When I use bpf2go to load the BPF application, the BTF information is incomplete.

[binw666]

Here is the code.

// xsk_def_xdp_prog.c
/* SPDX-License-Identifier: GPL-2.0 */

#include <linux/bpf.h>
#include <bpf/bpf_helpers.h>
#include <xdp/xdp_helpers.h>

#include "xsk_def_xdp_prog.h"

#define DEFAULT_QUEUE_IDS 64

struct {
	__uint(type, BPF_MAP_TYPE_XSKMAP);
	__uint(key_size, sizeof(int));
	__uint(value_size, sizeof(int));
	__uint(max_entries, DEFAULT_QUEUE_IDS);
} xsks_map SEC(".maps");

struct {
	__uint(priority, 20);
	__uint(XDP_PASS, 1);
} XDP_RUN_CONFIG(xsk_def_prog);

/* Program refcount, in order to work properly,
 * must be declared before any other global variables
 * and initialized with '1'.
 */
volatile int refcnt = 1;

/* This is the program for post 5.3 kernels. */
SEC("xdp")
int xsk_def_prog(struct xdp_md *ctx)
{
	/* Make sure refcount is referenced by the program */
	if (!refcnt)
		return XDP_PASS;

	/* A set entry here means that the corresponding queue_id
	 * has an active AF_XDP socket bound to it.
	 */
	return bpf_redirect_map(&xsks_map, ctx->rx_queue_index, XDP_PASS);
}

char _license[] SEC("license") = "GPL";
__uint(xsk_prog_version, XSK_PROG_VERSION) SEC(XDP_METADATA_SECTION);

// "xsk_def_xdp_prog.h"
// SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)

#ifndef __LIBXDP_XSK_DEF_XDP_PROG_H
#define __LIBXDP_XSK_DEF_XDP_PROG_H

#define XDP_METADATA_SECTION "xdp_metadata"
#define XSK_PROG_VERSION 1

#endif /* __LIBXDP_XSK_DEF_XDP_PROG_H */

I use bpf2go to compile it and generate Go code.

go run github.com/cilium/ebpf/cmd/bpf2go -cc clang -cflags -O2 -g -Wall -c  xsk_def_xdp_prog ./xdp/xsk_def_xdp_prog.c

I use the loading function generated by bpf2go to load the XDP program and bind it to the interface. Then, when I try to get the BTF using the BTF ID, an error(reference to invalid type id: 7) occurs.

ifaceName := "ens1"
iface, err := net.InterfaceByName(ifaceName)
if err != nil {
t.Fatal(err)
}

obj := &xsk_def_xdp_progObjects{}
err = loadXsk_def_xdp_progObjects(obj, nil)
if err != nil {
t.Fatal(err)
}
defer obj.Close()
l, err := link.AttachXDP(link.XDPOptions{
Program:   obj.XskDefProg,
Interface: iface.Index,
Flags:     link.XDPGenericMode,
})
if err != nil {
t.Fatal(err)
}
defer l.Close()

link, err := netlink.LinkByIndex(iface.Index)
if err != nil {
t.Fatal(err)
}
prog, err := ebpf.NewProgramFromID(ebpf.ProgramID(link.Attrs().Xdp.ProgId))
if err != nil {
t.Fatal(err)
}
fmt.Println(prog)
info, err := prog.Info()
if err != nil {
t.Fatal(err)
}
fmt.Println(info)
btfID, has := info.BTFID()
if has {
handle, err := btf.NewHandleFromID(btfID)
if err != nil {
	t.Fatal(err)
}
spec, err := handle.Spec(&btf.Spec{})
// error: reference to invalid type id: 7
if err != nil {
	t.Fatal(err)
}
fmt.Println(spec)
}

Then, I used bpftool to load the same program and used bpftool to get the BTF for the program loaded by bpftool and the program loaded by bpf2go.

