fix: support bpf_core_type_exists()

[holyspectral]

Hi folks,

In the previous version, during relocations, when no corresponding target is found in BTF for bpf_core_type_exists(), coreCalculateFixups() will mark its relocation as poison and cause failure when verified by Linux verifier, instead of returning false at runtime:

19: (85) call unknown#195896080
invalid func unknown#195896080

This can be triggered with below ebpf program:

// +build ignore

#include "common.h"
#include "bpf_core_read.h"

char __license[] SEC("license") = "Dual MIT/GPL";

struct bpf_map_def SEC("maps") kprobe_map = {
	.type        = BPF_MAP_TYPE_ARRAY,
	.key_size    = sizeof(u32),
	.value_size  = sizeof(u64),
	.max_entries = 1,
};

struct bpf_ringbuf_2 {

};

SEC("kprobe/sys_execve")
int kprobe_execve() {
	u32 key     = 0;
	u64 initval = 1, *valp;

	int exist = bpf_core_type_exists(struct bpf_ringbuf_2);

	valp = bpf_map_lookup_elem(&kprobe_map, &key);
	if (!valp) {
		bpf_map_update_elem(&kprobe_map, &key, &initval, BPF_ANY);
		return 0;
	}
	__sync_fetch_and_add(valp, exist ? 1: 100000);

	return 0;
}

And load it with below userspace program:

package main

import (
	"os"
	"time"

	log "github.com/sirupsen/logrus"

	"github.com/cilium/ebpf"
	"github.com/cilium/ebpf/link"
)

type bpfObjects struct {
	bpfPrograms
	bpfMaps
}

type bpfPrograms struct {
	KprobeExecve *ebpf.Program `ebpf:"kprobe_execve"`
}

type bpfMaps struct {
	KprobeMap *ebpf.Map `ebpf:"kprobe_map"`
}

//go:generate go run github.com/cilium/ebpf/cmd/bpf2go@96aa1a7a929f1c954193b537b19e5e59bdd950ef -cc clang-12 -cflags $BPF_CFLAGS bpf ../../bpf/kprobe.c -- -I../../bpf/include
func main() {
	// cilium/ebpf does relocation in newProgramWithOptions().
	bpf_reader, err := os.Open("bpf_bpfel.o")
	if err != nil {
		log.WithError(err).Fatal("Failed to load bpf program.")
	}
	spec, err := ebpf.LoadCollectionSpecFromReader(bpf_reader)
	if err != nil {
		log.WithError(err).Fatal("Failed to load collection spec.")
	}

	objs := bpfObjects{}
	err = spec.LoadAndAssign(&objs, nil)
	if err != nil {
		log.WithError(err).Fatal("Failed to LoadAndAssign().")
	}

	defer objs.KprobeExecve.Close()
	defer objs.KprobeMap.Close()

	kp, err := link.Kprobe("sys_execve", objs.KprobeExecve, nil)
	if err != nil {
		log.WithError(err).Fatal("Failed to attach kprobe().")
	}
	defer kp.Close()

	ticker := time.NewTicker(1 * time.Second)
	defer ticker.Stop()
	for range ticker.C {
		const mapKey uint32 = 0
		var value uint64
		if err := objs.KprobeMap.Lookup(mapKey, &value); err != nil {
			log.WithError(err).Fatal("failed to read map")
		}
		log.Println(value)
	}
}

This issue is fixed by providing fixup for "checksForExistence" type of relocations even when no target is found.

[holyspectral]

Hi @ti-mo @lmb Would you like to take a look? Appreciated.

[lmb]

Thank you for your PR :) Could you extend internal/btf/testdata/relocs_read.c so that it exercises this code path?

[ti-mo]

Thanks!

[holyspectral]

@ti-mo @lmb Thank you!