Fix verifier error to run --filter-trace-tc + --filter-track-bpf-helpers

When to run `./pwru --filter-track-bpf-helpers --filter-trace-tc --filter-func '.*udp.*' --output-limit-lines 10 icmp`,
there will be a verifier error:

```log
2024/10/08 13:11:17 Attaching tc-bpf progs...
2024/10/08 13:11:17 failed to trace TC progs: failed to trace bpf progs: failed to load objects: Verifier error: load program: permission denied:
	func#0 @0
	func#1 @43
	func#2 @1062
	func#3 @1118
	0: R1=ctx() R10=fp0
	; int BPF_PROG(fentry_tc, struct sk_buff *skb) { @ kprobe_pwru.c:594
        ...
        ; u64 fp = PT_REGS_FP(ctx); @ kprobe_pwru.c:399
	69: (79) r1 = *(u64 *)(r10 -64)       ; frame1: R1_w=ctx() R10=fp0 fp-64=ctx()
	70: (79) r7 = *(u64 *)(r1 +32)
	func 'entry2' doesn't have 3-th argument
	invalid bpf_context access off=32 size=8
	processed 51 insns (limit 1000000) max_states_per_insn 0 total_states 1 peak_states 1 mark_read 1

program fentry_tc: load program: permission denied: func 'entry2' doesn't have 3-th argument: invalid bpf_context access off=32 size=8 (110 line(s) omitted)
```

That's because `fentry` does not support `PT_REGS_FP`. And, there's no
way for `fentry` to get FP.

So, let us skip getting FP for `fentry_tc`.

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

bpf/kprobe_pwru.c

@@ -434,13 +434,13 @@ set_output(void *ctx, struct sk_buff *skb, struct event_t *event) {
 }
 
 static __noinline bool
-handle_everything(struct sk_buff *skb, void *ctx, struct event_t *event, u64 *_stackid) {
+handle_everything(struct sk_buff *skb, void *ctx, struct event_t *event, u64 *_stackid, const bool is_kprobe) {
 	u8 tracked_by;
 	u64 skb_addr = (u64) skb;
 	u64 skb_head = (u64) BPF_CORE_READ(skb, head);
 	u64 stackid;
 
-	if (cfg->track_skb_by_stackid)
+	if (cfg->track_skb_by_stackid && is_kprobe)
 		stackid = _stackid ? *_stackid : get_stackid(ctx);
 
 	if (cfg->is_set) {
@@ -457,7 +457,7 @@ handle_everything(struct sk_buff *skb, void *ctx, struct event_t *event, u64 *_s
 			goto cont;
 		}
 
-		if (cfg->track_skb_by_stackid && bpf_map_lookup_elem(&stackid_skb, &stackid)) {
+		if (cfg->track_skb_by_stackid && is_kprobe && bpf_map_lookup_elem(&stackid_skb, &stackid)) {
 			tracked_by = TRACKED_BY_STACKID;
 			goto cont;
 		}
@@ -479,7 +479,7 @@ handle_everything(struct sk_buff *skb, void *ctx, struct event_t *event, u64 *_s
 			bpf_map_update_elem(&xdp_dhs_skb_heads, &skb_head, &skb_addr, BPF_ANY);
 	}
 
-	if (cfg->track_skb_by_stackid && tracked_by != TRACKED_BY_STACKID) {
+	if (cfg->track_skb_by_stackid && is_kprobe && tracked_by != TRACKED_BY_STACKID) {
 		u64 *old_stackid = bpf_map_lookup_elem(&skb_stackid, &skb);
 		if (old_stackid && *old_stackid != stackid) {
 			bpf_map_delete_elem(&stackid_skb, old_stackid);
@@ -499,7 +499,7 @@ static __always_inline int
 kprobe_skb(struct sk_buff *skb, struct pt_regs *ctx, bool has_get_func_ip, u64 *_stackid) {
 	struct event_t event = {};
 
-	if (!handle_everything(skb, ctx, &event, _stackid))
+	if (!handle_everything(skb, ctx, &event, _stackid, true))
 		return BPF_OK;
 
 	event.skb_addr = (u64) skb;
@@ -596,7 +596,7 @@ SEC("fentry/tc")
 int BPF_PROG(fentry_tc, struct sk_buff *skb) {
 	struct event_t event = {};
 
-	if (!handle_everything(skb, ctx, &event, NULL))
+	if (!handle_everything(skb, ctx, &event, NULL, false))
 		return BPF_OK;
 
 	event.skb_addr = (u64) skb;