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

[Daniel Borkmann]

On 1/12/26 4:45 AM, gyutae.opensource@xxxxxxxxxxxxx wrote:
> From: Gyutae Bae <gyutae.bae@xxxxxxxxxxxxx>
>
> 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.
Acked-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>