linux-kernel - Re: [PATCH v2] bpftool: Add 'prepend' option for tcx attach to insert at chain start

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Message-ID: <545a9978-41fb-485b-8ad8-fe759322d4a4@iogearbox.net>
Date: Tue, 6 Jan 2026 14:11:49 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: gyutae.opensource@...ercorp.com, Quentin Monnet <qmo@...nel.org>,
 bpf@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Gyutae Bae <gyutae.bae@...ercorp.com>,
 Siwan Kim <siwan.kim@...ercorp.com>, Daniel Xu <dxu@...uu.xyz>,
 Jiayuan Chen <jiayuan.chen@...ux.dev>, Tao Chen <chen.dylane@...ux.dev>,
 Kumar Kartikeya Dwivedi <memxor@...il.com>
Subject: Re: [PATCH v2] bpftool: Add 'prepend' option for tcx attach to insert
 at chain start

Hi Gyutae,

On 1/6/26 9:55 AM, gyutae.opensource@...ercorp.com wrote:
> From: Gyutae Bae <gyutae.bae@...ercorp.com>
> 
> 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