Re: [PATCH bpf-next v3 01/10] bpf: Add dynptr type for skb metadata

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2025-07-21 at 12:52 +0200, Jakub Sitnicki wrote:
> Add a dynptr type, similar to skb dynptr, but for the skb metadata access.
> 
> The dynptr provides an alternative to __sk_buff->data_meta for accessing
> the custom metadata area allocated using the bpf_xdp_adjust_meta() helper.
> 
> More importantly, it abstracts away the fact where the storage for the
> custom metadata lives, which opens up the way to persist the metadata by
> relocating it as the skb travels through the network stack layers.
> 
> A notable difference between the skb and the skb_meta dynptr is that writes
> to the skb_meta dynptr don't invalidate either skb or skb_meta dynptr
> slices, since they cannot lead to a skb->head reallocation.
> 
> skb_meta dynptr ops are stubbed out and implemented by subsequent changes.
> 
> Only the program types which can access __sk_buff->data_meta today are
> allowed to create a dynptr for skb metadata at the moment. We need to
> modify the network stack to persist the metadata across layers before
> opening up access to other BPF hooks.
> 
> Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>


> @@ -2274,7 +2278,8 @@ static bool reg_is_pkt_pointer_any(const struct bpf_reg_state *reg)
>  static bool reg_is_dynptr_slice_pkt(const struct bpf_reg_state *reg)
>  {
>  	return base_type(reg->type) == PTR_TO_MEM &&
> -		(reg->type & DYNPTR_TYPE_SKB || reg->type & DYNPTR_TYPE_XDP);
> +	       (reg->type &
> +		(DYNPTR_TYPE_SKB | DYNPTR_TYPE_XDP | DYNPTR_TYPE_SKB_META));
>  }

Note: This function is used to identify pointers to packet data that
      might be stale after call to one of the functions in list [1].
      Once such pointers are identified, verifier would disallow
      access through these pointers.
      dynptr_from_skb_meta() is implemented as:

        bpf_dynptr_init(ptr, skb, BPF_DYNPTR_TYPE_SKB_META, 0, skb_metadata_len(skb));

      here any read or write goes through skb object, not a pointer derived from it.
      Given above, is it still necessary to list DYNPTR_FROM_SKB_META here?
      Or some functions from [1] can change skb_metadata_len(skb)?

[1] https://elixir.bootlin.com/linux/v6.15.7/source/net/core/filter.c#L7989

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