Re: [PATCH RFC net-next 1/6] bpf: Introduce per-packet metadata storage for BPF programs

[Stanislav Fomichev <sdf.kernel@gmail.com>]

On 07/14, Jakub Sitnicki wrote:
> BPF programs attached at different points in the network stack have no way
> to pass data between each other on a per-packet basis, other than by
> stashing it into a shared BPF map. xdp/skb->data_meta works for XDP-to-TC
> handoff, but is not available to programs running at later hooks like
> cgroup/skb, sock_ops, socket filters, tracing or LSM.
> 
> Add a new skb extension (struct bpf_skb_ext) that provides up to 256 bytes
> of per-packet storage. Size is configurable at build time though the
> CONFIG_BPF_SKB_EXT_SIZE option. The storage is embedded inside the
> extension chunk itself.
> 
> Expose the storage to BPF programs via bpf_dynptr_from_skb_ext() kfunc.
> The caller passes BPF_SKB_EXT_F_CREATE to allocate or COW (unshare) the
> extension and get a read-write dynptr. Without the flag, it gets a
> read-only dynptr to the existing extension, or -ENOENT if none exists.
> 
> Guard the feature behind a new CONFIG_BPF_SKB_EXT option.
> 
> Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
> ---
>  include/linux/bpf.h      |  10 ++++
>  include/linux/filter.h   |  26 ++++++++++
>  include/linux/skbuff.h   |   3 ++
>  include/uapi/linux/bpf.h |   5 ++
>  kernel/bpf/helpers.c     |   7 +++
>  kernel/bpf/log.c         |   2 +
>  kernel/bpf/verifier.c    |  10 +++-
>  net/Kconfig              |  20 ++++++++
>  net/core/filter.c        | 129 +++++++++++++++++++++++++++++++++++++++++++++++
>  net/core/skbuff.c        |   3 ++
>  10 files changed, 214 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 7719f6528445..6b918a5b61bf 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1484,6 +1484,8 @@ enum bpf_dynptr_type {
>  	BPF_DYNPTR_TYPE_SKB_META,
>  	/* Underlying data is a file */
>  	BPF_DYNPTR_TYPE_FILE,
> +	/* Underlying data is a bpf_skb_ext chunk */
> +	BPF_DYNPTR_TYPE_SKB_EXT,
>  };
>  
>  int bpf_dynptr_check_size(u64 size);
> @@ -4209,4 +4211,12 @@ static inline int bpf_map_check_op_flags(struct bpf_map *map, u64 flags, u64 all
>  	return 0;
>  }
>  
> +#ifdef CONFIG_BPF_SKB_EXT
> +
> +struct bpf_skb_ext {
> +	u8 buf[CONFIG_BPF_SKB_EXT_SIZE] __aligned(8);
> +};

Can we do a dynamic size from the start? Say, some sysfs knob, 0 by
default. Once written, it's locks in the size and can't be changed.
Then your new 'flags' field can be used to indicate whether the area
actually has been allocated or not?