I'm trying to write a sample eBPF program which can access __sk_buff member and dump it into /sys/kernel/debug/tracing/trace.
#include <uapi/linux/bpf.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_endian.h>
SEC("dump_skb_member")
int test_prog(struct __sk_buff *skb)
{
char fmt[] = "packet: local %u remote %u\n";
__u32 local_ip4 = bpf_htonl(skb->local_ip4);
__u32 remote_ip4 = bpf_htonl(skb->remote_ip4);
bpf_trace_printk(fmt, sizeof(fmt), local_ip4, remote_ip4);
return BPF_OK;
}
char _license[] SEC("license") = "GPL";
When i compile this code, and load this program
ip route add 192.168.56.104 encap bpf out obj sample.o section dump_skb_member dev enp0s8
An error is thrown.
Prog section 'dump_skb_member' rejected: Permission denied (13)!
- Type: 11
- Instructions: 21 (0 over limit)
- License: GPL
Verifier analysis:
0: (b7) r2 = 685349
1: (63) *(u32 *)(r10 -8) = r2
2: (18) r2 = 0x2065746f6d657220
4: (7b) *(u64 *)(r10 -16) = r2
5: (18) r2 = 0x7525206c61636f6c
7: (7b) *(u64 *)(r10 -24) = r2
8: (18) r2 = 0x203a74656b636170
10: (7b) *(u64 *)(r10 -32) = r2
11: (61) r4 = *(u32 *)(r1 +92)
invalid bpf_context access off=92 size=4
processed 9 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
Error fetching program/map!
But if i don't call bpf_trace_printk to dump member, it can be loaded.
My question is why the error is caused by calling bpf_trace_printk?