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

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Message-Id: <20250630-skb-metadata-thru-dynptr-v1-7-f17da13625d8@cloudflare.com>
Date: Mon, 30 Jun 2025 16:55:40 +0200
From: Jakub Sitnicki <jakub@...udflare.com>
To: bpf@...r.kernel.org
Cc: 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: [PATCH bpf-next 07/13] net: Clear skb metadata on handover from
 device to protocol

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);
 skip_classify:
 	if (pfmemalloc && !skb_pfmemalloc_protocol(skb))
 		goto drop;

-- 
2.43.0