From: "Toke Høiland-Jørgensen" <toke@kernel.org>
To: "Asbjørn Sloth Tønnesen" <ast@fiberby.net>, bpf@vger.kernel.org
Cc: "Asbjørn Sloth Tønnesen" <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
Date: Thu, 08 Aug 2024 11:24:25 +0200 [thread overview]
Message-ID: <87sevfpdti.fsf@toke.dk> (raw)
In-Reply-To: <20240808075906.1849564-1-ast@fiberby.net>
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>
next prev parent reply other threads:[~2024-08-08 9:24 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 [this message]
2024-08-14 1:15 ` Martin KaFai Lau
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