attach bpf program

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Mon, 08 Jun 2020 20:30:08 +0200

Jesper Dangaard Brouer <[email-masked]> writes:

> The recent commit fbee97feed9b ("bpf: Add support to attach bpf program to a
> devmap entry"), introduced ability to attach (and run) a separate XDP
> bpf_prog for each devmap entry. A bpf_prog is added via a file-descriptor.
> As zero were a valid FD, not using the feature requires using value minus-1.
> The UAPI is extended via tail-extending struct bpf_devmap_val and using
> map->value_size to determine the feature set.
>
> This will break older userspace applications not using the bpf_prog feature.
> Consider an old userspace app that is compiled against newer kernel
> uapi/bpf.h, it will not know that it need to initialise the member
> bpf_prog.fd to minus-1. Thus, users will be forced to update source code to
> get program running on newer kernels.
>
> As previous patch changed BPF-syscall to avoid returning file descriptor
> value zero, we can remove the minus-1 checks, and have zero mean feature
> isn't used.
>
> Fixes: fbee97feed9b ("bpf: Add support to attach bpf program to a devmap entry")
> Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> ---
>  include/uapi/linux/bpf.h       |   13 +++++++++++++
>  kernel/bpf/devmap.c            |   17 ++++-------------
>  tools/include/uapi/linux/bpf.h |    5 -----
>  3 files changed, 17 insertions(+), 18 deletions(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index c65b374a5090..19684813faae 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3761,6 +3761,19 @@ struct xdp_md {
>  	__u32 egress_ifindex;  /* txq->dev->ifindex */
>  };
>  
> +/* DEVMAP map-value layout
> + *
> + * The struct data-layout of map-value is a configuration interface.
> + * New members can only be added to the end of this structure.
> + */
> +struct bpf_devmap_val {
> +	__u32 ifindex;   /* device index */
> +	union {
> +		int   fd;  /* prog fd on map write */
> +		__u32 id;  /* prog id on map read */
> +	} bpf_prog;
> +};
> +
>  enum sk_action {
>  	SK_DROP = 0,
>  	SK_PASS,
> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index 854b09beb16b..d268a8e693cb 100644
> --- a/kernel/bpf/devmap.c
> +++ b/kernel/bpf/devmap.c
> @@ -60,15 +60,6 @@ struct xdp_dev_bulk_queue {
>  	unsigned int count;
>  };
>  
> -/* DEVMAP values */
> -struct bpf_devmap_val {
> -	u32 ifindex;   /* device index */
> -	union {
> -		int fd;  /* prog fd on map write */
> -		u32 id;  /* prog id on map read */
> -	} bpf_prog;
> -};
> -
>  struct bpf_dtab_netdev {
>  	struct net_device *dev; /* must be first member, due to tracepoint */
>  	struct hlist_node index_hlist;
> @@ -618,7 +609,7 @@ static struct bpf_dtab_netdev *__dev_map_alloc_node(struct net *net,
>  	if (!dev->dev)
>  		goto err_out;
>  
> -	if (val->bpf_prog.fd >= 0) {
> +	if (val->bpf_prog.fd > 0) {
>  		prog = bpf_prog_get_type_dev(val->bpf_prog.fd,
>  					     BPF_PROG_TYPE_XDP, false);
>  		if (IS_ERR(prog))
> @@ -652,8 +643,8 @@ static int __dev_map_update_elem(struct net *net, struct bpf_map *map,
>  				 void *key, void *value, u64 map_flags)
>  {
>  	struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
> -	struct bpf_devmap_val val = { .bpf_prog.fd = -1 };
>  	struct bpf_dtab_netdev *dev, *old_dev;
> +	struct bpf_devmap_val val = {0};

nit: I prefer {} to {0} - 'git grep' indicates that {} is also the most
commonly used in kernel/bpf/

>  	u32 i = *(u32 *)key;
>  
>  	if (unlikely(map_flags > BPF_EXIST))
> @@ -669,7 +660,7 @@ static int __dev_map_update_elem(struct net *net, struct bpf_map *map,
>  	if (!val.ifindex) {
>  		dev = NULL;
>  		/* can not specify fd if ifindex is 0 */
> -		if (val.bpf_prog.fd != -1)
> +		if (val.bpf_prog.fd > 0)
>  			return -EINVAL;
>  	} else {
>  		dev = __dev_map_alloc_node(net, dtab, &val, i);
> @@ -699,8 +690,8 @@ static int __dev_map_hash_update_elem(struct net *net, struct bpf_map *map,
>  				     void *key, void *value, u64 map_flags)
>  {
>  	struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
> -	struct bpf_devmap_val val = { .bpf_prog.fd = -1 };
>  	struct bpf_dtab_netdev *dev, *old_dev;
> +	struct bpf_devmap_val val = {0};

Same here.

>  	u32 idx = *(u32 *)key;
>  	unsigned long flags;
>  	int err = -EEXIST;
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index c65b374a5090..868e9efe9c09 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -3761,11 +3761,6 @@ struct xdp_md {
>  	__u32 egress_ifindex;  /* txq->dev->ifindex */
>  };
>  
> -enum sk_action {
> -	SK_DROP = 0,
> -	SK_PASS,
> -};
> -
>  /* user accessible metadata for SK_MSG packet hook, new fields must
>   * be added to the end of this structure
>   */