modsign: Make sign-file determine the format of the X.509 cert

Make sign-file determine the format of the X.509 certificate by reading the
first two bytes and seeing if the first byte is 0x30 and the second
0x81-0x84.  If this is the case, assume it's DER encoded, otherwise assume
it to be PEM encoded.

Without this, it gets awkward to deal with the error messages from
d2i_X509_bio() when we want to call BIO_reset() and then PEM_read_bio() in
case the certificate was PEM encoded rather than X.509 encoded.
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Ben Hutchings <ben@decadent.org.uk>
cc: David Woodhouse <dwmw2@infradead.org>
cc: Juerg Haefliger <juerg.haefliger@hpe.com>
cc: Ben Hutchings <ben@decadent.org.uk>

scripts/sign-file.c

 /* Sign a module file using the given key.
  *
- * Copyright © 2014-2015 Red Hat, Inc. All Rights Reserved.
+ * Copyright © 2014-2016 Red Hat, Inc. All Rights Reserved.
  * Copyright © 2015      Intel Corporation.
  * Copyright © 2016      Hewlett Packard Enterprise Development LP
  *
@@ -167,19 +167,37 @@ static EVP_PKEY *read_private_key(const char *private_key_name)
 
 static X509 *read_x509(const char *x509_name)
 {
+	unsigned char buf[2];
 	X509 *x509;
 	BIO *b;
+	int n;
 
 	b = BIO_new_file(x509_name, "rb");
 	ERR(!b, "%s", x509_name);
-	x509 = d2i_X509_bio(b, NULL); /* Binary encoded X.509 */
-	if (!x509) {
-		ERR(BIO_reset(b) != 1, "%s", x509_name);
-		x509 = PEM_read_bio_X509(b, NULL, NULL,
-					 NULL); /* PEM encoded X.509 */
-		if (x509)
-			drain_openssl_errors();
+
+	/* Look at the first two bytes of the file to determine the encoding */
+	n = BIO_read(b, buf, 2);
+	if (n != 2) {
+		if (BIO_should_retry(b)) {
+			fprintf(stderr, "%s: Read wanted retry\n", x509_name);
+			exit(1);
+		}
+		if (n >= 0) {
+			fprintf(stderr, "%s: Short read\n", x509_name);
+			exit(1);
+		}
+		ERR(1, "%s", x509_name);
 	}
+
+	ERR(BIO_reset(b) != 0, "%s", x509_name);
+
+	if (buf[0] == 0x30 && buf[1] >= 0x81 && buf[1] <= 0x84)
+		/* Assume raw DER encoded X.509 */
+		x509 = d2i_X509_bio(b, NULL);
+	else
+		/* Assume PEM encoded X.509 */
+		x509 = PEM_read_bio_X509(b, NULL, NULL, NULL);
+
 	BIO_free(b);
 	ERR(!x509, "%s", x509_name);