Re: [PATCH bpf-next v1 1/3] bpf: Add skb dynptrs

[Jakub Kicinski <kuba@kernel.org>]

(consider cross-posting network-related stuff to netdev@)

On Tue, 26 Jul 2022 11:47:04 -0700 Joanne Koong wrote:
> Add skb dynptrs, which are dynptrs whose underlying pointer points
> to a skb. The dynptr acts on skb data. skb dynptrs have two main
> benefits. One is that they allow operations on sizes that are not
> statically known at compile-time (eg variable-sized accesses).
> Another is that parsing the packet data through dynptrs (instead of
> through direct access of skb->data and skb->data_end) can be more
> ergonomic and less brittle (eg does not need manual if checking for
> being within bounds of data_end).

Is there really a need for dynptr_from_{skb,xdp} to be different
function IDs? I was hoping this work would improve portability of
networking BPF programs across the hooks.

> For bpf prog types that don't support writes on skb data, the dynptr is
> read-only (writes and data slices are not permitted). For reads on the
> dynptr, this includes reading into data in the non-linear paged buffers
> but for writes and data slices, if the data is in a paged buffer, the
> user must first call bpf_skb_pull_data to pull the data into the linear
> portion.
> 
> Additionally, any helper calls that change the underlying packet buffer
> (eg bpf_skb_pull_data) invalidates any data slices of the associated
> dynptr.

Grepping the verifier did not help me find that, would you mind
pointing me to the code?

> Right now, skb dynptrs can only be constructed from skbs that are
> the bpf program context - as such, there does not need to be any
> reference tracking or release on skb dynptrs.