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

On Mon, Jun 30, 2025 at 09:25 AM -07, Stanislav Fomichev wrote:

On 06/30, Jakub Sitnicki wrote:

quoted

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@cloudflare.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?

Correct, only tc_cls_act_is_valid_access tags the register as
PTR_TO_PACKET_META when loading __skb->data_meta, which allows access.

Something worth adding to the description. Thanks.