bpf: add bpf_modify_return_test_tp() kfunc triggering tracepoint

Add a simple bpf_modify_return_test_tp() kfunc, available to all program
types, that is useful for various testing and benchmarking scenarios, as
it allows to trigger most tracing BPF program types from BPF side,
allowing to do complex testing and benchmarking scenarios.

It is also attachable to for fmod_ret programs, making it a good and
simple way to trigger fmod_ret program under test/benchmark.

Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

include/trace/events/bpf_test_run.h

···
 
 #include <linux/tracepoint.h>
 
+TRACE_EVENT(bpf_trigger_tp,
+
+	TP_PROTO(int nonce),
+
+	TP_ARGS(nonce),
+
+	TP_STRUCT__entry(
+		__field(int, nonce)
+	),
+
+	TP_fast_assign(
+		__entry->nonce = nonce;
+	),
+
+	TP_printk("nonce %d", __entry->nonce)
+);
+
 DECLARE_EVENT_CLASS(bpf_test_finish,
 
 	TP_PROTO(int *err),

kernel/bpf/helpers.c

···
 BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
 BTF_ID_FLAGS(func, bpf_dynptr_size)
 BTF_ID_FLAGS(func, bpf_dynptr_clone)
+BTF_ID_FLAGS(func, bpf_modify_return_test_tp)
 BTF_KFUNCS_END(common_btf_ids)
 
 static const struct btf_kfunc_id_set common_kfunc_set = {

net/bpf/test_run.c

···
 	return a + *b + c + d + (long)e + f + g;
 }
 
+__bpf_kfunc int bpf_modify_return_test_tp(int nonce)
+{
+	trace_bpf_trigger_tp(nonce);
+
+	return nonce;
+}
+
 int noinline bpf_fentry_shadow_test(int a)
 {
 	return a + 1;
···
 BTF_KFUNCS_START(bpf_test_modify_return_ids)
 BTF_ID_FLAGS(func, bpf_modify_return_test)
 BTF_ID_FLAGS(func, bpf_modify_return_test2)
+BTF_ID_FLAGS(func, bpf_modify_return_test_tp)
 BTF_ID_FLAGS(func, bpf_fentry_test1, KF_SLEEPABLE)
 BTF_KFUNCS_END(bpf_test_modify_return_ids)