# bpftool prog list
...
162: xdp  name xsk_def_prog  tag 8f9c40757cb0a6a2  gpl
        loaded_at 2024-07-14T17:10:10+0800  uid 0
        xlated 96B  jited 64B  memlock 4096B  map_ids 245,243
        btf_id 308
174: xdp  name xsk_def_prog  tag 8f9c40757cb0a6a2  gpl
        loaded_at 2024-07-14T17:12:50+0800  uid 0
        xlated 96B  jited 64B  memlock 4096B  map_ids 259,260
        btf_id 324
        pids __debug_bin1895(3178060)
# bpftool btf dump id 308
[1] PTR '(anon)' type_id=3
[2] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[3] ARRAY '(anon)' type_id=2 index_type_id=4 nr_elems=17
[4] INT '__ARRAY_SIZE_TYPE__' size=4 bits_offset=0 nr_bits=32 encoding=(none)
[5] PTR '(anon)' type_id=6
[6] ARRAY '(anon)' type_id=2 index_type_id=4 nr_elems=4
[7] PTR '(anon)' type_id=8
[8] ARRAY '(anon)' type_id=2 index_type_id=4 nr_elems=64
[9] STRUCT '(anon)' size=32 vlen=4
        'type' type_id=1 bits_offset=0
        'key_size' type_id=5 bits_offset=64
        'value_size' type_id=5 bits_offset=128
        'max_entries' type_id=7 bits_offset=192
[10] VAR 'xsks_map' type_id=9, linkage=global
[11] PTR '(anon)' type_id=12
[12] STRUCT 'xdp_md' size=24 vlen=6
        'data' type_id=13 bits_offset=0
        'data_end' type_id=13 bits_offset=32
        'data_meta' type_id=13 bits_offset=64
        'ingress_ifindex' type_id=13 bits_offset=96
        'rx_queue_index' type_id=13 bits_offset=128
        'egress_ifindex' type_id=13 bits_offset=160
[13] TYPEDEF '__u32' type_id=14
[14] INT 'unsigned int' size=4 bits_offset=0 nr_bits=32 encoding=(none)
[15] FUNC_PROTO '(anon)' ret_type_id=2 vlen=1
        'ctx' type_id=11
[16] FUNC 'xsk_def_prog' type_id=15 linkage=global
[17] VOLATILE '(anon)' type_id=2
[18] VAR 'refcnt' type_id=17, linkage=global
[19] INT 'char' size=1 bits_offset=0 nr_bits=8 encoding=SIGNED
[20] ARRAY '(anon)' type_id=19 index_type_id=4 nr_elems=4
[21] VAR '_license' type_id=20, linkage=global
[22] STRUCT '(anon)' size=16 vlen=2
        'priority' type_id=23 bits_offset=0
        'XDP_PASS' type_id=25 bits_offset=64
[23] PTR '(anon)' type_id=24
[24] ARRAY '(anon)' type_id=2 index_type_id=4 nr_elems=20
[25] PTR '(anon)' type_id=26
[26] ARRAY '(anon)' type_id=2 index_type_id=4 nr_elems=1
[27] VAR '_xsk_def_prog' type_id=22, linkage=global
[28] VAR 'xsk_prog_version' type_id=25, linkage=global
[29] DATASEC '.data' size=4 vlen=1
        type_id=18 offset=0 size=4 (VAR 'refcnt')
[30] DATASEC '.maps' size=32 vlen=1
        type_id=10 offset=0 size=32 (VAR 'xsks_map')
[31] DATASEC '.xdp_run_config' size=16 vlen=1
        type_id=27 offset=0 size=16 (VAR '_xsk_def_prog')
[32] DATASEC 'license' size=4 vlen=1
        type_id=21 offset=0 size=4 (VAR '_license')
[33] DATASEC 'xdp_metadata' size=8 vlen=1
        type_id=28 offset=0 size=8 (VAR 'xsk_prog_version')
# bpftool btf dump id 324
[1] FUNC 'xsk_def_prog' type_id=7 linkage=global
[2] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[3] INT 'unsigned int' size=4 bits_offset=0 nr_bits=32 encoding=(none)
[4] TYPEDEF '__u32' type_id=3
[5] STRUCT 'xdp_md' size=24 vlen=6
        'data' type_id=4 bits_offset=0
        'data_end' type_id=4 bits_offset=32
        'data_meta' type_id=4 bits_offset=64
        'ingress_ifindex' type_id=4 bits_offset=96
        'rx_queue_index' type_id=4 bits_offset=128
        'egress_ifindex' type_id=4 bits_offset=160
[6] PTR '(anon)' type_id=5
[7] FUNC_PROTO '(anon)' ret_type_id=2 vlen=1
        'ctx' type_id=6

I found that the BTF for the program loaded by bpf2go is incomplete. I don't know why this is happening. How can I make the BTF complete?

[ti-mo]

2 observations here:

1. I don't think your use of spec, err := handle.Spec(&btf.Spec{}) is correct, afaik you need to call handle.Spec(nil) since the Handle doesn't represent a kmod BTF blob. The parameter is to specify a base BTF that a kmod BTF needs to parse successfully. That may fix your application and should let you obtain the BTF for the loaded BPF program.

2. ebpf-go has a fundamentally different way of passing (user) program BTF to the kernel. Unlike libbpf, it completely parses the BTF blob into Go structures and attaches each funcinfo and lineinfo and the types they point to directly to each asm.Instruction in ProgramSpec.Instructions. When loading a program into the kernel, it generates a new BTF blob from scratch using only the type and func/lineinfo for that specific program. The same happens for maps, where loading each MapSpec will generate a small BTF blob containing only the types in MapSpec.Key and .Value. There is no way to disable this, this is by design and makes modifying ProgramSpecs at runtime significantly easier.

[binw666]

Thank you very much for your response. I successfully fixed the error using handle.Spec(nil). I also understood your explanation regarding how ebpf-go passes BTF to the kernel. In this case, I would like to expose version information in the BTF using xsk_prog_version. If this is not feasible in ebpf-go, do you know of any suitable alternatives?

[ti-mo]

I see, you're trying to extract a version number from a loaded program. A simple solution would be renaming the prog to xsk_prog_v1 or something; I think you could paste in the number using the preprocessor (xsk_prog_v ## VERSION).

Alternatively, we're looking at exposing decl tags in many BTF types (there's an open PR for it), so if you declare a decl tag on a function it would be accessible in a btf.FuncProto.Tags field.

[binw666]

I understand. Thank you again for your response.