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Date: Tue, 13 Aug 2024 18:15:54 -0700
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Subject: Re: [PATCH bpf-next] selftests/bpf: Avoid subtraction after htons()
 in ipip tests
To: =?UTF-8?Q?Asbj=C3=B8rn_Sloth_T=C3=B8nnesen?= <ast@fiberby.net>,
 =?UTF-8?Q?Toke_H=C3=B8iland-J=C3=B8rgensen?= <toke@kernel.org>
Cc: bpf@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
 Daniel Borkmann <daniel@iogearbox.net>, Andrii Nakryiko <andrii@kernel.org>,
 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
References: <20240808075906.1849564-1-ast@fiberby.net>
 <87sevfpdti.fsf@toke.dk>
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From: Martin KaFai Lau <martin.lau@linux.dev>
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On 8/8/24 2:24 AM, Toke Høiland-Jørgensen wrote:
> Asbjørn Sloth Tønnesen <ast@fiberby.net> 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@fiberby.net>
> 
> Reviewed-by: Toke Høiland-Jørgensen <toke@kernel.org>

Applied to bpf-next/net. Thanks.