selftests/bpf: Fix flaky test ptr_untrusted

Somehow recently I frequently hit the following test failure
with either ./test_progs or ./test_progs-cpuv4:
  serial_test_ptr_untrusted:PASS:skel_open 0 nsec
  serial_test_ptr_untrusted:PASS:lsm_attach 0 nsec
  serial_test_ptr_untrusted:PASS:raw_tp_attach 0 nsec
  serial_test_ptr_untrusted:FAIL:cmp_tp_name unexpected cmp_tp_name: actual -115 != expected 0
  torvalds#182     ptr_untrusted:FAIL

Further investigation found the failure is due to
  bpf_probe_read_user_str()
where reading user-level string attr->raw_tracepoint.name
is not successfully, most likely due to the
string itself still in disk and not populated into memory yet.

One solution is do a printf() call of the string before doing bpf
syscall which will force the raw_tracepoint.name into memory.
But I think a more robust solution is to use bpf_copy_from_user()
which is used in sleepable program and can tolerate page fault,
and the fix here used the latter approach.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240204194452.2785936-1-yonghong.song@linux.dev

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

@@ -6,13 +6,13 @@
 
 char tp_name[128];
 
-SEC("lsm/bpf")
+SEC("lsm.s/bpf")
 int BPF_PROG(lsm_run, int cmd, union bpf_attr *attr, unsigned int size)
 {
 	switch (cmd) {
 	case BPF_RAW_TRACEPOINT_OPEN:
-		bpf_probe_read_user_str(tp_name, sizeof(tp_name) - 1,
-					(void *)attr->raw_tracepoint.name);
+		bpf_copy_from_user(tp_name, sizeof(tp_name) - 1,
+				   (void *)attr->raw_tracepoint.name);
 		break;
 	default:
 		break;