Re: [PATCH net-next v4] net: dropreason: add SKB_DROP_REASON_IP_TTL_EXCEEDED

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

Hi Junjie,
On Mon, Sep 07, 2026 at 11:01:33AM +0800, Junjie Cao wrote:
> The forwarding paths report an expired TTL or hop limit as
> SKB_DROP_REASON_IP_INHDR, the reason otherwise used for a header that is
> malformed (ip_input.c, exthdrs.c, br_netfilter). Nothing else in the drop
> path separates the two: IPSTATS_MIB_INHDRERRORS covers both, and the TTL
> check runs before NF_INET_FORWARD, so netfilter tracing stops at
> PREROUTING and never sees the drop.
> 
> The Fedora bug linked below shows how that reads in practice. The
> reporter took kfree_skb(reason=IP_INHDR, loc=ip_forward) to mean the
> software header checksum check had failed, and worked through RX checksum
> offload, tc csum actions and both libvirt firewall backends before the
> drops turned out to be replies arriving with TTL 1. ip_forward() never
> verifies the header checksum; that runs earlier, in ip_rcv_core(), and
> reports IP_CSUM.
> 
> TTL expiry is not a corner case -- every traceroute through a Linux
> router goes through too_many_hops.
> 
> The three loopback hop limit checks in exthdrs.c drop with no reason at
> all; give them the new one.
> 
> IPSTATS_MIB_INHDRERRORS stays as it is: RFC 1213 counts time-to-live
> exceeded under ipInHdrErrors. The drop reason has no such constraint.
> 
> Link: https://bugzilla.redhat.com/show_bug.cgi?id=2517131
> Signed-off-by: Junjie Cao <junjie.cao@intel.com>
> Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
> ---
> v4: kernel-doc says "<= 1" instead of "hit zero" (Jiayuan Chen)
> v3: https://lore.kernel.org/netdev/20260904030112.450920-1-junjie.cao@intel.com/
> v2: https://lore.kernel.org/netdev/20260901020613.417495-1-junjie.cao@intel.com/
> v1: https://lore.kernel.org/netdev/20260825073906.336072-1-junjie.cao@intel.com/
>  include/net/dropreason-core.h | 6 ++++++
>  net/ipv4/ip_forward.c         | 2 +-
>  net/ipv6/exthdrs.c            | 6 +++---
>  net/ipv6/ip6_output.c         | 2 +-
>  4 files changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
> index 2f312d1f67d6..3d6aec203c3f 100644
> --- a/include/net/dropreason-core.h
> +++ b/include/net/dropreason-core.h
> @@ -128,6 +128,7 @@
>  	FN(PSP_INPUT)			\
>  	FN(PSP_OUTPUT)			\
>  	FN(RECURSION_LIMIT)		\
> +	FN(IP_TTL_EXCEEDED)		\
>  	FNe(MAX)
>  
>  /**
> @@ -606,6 +607,11 @@ enum skb_drop_reason {
>  	SKB_DROP_REASON_PSP_OUTPUT,
>  	/** @SKB_DROP_REASON_RECURSION_LIMIT: Dead loop on virtual device. */
>  	SKB_DROP_REASON_RECURSION_LIMIT,
> +	/**
> +	 * @SKB_DROP_REASON_IP_TTL_EXCEEDED: IPv4 TTL or IPv6 hop limit <= 1
> +	 * (see IPSTATS_MIB_INHDRERRORS)
> +	 */
> +	SKB_DROP_REASON_IP_TTL_EXCEEDED,
>  	/**
>  	 * @SKB_DROP_REASON_MAX: the maximum of core drop reasons, which
>  	 * shouldn't be used as a real 'reason' - only for tracing code gen
> diff --git a/net/ipv4/ip_forward.c b/net/ipv4/ip_forward.c
> index 8b65f12583eb..b242561d37e7 100644
> --- a/net/ipv4/ip_forward.c
> +++ b/net/ipv4/ip_forward.c
> @@ -174,7 +174,7 @@ int ip_forward(struct sk_buff *skb)
>  	/* Tell the sender its packet died... */
>  	__IP_INC_STATS(net, IPSTATS_MIB_INHDRERRORS);
>  	icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
> -	SKB_DR_SET(reason, IP_INHDR);
> +	SKB_DR_SET(reason, IP_TTL_EXCEEDED);
>  drop:
>  	kfree_skb_reason(skb, reason);
>  	return NET_RX_DROP;

I saw

ip_vs_forward_icmp()
  - ip_vs_in_icmp_v6
      - ip_vs_icmp_xmit_v6()
        - __ip_vs_get_out_rt_v6()
          - decrement_ttl()

Also sends ICMP_EXC_TTL/ICMPV6_EXC_HOPLIMIT messages, But not changed
in this patch. Should we also update them? Or there is no plan to
change the ipvs code yet?

Thanks
Hangbin