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

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

On 7/29/25 3:06 AM, Mahe Tardy wrote:
> 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?

I also don't think icmp[v6]_send touches the skb. I am still not sure if 
ip[6]_route_reply_fetch_dst is needed.

> 
>>
>>>     (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.

 From looking at how fl6.{daddr,saddr} are filled and passed to 
icmpv6_route_lookup in icmpv6_send(), the icmpv6_send() should have done the 
reverse/invert route lookup. I also don't see icmpv6_send uses the skb_dst() of 
the original skb. Did I misread the code? The kfunc does not work without 
ip[6]_route_reply_fetch_dst()? Again, I have not checked the v4 icmp_send. fwiw, 
the selftest should have both v4 and v6 test.

Note that at cgroup/egress, the skb->_skb_refdst should have been set.

The same should be true for cgroup/ingress for inet proto but it seems 
BPF_CGROUP_RUN_PROG_"INET"_INGRESS is not called from INET only now. e.g. 
sk_filter() can be called from af_netlink. It seems like there is a bug.

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