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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Thu, 08 Aug 2024 11:24:25 +0200

Asbjørn Sloth Tønnesen <[email-masked]> 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 <[email-masked]>

Reviewed-by: Toke Høiland-Jørgensen <[email-masked]>