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Received: from sslproxy08.your-server.de ([78.47.166.52]) by www62.your-server.de with esmtpsa (TLS1.3) tls TLS_AES_256_GCM_SHA384 (Exim 4.96.2) (envelope-from ) id 1vd6qm-000Esp-06; Tue, 06 Jan 2026 14:11:52 +0100 Received: from localhost ([127.0.0.1]) by sslproxy08.your-server.de with esmtpsa (TLS1.3) tls TLS_AES_256_GCM_SHA384 (Exim 4.96) (envelope-from ) id 1vd6qk-000Fm9-1l; Tue, 06 Jan 2026 14:11:50 +0100 Message-ID: <545a9978-41fb-485b-8ad8-fe759322d4a4@iogearbox.net> Date: Tue, 6 Jan 2026 14:11:49 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] bpftool: Add 'prepend' option for tcx attach to insert at chain start To: gyutae.opensource@navercorp.com, Quentin Monnet , bpf@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Alexei Starovoitov , Andrii Nakryiko , Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , Gyutae Bae , Siwan Kim , Daniel Xu , Jiayuan Chen , Tao Chen , Kumar Kartikeya Dwivedi References: <43c23468-530b-45f3-af22-f03484e5148c@kernel.org> <20260106085527.4774-1-gyutae.opensource@navercorp.com> Content-Language: en-US From: Daniel Borkmann Autocrypt: addr=daniel@iogearbox.net; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Virus-Scanned: Clear (ClamAV 1.0.9/27872/Tue Jan 6 08:24:33 2026) Hi Gyutae, On 1/6/26 9:55 AM, gyutae.opensource@navercorp.com wrote: > From: Gyutae Bae > > Add support for the 'prepend' option when attaching tcx_ingress and > tcx_egress programs. This option allows inserting a BPF program at > the beginning of the TCX chain instead of appending it at the end. > > The implementation queries the first program ID in the chain and uses > BPF_F_BEFORE flag with the relative_id to insert the new program before > the existing first program. If the chain is empty, the program is simply > attached normally. > > This change includes: > - Add get_first_tcx_prog_id() helper to retrieve the first program ID > - Modify do_attach_tcx() to support prepend insertion using BPF_F_BEFORE > - Update documentation to describe the new 'prepend' option > - Add bash completion support for the 'prepend' option on tcx attach types > - Add example usage in the documentation > > The 'prepend' option is only valid for tcx_ingress and tcx_egress attach > types. For XDP attach types, the existing 'overwrite' option remains > available. > > Example usage: > # bpftool net attach tcx_ingress name tc_prog dev lo prepend > > This feature is useful when the order of program execution in the TCX > chain matters and users need to ensure certain programs run first. Could we make this a bit more generic? The internal API has BPF_F_BEFORE and BPF_F_AFTER flags, so we could also support relative ids. Alternatively "prepend" / "append" is imho also ok and the "before" / "after" could be added at a later point to bpftool. BPF_F_BEFORE as a standalone flag (and BPF_F_AFTER as a standalone) flag will have prepend and append behavior, so your approach of adding get_first_tcx_prog_id() helper to retrieve the first program id is not necessary, see also tcx BPF selftests [0]. Thanks, Daniel [0] tools/testing/selftests/bpf/prog_tests/tc_links.c