Avoid printing garvage skb written by previous writer

After we consume all of the print_skb_map slot, we start to reuse the
slot. On the reused buffer, there's a previous skb string and we don't
clear that. So, when the new skb string is shorter than the previous
one, we print the garvage string. Keep the length of the string in the
map and truncate unnecessary part from the output to avoid it.

Signed-off-by: Yutaro Hayakawa <[email protected]>

Author: YutaroHayakawa

bpf/kprobe_pwru.c

@@ -100,11 +100,15 @@ struct {
 } print_stack_map SEC(".maps");
 
 #ifdef OUTPUT_SKB
+struct print_skb_value {
+	u32 len;
+	char str[PRINT_SKB_STR_SIZE];
+};
 struct {
 	__uint(type, BPF_MAP_TYPE_ARRAY);
 	__uint(max_entries, 256);
 	__type(key, u32);
-	__type(value, char[PRINT_SKB_STR_SIZE]);
+	__type(value, struct print_skb_value);
 } print_skb_map SEC(".maps");
 #endif
 
@@ -230,22 +234,26 @@ 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 = {};
+	struct print_skb_value *v;
 	typeof(print_skb_id) id;
-	char *str;
+	long n;
 
 	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) {
+	v = bpf_map_lookup_elem(&print_skb_map, (u32 *) &id);
+	if (!v) {
 		return;
 	}
 
-	if (bpf_snprintf_btf(str, PRINT_SKB_STR_SIZE, &p, sizeof(p), 0) < 0) {
+	n = bpf_snprintf_btf(v->str, PRINT_SKB_STR_SIZE, &p, sizeof(p), 0);
+	if (n < 0) {
 		return;
 	}
 
+	v->len = n;
+
 	*event_id = id;
 #endif
 }

internal/pwru/output.go

@@ -5,6 +5,7 @@
 package pwru
 
 import (
+	"encoding/binary"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -247,10 +248,20 @@ func getStackData(event *Event, o *output) (stackData string) {
 
 func getSkbData(event *Event, o *output) (skbData string) {
 	id := uint32(event.PrintSkbId)
-	if str, err := o.printSkbMap.LookupBytes(&id); err == nil {
-		skbData = string(str)
+
+	b, err := o.printSkbMap.LookupBytes(&id)
+	if err != nil {
+		return ""
+	}
+
+	length := binary.NativeEndian.Uint32(b[:4])
+
+	// Bounds check
+	if int(length+4) > len(b) {
+		return ""
 	}
-	return skbData
+
+	return "\n" + string(b[4:4+length])
 }
 
 func getMetaData(event *Event, o *output) (metaData string) {