selftests/bpf: Fix verify_pkcs7_sig on s390x

Use bpf_probe_read_kernel() instead of bpf_probe_read(), which is not
defined on all architectures.

While at it, improve the error handling: do not hide the verifier log,
and check the return values of bpf_probe_read_kernel() and
bpf_copy_from_user().
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-10-iii@linux.ibm.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/verify_pkcs7_sig.c

@@ -61,6 +61,9 @@ static bool kfunc_not_supported;
 static int libbpf_print_cb(enum libbpf_print_level level, const char *fmt,
 			   va_list args)
 {
+	if (level == LIBBPF_WARN)
+		vprintf(fmt, args);
+
 	if (strcmp(fmt, "libbpf: extern (func ksym) '%s': not found in kernel or module BTFs\n"))
 		return 0;
 

tools/testing/selftests/bpf/progs/test_verify_pkcs7_sig.c

@@ -59,10 +59,14 @@ int BPF_PROG(bpf, int cmd, union bpf_attr *attr, unsigned int size)
 	if (!data_val)
 		return 0;
 
-	bpf_probe_read(&value, sizeof(value), &attr->value);
-
-	bpf_copy_from_user(data_val, sizeof(struct data),
-			   (void *)(unsigned long)value);
+	ret = bpf_probe_read_kernel(&value, sizeof(value), &attr->value);
+	if (ret)
+		return ret;
+
+	ret = bpf_copy_from_user(data_val, sizeof(struct data),
+				 (void *)(unsigned long)value);
+	if (ret)
+		return ret;
 
 	if (data_val->data_len > sizeof(data_val->data))
 		return -EINVAL;