netdev - Re: [PATCH bpf-next 03/13] bpf: Add new variant of skb dynptr for the metadata area

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Message-ID: <CAEf4BzYjUc_ppemufs98YX+hvQ7vmSkBayuhsATkqCwOzh90aQ@mail.gmail.com>
Date: Tue, 1 Jul 2025 13:59:15 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jakub Sitnicki <jakub@...udflare.com>
Cc: bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>, 
	Arthur Fabre <arthur@...hurfabre.com>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	Jesse Brandeburg <jbrandeburg@...udflare.com>, Joanne Koong <joannelkoong@...il.com>, 
	Lorenzo Bianconi <lorenzo@...nel.org>, Toke Høiland-Jørgensen <thoiland@...hat.com>, 
	Yan Zhai <yan@...udflare.com>, netdev@...r.kernel.org, kernel-team@...udflare.com, 
	Stanislav Fomichev <sdf@...ichev.me>
Subject: Re: [PATCH bpf-next 03/13] bpf: Add new variant of skb dynptr for the
 metadata area

On Mon, Jun 30, 2025 at 7:56 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
>
> Add a new flag for the bpf_dynptr_from_skb helper to let users to create
> dynptrs to skb metadata area. Access paths are stubbed out. Implemented by
> the following changes.
>
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> ---
>  include/uapi/linux/bpf.h |  9 ++++++++
>  net/core/filter.c        | 60 +++++++++++++++++++++++++++++++++++++++++-------
>  2 files changed, 61 insertions(+), 8 deletions(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 719ba230032f..ab5730d2fb29 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -7591,4 +7591,13 @@ enum bpf_kfunc_flags {
>         BPF_F_PAD_ZEROS = (1ULL << 0),
>  };
>
> +/**
> + * enum bpf_dynptr_from_skb_flags - Flags for bpf_dynptr_from_skb()
> + *
> + * @BPF_DYNPTR_F_SKB_METADATA: Create dynptr to the SKB metadata area
> + */
> +enum bpf_dynptr_from_skb_flags {
> +       BPF_DYNPTR_F_SKB_METADATA = (1ULL << 0),
> +};
> +
>  #endif /* _UAPI__LINUX_BPF_H__ */
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 1fee51b72220..3c2948517838 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -11967,12 +11967,27 @@ bpf_sk_base_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>         return func;
>  }
>
> +enum skb_dynptr_offset {
> +       SKB_DYNPTR_METADATA     = -1,
> +       SKB_DYNPTR_PAYLOAD      = 0,
> +};

I'm missing why you need to do it in this hacky way instead of just
having both bpf_dynptr_from_skb() and bpf_dynptr_from_skb_metadata()
(or whatever we bikeshed it into), which will create
BPF_DYNPTR_TYPE_SKB or new BPF_DYNPTR_TYPE_SKB_META dynptr kind,
respectively. Why so complicated?

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