cryptsetup: Implement offset and skip options

These are useful for plain devices as they don't have any metadata by
themselves. Instead of using an unreliable hardcoded device name in crypttab
you can then put static metadata at the start of the partition for a stable
UUID or label.

https://bugs.freedesktop.org/show_bug.cgi?id=87717
https://bugs.debian.org/751707
https://launchpad.net/bugs/953875

src/cryptsetup/cryptsetup.c

@@ -50,12 +50,12 @@ static bool arg_discards = false;
 static bool arg_tcrypt_hidden = false;
 static bool arg_tcrypt_system = false;
 static char **arg_tcrypt_keyfiles = NULL;
+static uint64_t arg_offset = 0;
+static uint64_t arg_skip = 0;
 static usec_t arg_timeout = 0;
 
 /* Options Debian's crypttab knows we don't:
 
-    offset=
-    skip=
     precheck=
     check=
     checkargs=
@@ -185,6 +185,20 @@ static int parse_one_option(const char *option) {
                         return 0;
                 }
 
+        } else if (startswith(option, "offset=")) {
+
+                if (safe_atou64(option+7, &arg_offset) < 0) {
+                        log_error("offset= parse failure, refusing.");
+                        return -EINVAL;
+                }
+
+        } else if (startswith(option, "skip=")) {
+
+                if (safe_atou64(option+5, &arg_skip) < 0) {
+                        log_error("skip= parse failure, refusing.");
+                        return -EINVAL;
+                }
+
         } else if (!streq(option, "none"))
                 log_error("Encountered unknown /etc/crypttab option '%s', ignoring.", option);
 
@@ -209,6 +223,14 @@ static int parse_options(const char *options) {
                         return r;
         }
 
+        /* sanity-check options */
+        if (arg_type != NULL && !streq(arg_type, CRYPT_PLAIN)) {
+                if (arg_offset)
+                      log_warning("offset= ignored with type %s", arg_type);
+                if (arg_skip)
+                      log_warning("skip= ignored with type %s", arg_type);
+        }
+
         return 0;
 }
 
@@ -410,7 +432,10 @@ static int attach_luks_or_plain(struct crypt_device *cd,
         }
 
         if ((!arg_type && r < 0) || streq_ptr(arg_type, CRYPT_PLAIN)) {
-                struct crypt_params_plain params = {};
+                struct crypt_params_plain params = {
+                        .offset = arg_offset,
+                        .skip = arg_skip,
+                };
                 const char *cipher, *cipher_mode;
                 _cleanup_free_ char *truncated_cipher = NULL;