Re: [PATCH bpf-next v3 3/4] bpf: add bpf_icmp_send_unreach cgroup_skb kfunc

[Martin KaFai Lau <martin.lau@linux.dev>]

On 7/28/25 2:43 AM, Mahe Tardy wrote:
> This is needed in the context of Tetragon to provide improved feedback
> (in contrast to just dropping packets) to east-west traffic when blocked
> by policies using cgroup_skb programs.
> 
> This reuse concepts from netfilter reject target codepath with the
> differences that:
> * Packets are cloned since the BPF user can still return SK_PASS from
>    the cgroup_skb progs and the current skb need to stay untouched

This needs more details. Which field(s) of the skb are changed by the kfunc, the 
skb_dst_set in ip[6]_route_reply_fetch_dst() and/or the code path in the 
icmp[v6]_send() ?

>    (cgroup_skb hooks only allow read-only skb payload).
> * Since cgroup_skb programs are called late in the stack, checksums do
>    not need to be computed or verified, and IPv4 fragmentation does not
>    need to be checked (ip_local_deliver should take care of that
>    earlier).
> 
> Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>
> ---
>   net/core/filter.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 61 insertions(+)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 7a72f766aacf..050872324575 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -85,6 +85,10 @@
>   #include <linux/un.h>
>   #include <net/xdp_sock_drv.h>
>   #include <net/inet_dscp.h>
> +#include <linux/icmp.h>
> +#include <net/icmp.h>
> +#include <net/route.h>
> +#include <net/ip6_route.h>
> 
>   #include "dev.h"
> 
> @@ -12148,6 +12152,53 @@ __bpf_kfunc int bpf_sock_ops_enable_tx_tstamp(struct bpf_sock_ops_kern *skops,
>   	return 0;
>   }
> 
> +__bpf_kfunc int bpf_icmp_send_unreach(struct __sk_buff *__skb, int code)
> +{
> +	struct sk_buff *skb = (struct sk_buff *)__skb;
> +	struct sk_buff *nskb;
> +
> +	switch (skb->protocol) {
> +	case htons(ETH_P_IP):
> +		if (code < 0 || code > NR_ICMP_UNREACH)
> +			return -EINVAL;
> +
> +		nskb = skb_clone(skb, GFP_ATOMIC);
> +		if (!nskb)
> +			return -ENOMEM;
> +
> +		if (ip_route_reply_fetch_dst(nskb) < 0) {
> +			kfree_skb(nskb);
> +			return -EHOSTUNREACH;
> +		}
> +
> +		icmp_send(nskb, ICMP_DEST_UNREACH, code, 0);
> +		kfree_skb(nskb);
> +		break;
> +#if IS_ENABLED(CONFIG_IPV6)
> +	case htons(ETH_P_IPV6):
> +		if (code < 0 || code > ICMPV6_REJECT_ROUTE)
> +			return -EINVAL;
> +
> +		nskb = skb_clone(skb, GFP_ATOMIC);
> +		if (!nskb)
> +			return -ENOMEM;
> +
> +		if (ip6_route_reply_fetch_dst(nskb) < 0) {

 From a very quick look at icmpv6_send(), it does its own route lookup. I 
haven't looked at the v4 yet.

I am likely missing some details. Can you explain why it needs to do a lookup 
before calling icmpv6_send()?

> +			kfree_skb(nskb);
> +			return -EHOSTUNREACH;
> +		}
> +
> +		icmpv6_send(nskb, ICMPV6_DEST_UNREACH, code, 0);
> +		kfree_skb(nskb);
> +		break;
> +#endif
> +	default:
> +		return -EPROTONOSUPPORT;
> +	}
> +
> +	return SK_DROP;
> +}
> +