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

[Jakub Sitnicki <jakub@cloudflare.com>]

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