Re: [PATCH bpf-next 2/4] bpf: Craft non-linear skbs in BPF_PROG_TEST_RUN

[Martin KaFai Lau <martin.lau@linux.dev>]

On 9/8/25 10:41 AM, Paul Chaignon wrote:
>>>> How about letting users specify the linear size through ctx->data_end? I am
>>>> working on a set that introduces a kfunc, bpf_xdp_pull_data(). A part of is
>>>> to support non-linear xdp_buff in test_run_xdp, and I am doing it through
>>>> ctx->data_end. Is it something reasonable for test_run_skb?
>>> Oh, nice! That was next on my list 🙂
>>>
>>> Why use data_end though? I guess it'd work for skb, but can't we just
>>> add a new field to the anonymous struct for BPF_PROG_TEST_RUN?
>>>
>> I choose to use ctx_in because it doesn't change the interface and
>> feels natural. kattr->test.ctx_in is already copied from users and
>> shows users' expectation about the input ctx. I think we should honor
>> that (as long as the value makes sense). WDYT?
> Ok, I think I see your point of view. To me, test.ctx_in*is* the
> context and not metadata about it. I'm worried it would be weird to
> users if we overload that field. I'm not against using that though if
> it's the consensus.

On the xdp side, the test_run_xdp has been using the ctx_in->data[_meta] to 
allow the user to specify the meta data (since commit 47316f4a3053). Using 
ctx_in->data_end [1]  to specify the end of the linear part seems to be a 
logical addition in xdp.

> 
>> Thanks for working on this. It would be great if there is some
>> consistency between test_run_skb and test_run_xdp.
> Definitely agree! Do you have a prototype anywhere I could check? If
> not, you can always flag any inconsistency when I send the v2.
[1]: https://lore.kernel.org/bpf/20250905173352.3759457-6-ameryhung@gmail.com/