cryptsetup: add keyfile-offset= support

This is useful if your keyfile is a block device, and you want to
use a specific part of it, such as an area between the MBR and the
first partition.

This feature is documented in the Arch wiki[0], and has been supported
by the Arch initscripts, so would be nice to get this into systemd.

This requires libcryptsetup >= 1.4.2 (released 12.4.2012).

Acked-by: Paul Menzel <paulepanter@users.sourceforge.net>

[0]:
<https://wiki.archlinux.org/index.php/System_Encryption_with_LUKS#
Storing_the_key_between_MBR_and_1st_partition>

src/cryptsetup/cryptsetup.c

@@ -37,6 +37,7 @@
 static const char *opt_type = NULL; /* LUKS1 or PLAIN */
 static char *opt_cipher = NULL;
 static unsigned opt_key_size = 0;
+static unsigned opt_keyfile_offset = 0;
 static char *opt_hash = NULL;
 static unsigned opt_tries = 0;
 static bool opt_readonly = false;
@@ -79,6 +80,13 @@ static int parse_one_option(const char *option) {
                         return 0;
                 }
 
+        } else if (startswith(option, "keyfile-offset=")) {
+
+                if (safe_atou(option+15, &opt_keyfile_offset) < 0) {
+                        log_error("keyfile-offset= parse failure, ignoring.");
+                        return 0;
+                }
+
         } else if (startswith(option, "hash=")) {
                 char *t;
 
@@ -462,7 +470,8 @@ int main(int argc, char *argv[]) {
                                  argv[3]);
 
                         if (key_file)
-                                k = crypt_activate_by_keyfile(cd, argv[2], CRYPT_ANY_SLOT, key_file, keyfile_size, flags);
+                                k = crypt_activate_by_keyfile_offset(cd, argv[2], CRYPT_ANY_SLOT, key_file, keyfile_size,
+                                            opt_keyfile_offset, flags);
                         else {
                                 char **p;