Re: [PATCH bpf-next 04/15] xdp: add a kfunc to delete skb core extension entries

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 11/10/26 04:57, Tianyi Gao wrote:
> bpf_xdp_skb_ext_del() deletes the stored entry for an id. A program on
> cpumap or veth can use it to drop RX metadata that an earlier program
> stored.
> 
> Without the XDP_FLAGS_HAS_SKB_EXT flag, the kfunc does nothing, because
> the header bytes may then belong to the packet.
> 
> The kfunc only clears XDP_SKB_EXT_ENTRY_ACTIVE, so a later store of the
> same id reuses its offset. The flag and the size of the area stay the
> same, also after the last delete. To give the bytes back, the kfunc
> would have to move the entries after the deleted one. The cost is
> headroom that bpf_xdp_adjust_head() and bpf_xdp_adjust_meta() cannot
> use.
> 
> Signed-off-by: Tianyi Gao <tianyi@cloudflare.com>
> ---
> __skb_ext_del() frees struct skb_ext after the last delete, because it
> comes from a slab cache. The skb core extension area uses existing
> headroom, so it is not freed.
> ---
>   include/net/xdp.h |  8 ++++++++
>   net/core/xdp.c    | 26 ++++++++++++++++++++++++++
>   2 files changed, 34 insertions(+)

I think this approach of simply disabling the entry is okay.
This is trading off performance vs small area memory usage.

Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>

--Jesper