Re: [PATCH net-next 2/5] seg6: handle a consumed SRH in SRv6 End with the PSP flavor

[Hangbin Liu <hangbin.liu@linux.dev>]

Hi Andrea,
On Sun, Oct 11, 2026 at 09:23:07PM +0200, Andrea Mayer wrote:
> With the seg6 inline mode, a packet can carry more than one SRH. The
> SRHs with Segments Left 0 are considered consumed SRHs, and they can
> come before the SRH to process (Segments Left > 0), as in this packet:
> 
>   +------+----------------------+----------------------+---------+
>   | IPv6 | SRH, Segments Left 0 | SRH, Segments Left 1 | payload |
>   +------+----------------------+----------------------+---------+
> 
> End without flavors skips the consumed SRH and processes the next
> one [1]. With the PSP flavor, End should do the same. When the SRH to
> process arrives with Segments Left 1, as in the diagram, End with PSP
> should then also remove it [2]. However, End with PSP alone currently
> looks only at the first routing header of the packet, finds Segments
> Left 0 there, and drops the packet.
> 
> Make End with PSP alone skip the consumed SRHs, and any other routing
> header with Segments Left 0, and process the SRH that follows them. If
> there is no SRH to process, the packet is dropped as before. When the
> SRH to process arrives with Segments Left 1, End with PSP now removes
> it even if consumed SRHs come before it.
> 
> Found by manual code inspection and reproduced in a VM without kernel
> changes or error injection. A following commit adds tests for this case
> to srv6_end_flavors_test.sh. Without this change, the new tests with a
> consumed SRH fail for End with PSP alone. With this series, all tests
> pass and KASAN reports no error.
> 
> [1] - https://datatracker.ietf.org/doc/html/rfc8986#section-4.1
> [2] - https://datatracker.ietf.org/doc/html/rfc8986#section-4.16.1.2
> 
> Fixes: bdf3c0b9c10b ("seg6: add PSP flavor support for SRv6 End behavior")
> Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
> ---
>  net/ipv6/seg6_local.c | 29 +++++++++++++++++++++++++----
>  1 file changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
> index da8225591fe3..26cd4facbe36 100644
> --- a/net/ipv6/seg6_local.c
> +++ b/net/ipv6/seg6_local.c
> @@ -775,11 +775,14 @@ static bool seg6_pop_srh(struct sk_buff *skb, int srhoff)
>  		skb_set_transport_header(skb, thoff);
>  
>  	iph = ipv6_hdr(skb);
> -	if (iph->nexthdr == NEXTHDR_ROUTING) {
> +	if (srhoff == sizeof(*iph)) {
>  		iph->nexthdr = srh_nexthdr;
>  	} else {
>  		/* we must look for the extension header (EXTH, for short) that
> -		 * immediately precedes the SRH we have just removed.
> +		 * immediately precedes the SRH we have just removed, i.e. the
> +		 * EXTH that ends at srhoff. We find it by its position and not
> +		 * by its nexthdr, since another routing header (e.g. an SRH
> +		 * with SL = 0) may come before the removed SRH.
>  		 * Then, we update the value of the EXTH nexthdr with the one
>  		 * contained in the SRH nexthdr.
>  		 */
> @@ -807,7 +810,7 @@ static bool seg6_pop_srh(struct sk_buff *skb, int srhoff)
>  				break;
>  			}
>  
> -			if (hp->nexthdr == NEXTHDR_ROUTING) {
> +			if (off == srhoff) {
>  				hp->nexthdr = srh_nexthdr;
>  				break;
>  			}
> @@ -823,6 +826,24 @@ static bool seg6_pop_srh(struct sk_buff *skb, int srhoff)
>  	return true;
>  }
>  
> +/* look up the SRH to be processed by the RFC8986 flavors. As End does, a
> + * routing header with SL = 0 is skipped (e.g. an SRH already consumed).
> + * This function returns NULL when no valid SRH is found.
> + */
> +static struct ipv6_sr_hdr *seg6_flv8986_get_srh(struct sk_buff *skb)
> +{
> +	struct ipv6_sr_hdr *srh;
> +
> +	srh = seg6_get_srh(skb, IP6_FH_F_SKIP_RH);
> +	if (srh)
> +		return srh;
> +
> +	/* we take the first routing header without skipping it. In this way,
> +	 * seg6_get_srh_pktinfo() reports SL = 0 and not a missing SRH.
> +	 */
> +	return seg6_get_srh(skb, 0);
> +}
> +
>  /* process the packet on the basis of the RFC8986 flavors set for the given
>   * SRv6 End behavior instance.
>   */
> @@ -837,7 +858,7 @@ static int end_flv8986_core(struct sk_buff *skb, struct seg6_local_lwt *slwt)
>  	__u32 flvmask;
>  	int srhoff;
>  
> -	srh = seg6_get_srh(skb, 0);
> +	srh = seg6_flv8986_get_srh(skb);

Hmm, here we always try skip the SL = 0 SRH when multi SRH exists. What if
we want to add USP flavor support in future? Then we will not able to remove
the first SRH which SL = 0, right? e.g.

   +------+----------------------+----------------------+---------+
   | IPv6 | SRH, Segments Left 0 | SRH, Segments Left 2 | payload |
   +------+----------------------+----------------------+---------+

Thanks
Hangbin

>  	srhoff = srh ? ((unsigned char *)srh - skb->data) : 0;
>  	pinfo = seg6_get_srh_pktinfo(srh);
>  #ifdef CONFIG_IPV6_SEG6_HMAC
> -- 
> 2.43.0
>