failed to `--output-skb`

[Asphaltt]

# ./pwru --output-skb
2021/11/25 16:47:00 Loading objects: field KprobeSkb1: program kprobe_skb_1: load program: invalid argument: BPF_STX uses reserved fields
processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0

The running OS:

# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 21.04
Release:        21.04
Codename:       hirsute

# uname -a
Linux pagani 5.11.0-31-generic #33-Ubuntu SMP Wed Aug 11 13:19:04 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

bpf:

# cat /boot/config-$(uname -r) | grep BPF
CONFIG_CGROUP_BPF=y
CONFIG_BPF=y
CONFIG_BPF_LSM=y
CONFIG_BPF_SYSCALL=y
CONFIG_ARCH_WANT_DEFAULT_BPF_JIT=y
CONFIG_BPF_JIT_ALWAYS_ON=y
CONFIG_BPF_JIT_DEFAULT_ON=y
# CONFIG_BPF_PRELOAD is not set
CONFIG_IPV6_SEG6_BPF=y
CONFIG_NETFILTER_XT_MATCH_BPF=m
CONFIG_BPFILTER=y
CONFIG_BPFILTER_UMH=m
CONFIG_NET_CLS_BPF=m
CONFIG_NET_ACT_BPF=m
CONFIG_BPF_JIT=y
CONFIG_BPF_STREAM_PARSER=y
CONFIG_LWTUNNEL_BPF=y
CONFIG_HAVE_EBPF_JIT=y
CONFIG_BPF_EVENTS=y
CONFIG_BPF_KPROBE_OVERRIDE=y
CONFIG_TEST_BPF=m

# cat /boot/config-$(uname -r) | grep BTF
CONFIG_VIDEO_SONY_BTF_MPX=m
CONFIG_DEBUG_INFO_BTF=y
CONFIG_PAHOLE_HAS_SPLIT_BTF=y
CONFIG_DEBUG_INFO_BTF_MODULES=y

llvm:

# llc --version
LLVM (http://llvm.org/):
  LLVM version 12.0.0

  Optimized build.
  Default target: x86_64-pc-linux-gnu
  Host CPU: skylake

  Registered Targets:
    bpf        - BPF (host endian)
    bpfeb      - BPF (big endian)
    bpfel      - BPF (little endian)

how error happens

I run Ubuntu 21.04 as a VirtualBox VM. go generate, go build and ./pwru --output-skb, then the error comes.

what I try

check the kprobe_pwru.c:

static __always_inline void
set_skb_btf(struct sk_buff *skb, typeof(print_skb_id) *event_id) {
#ifdef OUTPUT_SKB
	static struct btf_ptr p = {};
	typeof(print_skb_id) id;
	char *str;

	p.type_id = bpf_core_type_id_kernel(struct sk_buff);
	p.ptr = skb;
	id = __sync_fetch_and_add(&print_skb_id, 1) % 256;

	str = bpf_map_lookup_elem(&print_skb_map, (u32 *) &id);
	if (!str)
		return;

	if (bpf_snprintf_btf(str, PRINT_SKB_STR_SIZE, &p, sizeof(p), 0) < 0)
		return;

	*event_id = id;
#endif
}

check the kernel bpf verifier.c:

		if (BPF_CLASS(insn->code) == BPF_STX &&
		    ((BPF_MODE(insn->code) != BPF_MEM &&
		      BPF_MODE(insn->code) != BPF_XADD) || insn->imm != 0)) {
			verbose(env, "BPF_STX uses reserved fields\n");
			return -EINVAL;
		}

and learn BPF and XDP Reference Guide:

BPF_ST, BPF_STX: Both classes are for store operations. Similar to BPF_LDX the BPF_STX is the store counterpart and is used to 

store the data from a register into memory, which, again, can be stack memory, map value, packet data, etc. BPF_STX also holds 

special instructions for performing word and double-word based atomic add operations, which can be used for counters, for 

example. The BPF_ST class is similar to BPF_STX by providing instructions for storing data into memory only that the source 

operand is an immediate value.

also to check its assembly code:

# clang -I./bpf/headers -O2 -target bpf -c bpf/kprobe_pwru.c -o kprobe_pwru.elf -g -DOUTPUT_SKB
# llvm-objdump -S ./kprobe_pwru.elf > kprobe_pwru.elf.txt

But at the end, I don't know how to fix it.

[pchaigno]

Could you attach the object file pwru/kprobepwru_bpfel.o that's generated after running go generate && go build?

[Asphaltt]

Could you attach the object file pwru/kprobepwru_bpfel.o

How to attach it?

[pchaigno]

You can click on the text below at the bottom of any new comment or comment you are editing.

[Asphaltt]

Oh, sorry, I misunderstand the 'attach'. Attach the kprobes in object file to kernel.

Here it is. kprobepwru_bpfel.o.txt

(Can't upload the .o file, plz remove the suffix .txt to get .o file)

[brb]

@Asphaltt Does this still happen with the latest release?

[Asphaltt]

I try it with the latest release. It does not happen again.

[Asphaltt]

Probably, the reason of that the issue gone is because I upgraded my Ubuntu VM.

I have upgraded my Ubuntu VM to Ubuntu 22.04.

# lsb_release -a
Distributor ID:	Ubuntu
Description:	Ubuntu 22.04.1 LTS
Release:	22.04
Codename:	jammy

# uname -a
Linux pagani 5.15.0-46-generic #49-Ubuntu SMP Thu Aug 4 18:03:25 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

[brb]

I try it with the latest release. It does not happen again.

Thanks for confirming!