From: Martin KaFai Lau <martin.lau@linux.dev>
To: "Asbjørn Sloth Tønnesen" <ast@fiberby.net>,
"Toke Høiland-Jørgensen" <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
Subject: Re: [PATCH bpf-next] selftests/bpf: Avoid subtraction after htons() in ipip tests
Date: Tue, 13 Aug 2024 18:15:54 -0700 [thread overview]
Message-ID: <77a5dbd2-7792-486c-b161-76e2262b67a7@linux.dev> (raw)
In-Reply-To: <87sevfpdti.fsf@toke.dk>
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.
prev parent reply other threads:[~2024-08-14 1:16 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-08-08 7:59 [PATCH bpf-next] selftests/bpf: Avoid subtraction after htons() in ipip tests Asbjørn Sloth Tønnesen
2024-08-08 9:24 ` Toke Høiland-Jørgensen
2024-08-14 1:15 ` Martin KaFai Lau [this message]
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