Re: [PATCH bpf-next] selftests/bpf: Avoid subtraction after htons() in ipip tests

[Martin KaFai Lau]

On 8/8/24 2:24 AM, Toke Høiland-Jørgensen wrote:
> Asbjørn Sloth Tønnesen <ast@xxxxxxxxxxx> writes:
>
> > On little-endian systems, doing subtraction after htons()
> > leads to interesting results:
> >
> > Given:
> >    MAGIC_BYTES = 123 = 0x007B aka. in big endian: 0x7B00 = 31488
> >    sizeof(struct iphdr) = 20
> >
> > Before this patch:
> > __bpf_constant_htons(MAGIC_BYTES) - sizeof(struct iphdr) = 0x7AEC
> > 0x7AEC = htons(0xEC7A) = htons(60538)
> >
> > So these were outer IP packets with a total length of 123 bytes,
> > containing an inner IP packet with a total length of 60538 bytes.
>
> It's just using bag of holding technology!
>
> > After this patch:
> > __bpf_constant_htons(MAGIC_BYTES - sizeof(struct iphdr)) = htons(103)
> >
> > Now these packets are outer IP packets with a total length of 123 bytes,
> > containing an inner IP packet with a total length of 103 bytes.
> >
> > Signed-off-by: Asbjørn Sloth Tønnesen <ast@xxxxxxxxxxx>
>
> Reviewed-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>

Applied to bpf-next/net. Thanks.