Subject: Re: [PATCH bpf-next v6 3/6] bpf: add bpf_icmp_send kfunc

From: Stanislav Fomichev <[email-masked]>
Date: Mon, 18 May 2026 09:17:45 -0700

On 05/18, 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. We also extend this kfunc to tc
> program as a convenience.
> 
> This reuses concepts from netfilter reject target codepath with the
> differences that:
> * Packets are cloned since the BPF user can still let the packet pass
>   (SK_PASS from the cgroup_skb progs for example) and the current skb
>   need to stay untouched (cgroup_skb hooks only allow read-only skb
>   payload).
> * We protect against recursion since the kfunc, by generating an ICMP
>   error message, could retrigger the BPF prog that invoked it.
> 
> For now, we support cgroup_skb and tc program types. For cgroup_skb and
> tc egress, almost everything should be good. However for tc ingress:
> - packet will not be routed yet: need to set the net device for
>   icmp_send, thus the call to ip[6]_route_reply_fill_dst.
> - fragments could trigger hook: icmp_send will only reply to fragment 0.
> - ensure the ip headers is linearized before processing, and zero out
>   the SKB control block after cloning to prevent icmp_send()/icmpv6_send()
>   from misinterpreting garbage data as IP options.
> 
> Only ICMP_DEST_UNREACH and ICMPV6_DEST_UNREACH are currently supported.
> The interface accepts a type parameter to facilitate future extension to
> other ICMP control message types.
> 
> Signed-off-by: Mahe Tardy <[email-masked]>
> ---
>  net/core/filter.c | 118 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 118 insertions(+)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 9590877b0714..843fa775596b 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -84,6 +84,8 @@
>  #include <linux/un.h>
>  #include <net/xdp_sock_drv.h>
>  #include <net/inet_dscp.h>
> +#include <linux/icmpv6.h>
> +#include <net/icmp.h>
> 
>  #include "dev.h"
> 
> @@ -12464,6 +12466,110 @@ __bpf_kfunc int bpf_xdp_pull_data(struct xdp_md *x, u32 len)
>  	return 0;
>  }
> 
> +static DEFINE_PER_CPU(bool, bpf_icmp_send_in_progress);
> +
> +/**
> + * bpf_icmp_send - Send an ICMP control message
> + * @skb_ctx: Packet that triggered the control message
> + * @type: ICMP type (only ICMP_DEST_UNREACH/ICMPV6_DEST_UNREACH supported)
> + * @code: ICMP code (0-15 for IPv4, 0-6 for IPv6)
> + *
> + * Sends an ICMP control message in response to the packet. The original packet
> + * is cloned before sending the ICMP message, so the BPF program can still let
> + * the packet pass if desired.
> + *
> + * Currently only ICMP_DEST_UNREACH (IPv4) and ICMPV6_DEST_UNREACH (IPv6) are
> + * supported.
> + *
> + * Recursion protection: If called from a context that would trigger recursion
> + * (e.g., root cgroup processing its own ICMP packets), returns -EBUSY on
> + * re-entry.
> + *
> + * Return: 0 on success, negative error code on failure:
> + *         -EINVAL: Invalid code parameter
> + *         -EBADMSG: Packet too short or malformed
> + *         -ENOMEM: Memory allocation failed
> + *         -EBUSY: Recursion detected
> + *         -EHOSTUNREACH: Routing failed
> + *         -EPROTONOSUPPORT: Non-IP protocol
> + *         -EOPNOTSUPP: Unsupported ICMP type
> + */
> +__bpf_kfunc int bpf_icmp_send(struct __sk_buff *skb_ctx, int type, int code)
> +{
> +	struct sk_buff *skb = (struct sk_buff *)skb_ctx;
> +	struct sk_buff *nskb;
> +	bool *in_progress;
> +
> +	in_progress = this_cpu_ptr(&bpf_icmp_send_in_progress);
> +	if (*in_progress)
> +		return -EBUSY;
> +
> +	switch (skb->protocol) {
> +#if IS_ENABLED(CONFIG_INET)
> +	case htons(ETH_P_IP):
> +		if (type != ICMP_DEST_UNREACH)
> +			return -EOPNOTSUPP;
> +		if (code < 0 || code > NR_ICMP_UNREACH)
> +			return -EINVAL;
> +
> +		nskb = skb_clone(skb, GFP_ATOMIC);
> +		if (!nskb)
> +			return -ENOMEM;
> +
> +		if (!pskb_network_may_pull(nskb, sizeof(struct iphdr))) {
> +			kfree_skb(nskb);
> +			return -EBADMSG;
> +		}
> +
> +		if (!skb_dst(nskb) && ip_route_reply_fill_dst(nskb) < 0) {
> +			kfree_skb(nskb);
> +			return -EHOSTUNREACH;
> +		}
> +
> +		memset(IPCB(nskb), 0, sizeof(struct inet_skb_parm));
> +
> +		*in_progress = true;
> +		icmp_send(nskb, type, code, 0);
> +		*in_progress = false;

[..]

> +		kfree_skb(nskb);

I was going to suggest to use consume_skb here, I think it is a better fit?

But I'm not sure why you do the clone here, I don't see any requirement from
the icmp_send side, can you clarify? Is it because of the pull?