Re: [PATCH bpf-next v4 1/8] bpf: Add dynptr type for skb metadata

[Jakub Sitnicki <jakub@cloudflare.com>]

On Wed, Jul 23, 2025 at 06:54 PM -07, Martin KaFai Lau wrote:
> On 7/23/25 10:36 AM, Jakub Sitnicki wrote:
>> More importantly, it abstracts away the fact where the storage for the
>> custom metadata lives, which opens up the way to persist the metadata by
>> relocating it as the skb travels through the network stack layers.
>> A notable difference between the skb and the skb_meta dynptr is that writes
>> to the skb_meta dynptr don't invalidate either skb or skb_meta dynptr
>> slices, since they cannot lead to a skb->head reallocation.
>
> There is not much visibility on how the metadata will be relocated, so trying to
> think out loud. The "no invalidation after bpf_dynptr_write(&meta_dynptr, ..."
> behavior will be hard to change in the future. Will this still hold in the
> future when the metadata can be preserved?
>
> Also, following up on Kuba's point about clone skb, what if the bpf prog wants
> to write metadata to a clone skb in the future by using bpf_dynptr_write?

Good point. If we decide to implement a copy-on-write in the future,
then this will be a problem. Why tie our hands if it's not a huge
nuissance for the user? Let me restrict it.