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

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

On Mon, Jul 28, 2025 at 06:05:26PM -0700, Martin KaFai Lau wrote:
> 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() ?

Okay I can add that: "ip[6]_route_reply_fetch_dst set the dst of the skb
by using the saddr as a daddr and routing it", I don't think
icmp[v6]_send touches the skb?

> 
> >    (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()?

From my understanding, I need to do this to invert the daddr with the
saddr to send the unreach message back to the sender.

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