[PATCH RFC v2 bpf-next 0/3] bpf: add netfilter program type

[Florian Westphal <fw@xxxxxxxxx>]

Add minimal support to hook bpf programs to netfilter hooks,
e.g. PREROUTING or FORWARD.

For this the most relevant parts for registering a netfilter
hook via the in-kernel api are exposed to userspace via bpf_link.

The new program type is 'tracing style' and assumes skb dynptrs are used
rather than 'direct packet access'.

With this its possible to build a small test program such as:

#include "vmlinux.h"

extern int bpf_dynptr_from_skb(struct __sk_buff *skb, __u64 flags,
                               struct bpf_dynptr *ptr__uninit) __ksym;
extern void *bpf_dynptr_slice(const struct bpf_dynptr *ptr, uint32_t offset,
                                   void *buffer, uint32_t buffer__sz) __ksym;

SEC("netfilter")
int nf_test(struct bpf_nf_ctx *ctx)
{
	struct nf_hook_state *state = ctx->state;
	struct sk_buff *skb = ctx->skb;
	const struct iphdr *iph, _iph;
	const struct tcphdr *th, _th;
	struct bpf_dynptr ptr;

	if (bpf_dynptr_from_skb(skb, 0, &ptr))
		return NF_DROP;

	iph = bpf_dynptr_slice(&ptr, 0, &_iph, sizeof(_iph));
	if (!iph)
		return NF_DROP;

	th = bpf_dynptr_slice(&ptr, iph->ihl << 2, &_th, sizeof(_th));
	if (!th)
		return NF_DROP;

	bpf_printk("accept %x:%d->%x:%d, hook %d ifin %d\n", iph->saddr, bpf_ntohs(th->source), iph->daddr, bpf_ntohs(th->dest), state->hook, state->in->ifindex);
        return NF_ACCEPT;
}

(output can be observed via /sys/kernel/tracing/trace_pipe).

At this point I think its fairly complete.  Known problems are:
- no test cases, I will look into this.  Might take some time
  though because I might have to extend libbpf first.
- nfnetlink_hook needs minor work so that it can dump the bpf
  program id. As-is, userspace could see that a bpf program
  is attached to e.g. forward and output, but it cannot tell
  which program.  This is fairly simple and doesn't need changes
  on bpf side.

I will work on these address those two next unless anyone spots
a fundamental issue with this rfc set.

Florian Westphal (3):
  bpf: add bpf_link support for BPF_NETFILTER programs
  libbpf: sync header file, add nf prog section name
  bpf: minimal support for programs hooked into netfilter framework

 include/linux/bpf_types.h           |   4 +
 include/linux/netfilter.h           |   1 +
 include/net/netfilter/nf_hook_bpf.h |   8 ++
 include/uapi/linux/bpf.h            |  12 ++
 kernel/bpf/btf.c                    |   5 +
 kernel/bpf/syscall.c                |   6 +
 kernel/bpf/verifier.c               |   3 +
 net/netfilter/Kconfig               |   3 +
 net/netfilter/Makefile              |   1 +
 net/netfilter/nf_bpf_link.c         | 192 ++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h      |  12 ++
 tools/lib/bpf/libbpf.c              |   1 +
 12 files changed, 248 insertions(+)
 create mode 100644 include/net/netfilter/nf_hook_bpf.h
 create mode 100644 net/netfilter/nf_bpf_link.c
-- 
2.39.2