Re: [PATCH net-next v2] net: dropreason: add SKB_DROP_REASON_IP_TTL_EXCEEDED

[David Ahern]

On 8/31/26 8:06 PM, Junjie Cao wrote:
> diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
> index 2f312d1f67d6..3046a2699479 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 hit
> +	 * zero on a packet being forwarded (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;

what about icmp_unreach, ip_expire, and other sources of
ICMP_TIME_EXCEEDED? The reason code applies to more than just forwarding
paths.

> diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
> index 51941ad656a3..74caaf8746ff 100644
> --- a/net/ipv6/exthdrs.c
> +++ b/net/ipv6/exthdrs.c
> @@ -464,7 +464,7 @@ static int ipv6_srh_rcv(struct sk_buff *skb, struct inet6_dev *idev)
>  			__IP6_INC_STATS(net, idev, IPSTATS_MIB_INHDRERRORS);
>  			icmpv6_send(skb, ICMPV6_TIME_EXCEED,
>  				    ICMPV6_EXC_HOPLIMIT, 0);
> -			kfree_skb(skb);
> +			kfree_skb_reason(skb, SKB_DROP_REASON_IP_TTL_EXCEEDED);
>  			return -1;
>  		}
>  		ipv6_hdr(skb)->hop_limit--;
> @@ -623,7 +623,7 @@ static int ipv6_rpl_srh_rcv(struct sk_buff *skb, struct inet6_dev *idev)
>  			__IP6_INC_STATS(net, idev, IPSTATS_MIB_INHDRERRORS);
>  			icmpv6_send(skb, ICMPV6_TIME_EXCEED,
>  				    ICMPV6_EXC_HOPLIMIT, 0);
> -			kfree_skb(skb);
> +			kfree_skb_reason(skb, SKB_DROP_REASON_IP_TTL_EXCEEDED);
>  			return -1;
>  		}
>  		ipv6_hdr(skb)->hop_limit--;
> @@ -815,7 +815,7 @@ static int ipv6_rthdr_rcv(struct sk_buff *skb)
>  			__IP6_INC_STATS(net, idev, IPSTATS_MIB_INHDRERRORS);
>  			icmpv6_send(skb, ICMPV6_TIME_EXCEED, ICMPV6_EXC_HOPLIMIT,
>  				    0);
> -			kfree_skb(skb);
> +			kfree_skb_reason(skb, SKB_DROP_REASON_IP_TTL_EXCEEDED);
>  			return -1;
>  		}
>  		ipv6_hdr(skb)->hop_limit--;
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index 8fc4766c8da9..0b6d78c8b6be 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -577,7 +577,7 @@ int ip6_forward(struct sk_buff *skb)
>  		icmpv6_send(skb, ICMPV6_TIME_EXCEED, ICMPV6_EXC_HOPLIMIT, 0);
>  		__IP6_INC_STATS(net, idev, IPSTATS_MIB_INHDRERRORS);
>  
> -		kfree_skb_reason(skb, SKB_DROP_REASON_IP_INHDR);
> +		kfree_skb_reason(skb, SKB_DROP_REASON_IP_TTL_EXCEEDED);
>  		return -ETIMEDOUT;
>  	}
>  

similarly for IPv6, there are more ICMPV6_TIME_EXCEED cases than just
the forwarding path.