Add features detection

It's to prevent running --output-skb or --output-skb-shared-info on
kernel which does not support bpf_snprintf_btf() helper.

Meanwhile, avoid verifier roaring on Ubuntu 20.04:

; instruction poisoned by CO-RE
938: (85) call unknown#195896080
invalid func unknown#195896080
processed 519 insns (limit 1000000) max_states_per_insn 0 total_states 29 peak_states 29 mark_read 27

Signed-off-by: Leon Hwang <hffilwlqm@gmail.com>

.gitignore

@@ -4,4 +4,5 @@ release
 tags
 kprobepwru_*
 kprobemultipwru_*
+featurespwru_*
 !pwru/

bpf/features.c

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+/* Copyright Leon Hwang */
+
+#include "vmlinux.h"
+#include "bpf/bpf_helpers.h"
+#include "bpf/bpf_core_read.h"
+
+struct pwru_features {
+	bool kprobe_happened;
+	bool has_snprintf_btf;
+} features;
+
+SEC("kprobe")
+int kprobe__map_lookup_elem(struct pt_regs *ctx)
+{
+	features.kprobe_happened = true;
+
+	/* Detect if bpf_snprintf_btf is available.
+	 * Since: c4d0bfb45068 ("bpf: Add bpf_snprintf_btf helper")
+	 */
+	features.has_snprintf_btf = bpf_core_enum_value_exists(enum bpf_func_id, BPF_FUNC_snprintf_btf);
+
+	return BPF_OK;
+}
+
+char __license[] SEC("license") = "Dual BSD/GPL";

build.go

@@ -4,5 +4,6 @@
 //go:generate sh -c "echo Generating for $TARGET_GOARCH"
 //go:generate go run github.com/cilium/ebpf/cmd/bpf2go -target $TARGET_GOARCH -cc clang -no-strip KProbePWRU ./bpf/kprobe_pwru.c -- -I./bpf/headers -Wno-address-of-packed-member
 //go:generate go run github.com/cilium/ebpf/cmd/bpf2go -target $TARGET_GOARCH -cc clang -no-strip KProbeMultiPWRU ./bpf/kprobe_pwru.c -- -DHAS_KPROBE_MULTI -I./bpf/headers -Wno-address-of-packed-member
+//go:generate go run github.com/cilium/ebpf/cmd/bpf2go -target $TARGET_GOARCH -cc clang -no-strip FeaturesPWRU ./bpf/features.c -- -I./bpf/headers -Wno-address-of-packed-member
 
 package main

internal/pwru/features.go

@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: Apache-2.0
+/* Copyright Leon Hwang */
+
+package pwru
+
+import (
+	"log"
+
+	"github.com/cilium/ebpf"
+	"github.com/cilium/ebpf/btf"
+	"github.com/cilium/ebpf/link"
+)
+
+func DetectFeatures(spec *ebpf.CollectionSpec, btfSpec *btf.Spec, f *Flags) {
+	bssMap, err := ebpf.NewMap(spec.Maps[".bss"])
+	if err != nil {
+		log.Fatalf("Failed to create bss map: %v", err)
+	}
+	defer bssMap.Close()
+
+	coll, err := ebpf.NewCollectionWithOptions(spec, ebpf.CollectionOptions{
+		Programs: ebpf.ProgramOptions{
+			KernelTypes: btfSpec,
+		},
+		MapReplacements: map[string]*ebpf.Map{
+			".bss": bssMap,
+		},
+	})
+	if err != nil {
+		log.Fatalf("Failed to create collection: %v", err)
+	}
+	defer coll.Close()
+
+	kp, err := link.Kprobe("map_lookup_elem", coll.Programs["kprobe__map_lookup_elem"], nil)
+	if err != nil {
+		log.Fatalf("Failed to kprobe map_lookup_elem: %v", err)
+	}
+	defer kp.Close()
+
+	var features struct {
+		Happened       uint8
+		HasSnprintfBtf uint8
+	}
+
+	// Trigger kprobe and retrieve features data
+	err = bssMap.Lookup(uint32(0), &features)
+	if err != nil {
+		log.Fatalf("Failed to lookup features data: %v", err)
+	}
+
+	if features.Happened == 0 {
+		log.Fatalf("Features detection was not triggered")
+	}
+
+	if (f.OutputSkb || f.OutputShinfo) && features.HasSnprintfBtf == 0 {
+		log.Fatalf("Unsupported to output skb or shinfo because bpf_snprintf_btf() is unavailable")
+	}
+}

main.go

@@ -67,6 +67,13 @@ func main() {
 		log.Fatalf("Failed to load BTF spec: %s", err)
 	}
 
+	featBpfSpec, err := LoadFeaturesPWRU()
+	if err != nil {
+		log.Fatalf("Failed to load features bpf spec: %v", err)
+	}
+
+	pwru.DetectFeatures(featBpfSpec, btfSpec, &flags)
+
 	if flags.AllKMods {
 		files, err := os.ReadDir("/sys/kernel/btf")
 		if err != nil {