netdev - Re: [External] [Patch bpf 2/2] selftests/bpf: Add a BPF selftest for bpf_skb_change

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Message-ID: <Z0i4i0Db8EnRjzZJ@pop-os.localdomain>
Date: Thu, 28 Nov 2024 10:38:03 -0800
From: Cong Wang <xiyou.wangcong@...il.com>
To: John Fastabend <john.fastabend@...il.com>
Cc: Zijian Zhang <zijianzhang@...edance.com>, netdev@...r.kernel.org,
	bpf@...r.kernel.org, Cong Wang <cong.wang@...edance.com>,
	Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [External] [Patch bpf 2/2] selftests/bpf: Add a BPF selftest for
 bpf_skb_change_tail()

On Tue, Nov 26, 2024 at 11:34:25PM -0800, John Fastabend wrote:
> Zijian Zhang wrote:
> > 
> > LGTM!
> > 
> > I think it will be better if the test could also cover the case you
> > indicated in the first patch, where skb_transport_offset is a negative
> > value.
> > 
> > Thanks,
> > Zijian
> > 
> 
> Hi Cong,
> 
> I agree it would be great to see the skb_transport_offset is
> negative pattern. Could we add it?

Hmm? It is already negative for sockmap, as I already mentioned in patch
1/1:

"skb_transport_offset() and skb_transport_offset() can be negative when
they are called after we pull the transport header, for example, when
we use eBPF sockmap (aka at the point of ->sk_data_ready())."

My test case uses skb verdict, which is one of the sockmap hooks.

Or I guess you mean positive? In that case, we would need hook different
locations, like TC. I can certainly add it, but once again, it would
make backporting this patchset even harder.

Thanks!