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

[Mahe Tardy <mahe.tardy@gmail.com>]

On Mon, May 18, 2026 at 09:17:45AM -0700, Stanislav Fomichev wrote:
> 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 <mahe.tardy@gmail.com>
> > ---
> >  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?

Yeah correct, I can replace it with consume_skb, didn't know about it,
thanks.

> 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?

From the icmp_send side I think it's fine, however, this part might
touch the original packet, especially ip_route_reply_fill_dst:


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));


All of this is mostly there because we allow this kfunc for tc and
especially tc ingress. At this stage, the skb might not have a routing
entry yet and icmp_send needs to know the dev from this or fail
silently. This is the original reason why I added the the net patches
(patch 1 and 2) and it was also spotted by Sashiko when I tried to
remove them[^1].

[^1]: https://lore.kernel.org/bpf/20260515202358.20252C2BCB0@smtp.kernel.org/