Re: [PATCH v3 4/6] net: filter: run cgroup eBPF ingress programs

If the cgroup associated with the receiving socket has an eBPF
programs installed, run them from sk_filter_trim_cap().

eBPF programs used in this context are expected to either return 1 to
let the packet pass, or != 1 to drop them. The programs have access to
the full skb, including the MAC headers.

Note that cgroup_bpf_run_filter() is stubbed out as static inline nop
for !CONFIG_CGROUP_BPF, and is otherwise guarded by a static key if
the feature is unused.

Signed-off-by: Daniel Mack <daniel@zonque.org>
---
  net/core/filter.c | 5 +++++
  1 file changed, 5 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index bc04e5c..163f75b 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c

@@ -78,6 +78,11 @@ int sk_filter_trim_cap(struct sock *sk, struct sk_buff *skb, unsigned int cap)
  	if (skb_pfmemalloc(skb) && !sock_flag(sk, SOCK_MEMALLOC))
  		return -ENOMEM;

+	err = cgroup_bpf_run_filter(sk, skb,
+				    BPF_ATTACH_TYPE_CGROUP_INET_INGRESS);

+	if (err)
+		return err;
+
  	err = security_sock_rcv_skb(sk, skb);
  	if (err)
  		return err;