Re: [PATCH v2 5/7] seg6: add End.M.GTP6.D.Di behavior

[Andrea Mayer <andrea@common-net.org>]

On Tue, 05 May 2026 01:30:15 +0900
Yuya Kusakabe <yuya.kusakabe@gmail.com> wrote:

Hi Yuya,

I do not repeat below the points from the cover letter thread and patch
1-4 replies (drop reasons, OIF/VRF removal, C helper, coding style, etc.).

The patch 4 review applies here, except for the parts where Section 6.4 is
implemented instead of Section 6.3 (which is incorrectly implemented in
patch 4).

> Add the End.M.GTP6.D.Di drop-in mode variant of End.M.GTP6.D
> (RFC 9433 Section 6.4).  Unlike End.M.GTP6.D, the drop-in variant
> does NOT fold the GTP-U identifiers into Args.Mob.Session: the
> original outer IPv6 destination is preserved at SRH[0] of the new
> SRH, so the destination side can keep the original address
> untouched while still benefiting from SR Policy steering.
>
> The augmented SRH builder/destroyer is shared with End.M.GTP6.D.
> The TEID and QFI parsed out of the inbound GTP-U header are
> intentionally discarded for this variant (matching RFC 9433
> Section 6.4).
>
> When net.netfilter.nf_hooks_lwtunnel=1, the inner T-PDU traverses
> NF_INET_PRE_ROUTING between the GTP-U strip and the SRv6 push,
> mirroring End.DX4 / End.DX6.
>
> Non-T-PDU GTP-U messages are forwarded the same way as in
> End.M.GTP6.D: passed through via the lwtunnel's saved orig_input
> to a downstream peer that owns the GTP-U control plane.
>
> Configuration:
>
>   ip -6 route add 2001:db8:f::/64 \
>       encap seg6local action End.M.GTP6.D.Di \
>           srh segs 2001:db8:2::e,2001:db8:3::e \
>           src 2001:db8:2::1 \
>       dev <dev>
>
> Link: https://www.rfc-editor.org/rfc/rfc9433.html#section-6.4
> Signed-off-by: Yuya Kusakabe <yuya.kusakabe@gmail.com>
> ---
>  include/uapi/linux/seg6_local.h                    |   2 +
>  net/ipv6/seg6_local.c                              | 222 +++++++++++
>  tools/testing/selftests/net/Makefile               |   1 +
>  .../selftests/net/srv6_end_m_gtp6_d_di_test.sh     | 427 +++++++++++++++++++++
>  4 files changed, 652 insertions(+)
>
> diff --git a/include/uapi/linux/seg6_local.h b/include/uapi/linux/seg6_local.h
> index 7d3d3d245b47..326da65ad5aa 100644
> --- a/include/uapi/linux/seg6_local.h
> +++ b/include/uapi/linux/seg6_local.h
> @@ -80,6 +80,8 @@ enum {
>  	SEG6_LOCAL_ACTION_END_M_GTP6_E	= 19,
>  	/* IPv6/GTP-U decap into SRv6 (RFC 9433 Section 6.3) */
>  	SEG6_LOCAL_ACTION_END_M_GTP6_D	= 20,
> +	/* IPv6/GTP-U decap into SRv6, drop-in mode (RFC 9433 Section 6.4) */
> +	SEG6_LOCAL_ACTION_END_M_GTP6_D_DI = 21,
>
>  	__SEG6_LOCAL_ACTION_MAX,
>  };
> diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
> index 09e912e17df8..a6cd57ebcbde 100644
> --- a/net/ipv6/seg6_local.c
> +++ b/net/ipv6/seg6_local.c

> + [snip]

input_action_end_m_gtp6_d_di() and its finish callback are largely
identical to the patch 4 functions (input_action_end_m_gtp6_d() and its
finish): the SRH check, GTP-U dispatch, outer strip, inner protocol
detection, and NF_HOOK invocation are identical. The duplication should be
reduced via shared helpers.

> +static int input_action_end_m_gtp6_d_di(struct sk_buff *skb,
> +					struct seg6_local_lwt *slwt)
> +{

> + [snip]

> +		gtp_hdrlen = seg6_mobile_parse_gtpu(skb,
> +						    upper_off + sizeof(*uh),
> +						    &teid, &qfi);
> +		if (gtp_hdrlen == -EOPNOTSUPP)
> +			return seg6_mobile_passthrough_non_tpdu(skb);
> +		if (gtp_hdrlen < 0) {
> +			reason = SKB_DROP_REASON_SEG6_MOBILE_BAD_GTPU;
> +			goto drop;
> +		}
> +		(void)teid;
> +		(void)qfi;

D.Di does not use teid or qfi, so these variables and the (void) casts are
dead code and should be avoided. For example, seg6_mobile_parse_gtpu() could
accept NULL for teid and qfi so callers that do not need them can pass NULL
directly.

Thanks,

Ciao,
Andrea

P.S. I am temporarily writing from another address due to a mail
delivery issue at my @uniroma2.it address. Please always Cc my default
andrea.mayer@uniroma2.it address on replies.