selftests/bpf: Add selftests verifying bpf_trace_printk() behaviour

Simple selftests that verifies bpf_trace_printk() returns a sensible
value and tracing messages appear.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/1594641154-18897-3-git-send-email-alan.maguire@oracle.com

tools/testing/selftests/bpf/progs/trace_printk.c

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (c) 2020, Oracle and/or its affiliates.
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+int trace_printk_ret = 0;
+int trace_printk_ran = 0;
+
+SEC("tp/raw_syscalls/sys_enter")
+int sys_enter(void *ctx)
+{
+	static const char fmt[] = "testing,testing %d\n";
+
+	trace_printk_ret = bpf_trace_printk(fmt, sizeof(fmt),
+					    ++trace_printk_ran);
+	return 0;
+}