bpf: Remove unnecessary OUTPUT_SKB macro

When call set_skb_btf() or set_shinfo_btf(), these functions are
protected by constant CFG.

Then, while loading kprobe_pwru into kernel, if no
--output-skb/--output-skb-shared-info, set_skb_btf()/set_shinfo_btf()
won't be verified. Next, these unreachable code will be eliminated by
verifier.

Signed-off-by: Leon Hwang <[email protected]>

bpf/kprobe_pwru.c

@@ -164,7 +164,6 @@ struct {
 	__uint(value_size, MAX_STACK_DEPTH * sizeof(u64));
 } print_stack_map SEC(".maps");
 
-#ifdef OUTPUT_SKB
 struct print_skb_value {
 	u32 len;
 	char str[PRINT_SKB_STR_SIZE];
@@ -197,7 +196,6 @@ struct {
 	__type(key, u64);
 	__type(value, struct print_shinfo_value);
 } print_shinfo_map SEC(".maps");
-#endif
 
 static __always_inline u32
 get_netns(struct sk_buff *skb) {
@@ -344,7 +342,6 @@ sync_fetch_and_add(void *id_map) {
 
 static __always_inline void
 set_skb_btf(struct sk_buff *skb, u64 *event_id) {
-#ifdef OUTPUT_SKB
 	static struct btf_ptr p = {};
 	static struct print_skb_value v = {};
 	u64 id;
@@ -359,12 +356,10 @@ set_skb_btf(struct sk_buff *skb, u64 *event_id) {
 	}
 
 	bpf_map_update_elem(&print_skb_map, event_id, &v, BPF_ANY);
-#endif
 }
 
 static __always_inline void
 set_shinfo_btf(struct sk_buff *skb, u64 *event_id) {
-#ifdef OUTPUT_SKB
 	struct skb_shared_info *shinfo;
 	static struct btf_ptr p = {};
 	static struct print_shinfo_value v = {};
@@ -393,7 +388,6 @@ set_shinfo_btf(struct sk_buff *skb, u64 *event_id) {
 	}
 
 	bpf_map_update_elem(&print_shinfo_map, event_id, &v, BPF_ANY);
-#endif
 }
 
 static __always_inline u64

build.go

@@ -2,9 +2,7 @@
 // Copyright (C) 2021 Authors of Cilium */
 
 //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 -- -DOUTPUT_SKB -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 -- -DOUTPUT_SKB -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 KProbePWRUWithoutOutputSKB ./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 KProbeMultiPWRUWithoutOutputSKB ./bpf/kprobe_pwru.c -- -D HAS_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 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
 
 package main

internal/pwru/utils.go

@@ -194,3 +194,22 @@ func HaveAvailableFilterFunctions() bool {
 	_, err := getAvailableFilterFunctions()
 	return err == nil
 }
+
+func HaveSnprintfBtf(kernelBtf *btf.Spec) bool {
+	types, err := kernelBtf.AnyTypesByName("bpf_func_id")
+	if err != nil {
+		return false
+	}
+
+	for _, t := range types {
+		if enum, ok := t.(*btf.Enum); ok {
+			for _, v := range enum.Values {
+				if v.Name == "BPF_FUNC_snprintf_btf" {
+					return true
+				}
+			}
+		}
+	}
+
+	return false
+}

main.go

@@ -67,6 +67,10 @@ func main() {
 		log.Fatalf("Failed to load BTF spec: %s", err)
 	}
 
+	if (flags.OutputSkb || flags.OutputShinfo) && !pwru.HaveSnprintfBtf(btfSpec) {
+		log.Fatal("Unsupported to output skb or shinfo because bpf_snprintf_btf() is unavailable")
+	}
+
 	if flags.AllKMods {
 		files, err := os.ReadDir("/sys/kernel/btf")
 		if err != nil {
@@ -119,12 +123,8 @@ func main() {
 	switch {
 	case (flags.OutputSkb || flags.OutputShinfo) && useKprobeMulti:
 		bpfSpec, err = LoadKProbeMultiPWRU()
-	case flags.OutputSkb || flags.OutputShinfo:
-		bpfSpec, err = LoadKProbePWRU()
-	case useKprobeMulti:
-		bpfSpec, err = LoadKProbeMultiPWRUWithoutOutputSKB()
 	default:
-		bpfSpec, err = LoadKProbePWRUWithoutOutputSKB()
+		bpfSpec, err = LoadKProbePWRU()
 	}
 	if err != nil {
 		log.Fatalf("Failed to load bpf spec: %v", err)