libbpf: sec 'sockops': failed to find program symbol at offset 0 - c

Question

I'd like to use the bpftool prog load to load my program into kernel. However, some errors occurred.
# bpftool prog load sockmap_update_kern.o "/sys/fs/bpf/bpf_sockmap"
libbpf: sec 'sockops': failed to find program symbol at offset 0
Error: failed to open object file
The program compiles fine with LLVM version 6.0.0
#include <linux/bpf.h>
#include "bpf_helpers.h"
struct bpf_map_def SEC("maps") sock_map = {
.type = BPF_MAP_TYPE_SOCKMAP,
.key_size = sizeof(int),
.value_size = sizeof(int),
.max_entries = 10,
};
SEC("sockops")
int sock_map_update(struct bpf_sock_ops *ops)
{
__u32 op, family;
int key;
op = ops->op;
family = ops->family;
switch (op){
case BPF_SOCK_OPS_ACTIVE_ESTABLISHED_CB:
case BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB:
if (family == 2){ //AF_INET
key = 1;
bpf_sock_map_update(ops, &sock_map, &key, BPF_ANY);
}
default:
break;
}
return 0;
}
char _license[] SEC("license") = "GPL";
# readelf -a sockmap_update_kern.o
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: REL (Relocatable file)
Machine: Linux BPF
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 456 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 64 (bytes)
Number of section headers: 8
Section header string table index: 1
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .strtab STRTAB 0000000000000000 00000170
0000000000000051 0000000000000000 0 0 1
[ 2] .text PROGBITS 0000000000000000 00000040
0000000000000000 0000000000000000 AX 0 0 4
[ 3] sockops PROGBITS 0000000000000000 00000040
0000000000000088 0000000000000000 AX 0 0 8
[ 4] .relsockops REL 0000000000000000 00000160
0000000000000010 0000000000000010 7 3 8
[ 5] maps PROGBITS 0000000000000000 000000c8
000000000000001c 0000000000000000 WA 0 0 4
[ 6] license PROGBITS 0000000000000000 000000e4
0000000000000004 0000000000000000 WA 0 0 1
[ 7] .symtab SYMTAB 0000000000000000 000000e8
0000000000000078 0000000000000018 1 2 8
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
p (processor specific)
There are no section groups in this file.
There are no program headers in this file.
There is no dynamic section in this file.
Relocation section '.relsockops' at offset 0x160 contains 1 entry:
Offset Info Type Sym. Value Sym. Name
000000000058 000300000001 unrecognized: 1 0000000000000000 sock_map
The decoding of unwind sections for machine type Linux BPF is not currently supported.
Symbol table '.symtab' contains 5 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000078 0 NOTYPE LOCAL DEFAULT 3 LBB0_3
2: 0000000000000000 0 NOTYPE GLOBAL DEFAULT 6 _license
3: 0000000000000000 0 NOTYPE GLOBAL DEFAULT 5 sock_map
4: 0000000000000000 0 NOTYPE GLOBAL DEFAULT 3 sock_map_update
No version information found in this file.
My kernel version is 5.3.0-42. I get it by apt-get install, so I guess there are some problems about the kernel. Could you give me some advise? Thanks in advance.
# uname -a
Linux iZ2zehe0r5ccv5sse5ib5fZ 5.3.0-42-generic #34~18.04.1-Ubuntu SMP Fri Feb 28 13:42:26 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux