Re: [PATCH net-next v2 2/5] seg6: add RFC8986 flavor support for End.X

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- [Medium] End.X now accepts the flavor pair NEXT-C-SID + PSP but only
  applies NEXT-C-SID.

--

seg6: add RFC8986 flavor support for End.X

This patch adds PSP flavor processing to the SRv6 End.X behavior by reusing
end_flv8986_core(). It also widens SEG6_LOCAL_END_X_FLV_SUPP_OPS so that
End.X advertises PSP support alongside NEXT-C-SID.

> diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
> index 4fcfcd48cfaed..521a9b0c09290 100644
> --- a/net/ipv6/seg6_local.c
> +++ b/net/ipv6/seg6_local.c
> @@ -121,7 +121,8 @@ struct bpf_lwt_prog {
>  
>  #define SEG6_LOCAL_END_FLV_SUPP_OPS	(SEG6_F_LOCAL_FLV_NEXT_CSID | \
>  					 SEG6_LOCAL_FLV8986_SUPP_OPS)
> -#define SEG6_LOCAL_END_X_FLV_SUPP_OPS	SEG6_F_LOCAL_FLV_NEXT_CSID
> +#define SEG6_LOCAL_END_X_FLV_SUPP_OPS	(SEG6_F_LOCAL_FLV_NEXT_CSID | \
> +					 SEG6_LOCAL_FLV8986_SUPP_OPS)
>  
>  struct seg6_flavors_info {
>  	/* Flavor operations */

[ ... ]

> @@ -923,12 +924,20 @@ static int input_action_end_x(struct sk_buff *skb, struct seg6_local_lwt *slwt)
>  {
>  	const struct seg6_flavors_info *finfo = &slwt->flv_info;
>  	__u32 fops = finfo->flv_ops;
> +	int ret;
> +
> +	if (!fops)
> +		return input_action_end_x_core(skb, slwt);
>  
>  	/* check for the presence of NEXT-C-SID since it applies first */
>  	if (seg6_next_csid_enabled(fops))
>  		return end_x_next_csid_core(skb, slwt);

[Severity: Medium]
Now that SEG6_LOCAL_END_X_FLV_SUPP_OPS is wider, what happens when an
End.X instance is configured with both next-csid and psp?

parse_nla_flavors() only rejects bits that fall outside the supported mask:

net/ipv6/seg6_local.c:parse_nla_flavors() {
    ...
	if (rc < 0 || (fops & ~supp_fops)) {
		NL_SET_ERR_MSG(extack, "Unsupported Flavor operation(s)");
		return -EOPNOTSUPP;
	}
    ...
}

So NEXT_CSID|PSP is now accepted for SEG6_LOCAL_ACTION_END_X. It gets
stored in flv_ops and reported back to userspace as configured.

On the data path, input_action_end_x() returns through the NEXT-C-SID
branch above and never reaches end_flv8986_core(). Once the C-SID argument
is used up, end_x_next_csid_core() falls back to plain End.X processing:

net/ipv6/seg6_local.c:end_x_next_csid_core() {
    ...
	if (seg6_next_csid_is_arg_zero(daddr, finfo))
		return input_action_end_x_core(skb, slwt);
    ...
}

input_action_end_x_core() only calls advance_nextseg() and then
input_action_end_x_finish(). It never calls seg6_pop_srh(), even when
SL == 1.

Does that mean the configured psp flavor is silently ignored, and packets
leave with the SRH still attached? A downstream node with seg6_enabled=0
would then drop them, and nothing reports an error at configuration time.

This also seems to contradict the comment above
seg6_flv_supp_ops_by_action():

 * ... if a user specifies a
 * flavor combination that is not supported by a given End* behavior, the
 * kernel refuses to instantiate the tunnel reporting the error.

Before this patch, End.X refused psp entirely. End already has the same
mismatch in input_action_end(); this patch carries it over to End.X.

The note below the --- line says this combination is not handled and that
Andrea is preparing a fix. That note won't be in the commit log, though.
Nothing in the code or the commit message rejects or documents the
combination, and net/ipv6/seg6_local.c is not changed again after this
patch.

Could parse_nla_flavors() reject NEXT_CSID combined with any RFC8986
flavor? Or could the End.X mask stay at SEG6_F_LOCAL_FLV_NEXT_CSID until
combined processing is in place?

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261009-srv6_flavors_endx-v2-0-56ac7f838f41%40kylinos.cn