selftests/bpf: Convert test printk to use rodata.

mainline inclusion
from mainline-5.14-rc1
commit eb0f1e0c
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5EUVD
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=eb0f1e0c7f103e262308d3f12a27a8331af3011a

-------------------------------------------------

Convert test trace_printk to more aggressively validate and use rodata.
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Acked-by: NAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210514003623.28033-21-alexei.starovoitov@gmail.com
(cherry picked from commit eb0f1e0c)
Signed-off-by: NWang Yufen <wangyufen@huawei.com>

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

@@ -21,6 +21,9 @@ void test_trace_printk(void)
 	if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
 		return;
 
+	ASSERT_EQ(skel->rodata->fmt[0], 'T', "invalid printk fmt string");
+	skel->rodata->fmt[0] = 't';
+
 	err = trace_printk__load(skel);
 	if (CHECK(err, "skel_load", "failed to load skeleton: %d\n", err))
 		goto cleanup;

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

@@ -10,11 +10,11 @@ char _license[] SEC("license") = "GPL";
 int trace_printk_ret = 0;
 int trace_printk_ran = 0;
 
-SEC("tp/raw_syscalls/sys_enter")
+const char fmt[] = "Testing,testing %d\n";
+
+SEC("fentry/__x64_sys_nanosleep")
 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;