linux-kernel - Re: [PATCH v4] bpftool: Add 'prepend' option for tcx attach to insert at chain start

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Message-ID: <b0a95722-23ed-4f1a-aeac-ded6983693b0@kernel.org>
Date: Mon, 12 Jan 2026 10:20:37 +0000
From: Quentin Monnet <qmo@...nel.org>
To: gyutae.opensource@...ercorp.com, bpf@...r.kernel.org,
 Daniel Borkmann <daniel@...earbox.net>
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 v4] bpftool: Add 'prepend' option for tcx attach to insert
 at chain start

2026-01-12 12:45 UTC+0900 ~ gyutae.opensource@...ercorp.com
> 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 uses BPF_F_BEFORE flag which automatically inserts
> the program at the beginning of the chain when no relative reference
> is specified.
> 
> This change includes:
> - 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
> - Add validation to reject 'overwrite' for non-XDP attach types
> 
> 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.
> 
> Co-developed-by: Siwan Kim <siwan.kim@...ercorp.com>
> Signed-off-by: Siwan Kim <siwan.kim@...ercorp.com>
> Signed-off-by: Gyutae Bae <gyutae.bae@...ercorp.com>
> Reviewed-by: Quentin Monnet <qmo@...nel.org>
> ---
> Hi Quentin.
> 
> Thank you for the review! I have added the validation for 'overwrite'
> option as you suggested.
> 
> I used a whitelist approach (rejecting non-XDP types) rather than
> a blacklist approach (rejecting TCX types) to be consistent with the
> 'prepend' validation style and to ensure that any future attach types
> will also be rejected by default unless explicitly allowed.


Looks good, thank you!
Quentin