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

Florian Westphal
2023-03-02T17:28:17Z

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