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From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@kernel.org>
To: =?utf-8?Q?Asbj=C3=B8rn?= Sloth =?utf-8?Q?T=C3=B8nnesen?=
 <ast@fiberby.net>, bpf@vger.kernel.org
Cc: =?utf-8?Q?Asbj=C3=B8rn?= Sloth =?utf-8?Q?T=C3=B8nnesen?=
 <ast@fiberby.net>, Alexei Starovoitov
 <ast@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>, Andrii Nakryiko
 <andrii@kernel.org>, Martin KaFai Lau <martin.lau@linux.dev>, Eduard
 Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>, Yonghong Song
 <yonghong.song@linux.dev>, John Fastabend <john.fastabend@gmail.com>, KP
 Singh <kpsingh@kernel.org>, Stanislav Fomichev <sdf@fomichev.me>, Hao Luo
 <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>, Mykola Lysenko
 <mykolal@fb.com>, Shuah Khan <shuah@kernel.org>, Tushar Vyavahare
 <tushar.vyavahare@intel.com>, Magnus Karlsson <magnus.karlsson@intel.com>,
 Willem de Bruijn <willemb@google.com>, netdev@vger.kernel.org,
 linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH bpf-next] selftests/bpf: Avoid subtraction after htons()
 in ipip tests
In-Reply-To: <20240808075906.1849564-1-ast@fiberby.net>
References: <20240808075906.1849564-1-ast@fiberby.net>
X-Clacks-Overhead: GNU Terry Pratchett
Date: Thu, 08 Aug 2024 11:24:25 +0200
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Asbj=C3=B8rn Sloth T=C3=B8nnesen <ast@fiberby.net> writes:

> On little-endian systems, doing subtraction after htons()
> leads to interesting results:
>
> Given:
>   MAGIC_BYTES =3D 123 =3D 0x007B aka. in big endian: 0x7B00 =3D 31488
>   sizeof(struct iphdr) =3D 20
>
> Before this patch:
> __bpf_constant_htons(MAGIC_BYTES) - sizeof(struct iphdr) =3D 0x7AEC
> 0x7AEC =3D htons(0xEC7A) =3D 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)) =3D 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: Asbj=C3=B8rn Sloth T=C3=B8nnesen <ast@fiberby.net>

Reviewed-by: Toke H=C3=B8iland-J=C3=B8rgensen <toke@kernel.org>