Can eBPF's perf_submit() be used in a socket_filter program as well? - bpf

Question

So I was trying to send some data from the kernel space program to the user space program using perf_submit.
I've done some studies and here(https://github.com/iovisor/bcc/issues/2423), yonghong-song answered(the last comment) that a socket_filter program can not access bpf_perf_event_output helper and therefore it can only be used for tracing program types.
However, on BCC reference site(https://github.com/iovisor/bcc/blob/master/docs/reference_guide.md#2-bpf_perf_output), if you ctrl+f and search for : 3. perf_submit()
, it says on the fifth line that "for SOCKET_FILTER programs, the struct __sk_buff *skb must be used instead."
I believe this infers that perf_submit() can be used for socket_filter programs as well?
So I have hard time figuring out if perf_submit() can indeed be used for a socket filter program. Maybe some functionalities have been added since Yonghong-song answered the question above?
I'm checking if perf_submit() would work with a socket filter and there's not really a line of code that grabs the data output by perf_submit because just addint perf_submit() in the kernel program already omitted an error.
Here's the code for my program :
from bcc import BPF
# Network interface to be monoitored
INTERFACE = "br-netrome"
bpf_text = """
#include <uapi/linux/ptrace.h>
#include <net/sock.h>
#include <bcc/proto.h>
#include <linux/bpf.h>
#define IP_TCP 6
#define IP_UDP 17
#define IP_ICMP 1
#define ETH_HLEN 14
BPF_PERF_OUTPUT(events); // has to be delcared outside any function
int packet_monitor(struct __sk_buff *skb) {
u8 *cursor = 0;
u64 saddr;
u64 daddr;
u64 ttl;
u64 hchecksum;
struct ethernet_t *ethernet = cursor_advance(cursor, sizeof(*ethernet));
if (!(ethernet -> type == 0x0800)) {
return 0; // drop
}
struct ip_t *ip = cursor_advance(cursor, sizeof(*ip));
/*
if (ip->nextp != IP_TCP)
{
if (ip -> nextp != IP_UDP)
{
if (ip -> nextp != IP_ICMP)
return 0;
}
}
*/
saddr = ip -> src;
daddr = ip -> dst;
ttl = ip -> ttl;
hchecksum = ip -> hchecksum;
events.perf_submit(skb, &saddr, sizeof(saddr));
// bpf_trace_printk("saddr = %llu, daddr = %llu, ttl = %llu", saddr, daddr, ttl); // only three arguments can be passed using printk
// bpf_trace_printk("Incoming packet!!\\n");
return -1;
}
and here is the error code :
R0=inv2048 R6=ctx(id=0,off=0,imm=0) R7=inv0 R10=fp0,call_-1
4: (20) r0 = *(u32 *)skb[26]
5: (7b) *(u64 *)(r10 -8) = r0
6: (18) r2 = 0xffff9bde204ffa00
8: (18) r7 = 0xffffffff
10: (bf) r4 = r10
11: (07) r4 += -8
12: (bf) r1 = r6
13: (18) r3 = 0xffffffff
15: (b7) r5 = 8
16: (85) call bpf_perf_event_output#25
unknown func bpf_perf_event_output#25
Traceback (most recent call last):
File "packet_monitor.py", line 68, in <module>
function_skb_matching = bpf.load_func("packet_monitor", BPF.SOCKET_FILTER)
File "/usr/lib/python2.7/dist-packages/bcc/__init__.py", line 397, in load_func
(func_name, errstr))

Answer 1

TL;DR. BPF programs of type BPF_PROG_TYPE_SOCKET_FILTER can use bpf_perf_event_output only starting with Linux 5.4.
Which helpers a given BPF program has access to is defined by the get_func_proto member of objects struct bpf_verifier_ops. You can find which bpf_verifier_ops object corresponds to which program type by reading function find_prog_type() and file bpf_types.h. In the case of BPF_PROG_TYPE_SOCKET_FILTER, the corresponding function is sk_filter_func_proto().
If you git blame that function on recent kernel sources, you will get something like the following (you can do the same with GitHub's blame feature):
$ git blame net/core/filter.c
[...]
2492d3b867043 (Daniel Borkmann 2017-01-24 01:06:27 +0100 6080) static const struct bpf_func_proto *
5e43f899b03a3 (Andrey Ignatov 2018-03-30 15:08:00 -0700 6081) sk_filter_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
2492d3b867043 (Daniel Borkmann 2017-01-24 01:06:27 +0100 6082) {
2492d3b867043 (Daniel Borkmann 2017-01-24 01:06:27 +0100 6083) switch (func_id) {
2492d3b867043 (Daniel Borkmann 2017-01-24 01:06:27 +0100 6084) case BPF_FUNC_skb_load_bytes:
2492d3b867043 (Daniel Borkmann 2017-01-24 01:06:27 +0100 6085) return &bpf_skb_load_bytes_proto;
4e1ec56cdc597 (Daniel Borkmann 2018-05-04 01:08:15 +0200 6086) case BPF_FUNC_skb_load_bytes_relative:
4e1ec56cdc597 (Daniel Borkmann 2018-05-04 01:08:15 +0200 6087) return &bpf_skb_load_bytes_relative_proto;
91b8270f2a4d1 (Chenbo Feng 2017-03-22 17:27:34 -0700 6088) case BPF_FUNC_get_socket_cookie:
91b8270f2a4d1 (Chenbo Feng 2017-03-22 17:27:34 -0700 6089) return &bpf_get_socket_cookie_proto;
6acc5c2910689 (Chenbo Feng 2017-03-22 17:27:35 -0700 6090) case BPF_FUNC_get_socket_uid:
6acc5c2910689 (Chenbo Feng 2017-03-22 17:27:35 -0700 6091) return &bpf_get_socket_uid_proto;
7c4b90d79d0f4 (Allan Zhang 2019-07-23 17:07:24 -0700 6092) case BPF_FUNC_perf_event_output:
7c4b90d79d0f4 (Allan Zhang 2019-07-23 17:07:24 -0700 6093) return &bpf_skb_event_output_proto;
2492d3b867043 (Daniel Borkmann 2017-01-24 01:06:27 +0100 6094) default:
2492d3b867043 (Daniel Borkmann 2017-01-24 01:06:27 +0100 6095) return bpf_base_func_proto(func_id);
2492d3b867043 (Daniel Borkmann 2017-01-24 01:06:27 +0100 6096) }
2492d3b867043 (Daniel Borkmann 2017-01-24 01:06:27 +0100 6097) }
[...]
As you can see, BPF_FUNC_perf_event_output was only recently added to the list of helpers these BPF programs can call. The commit which added this support, 7c4b90d79d0f4, was merged in Linux v5.4:
$ git describe --contains 7c4b90d79d0f4
v5.4-rc1~131^2~248^2~20