netdev - Re: [PATCH bpf-next 07/13] net: Clear skb metadata on handover from device to protocol

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Message-ID: <aGK6hdOwBSC7r4gF@mini-arch>
Date: Mon, 30 Jun 2025 09:25:41 -0700
From: Stanislav Fomichev <stfomichev@...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 07/13] net: Clear skb metadata on handover from
 device to protocol

On 06/30, Jakub Sitnicki wrote:
> With the extension of bpf_dynptr_from_skb(BPF_DYNPTR_F_SKB_METADATA), all
> BPF programs authorized to call this kfunc now have access to the skb
> metadata area.
> 
> These programs can read up to skb_shinfo(skb)->meta_len bytes located just
> before skb_mac_header(skb), regardless of what data is currently there.
> 
> However, as the network stack processes the skb, headers may be added or
> removed. Hence, we cannot assume that skb_mac_header() always marks the end
> of the metadata area.
> 
> To avoid potential pitfalls, reset the skb metadata length to zero before
> passing the skb to the protocol layers. This is a temporary measure until
> we can make metadata persist through protocol processing.
> 
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> ---
>  net/core/dev.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index be97c440ecd5..4a2389997535 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5839,6 +5839,7 @@ static int __netif_receive_skb_core(struct sk_buff **pskb, bool pfmemalloc,
>  	}
>  #endif
>  	skb_reset_redirect(skb);
> +	skb_metadata_clear(skb);

And the assumption that it's not gonna break the existing cases is
because there is currently no way to read that metadata out afterwards?