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

2 files changed, 52 insertions, 3 deletions

diff --git a/man/crypttab.xml b/man/crypttab.xml
index 3e249ad23e..d4ff760adc 100644
--- a/man/crypttab.xml
+++ b/man/crypttab.xml
@@ -146,6 +146,30 @@
       </varlistentry>
 
       <varlistentry>
+        <term><option>offset=</option></term>
+
+        <listitem><para>Start offset in the backend device, in 512-byte sectors.
+        This option is only relevant for plain devices.
+        </para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><option>skip=</option></term>
+
+        <listitem><para>How many 512-byte sectors of the encrypted data to skip
+        at the beginning. This is different from the <option>--offset</option>
+        option with respect to the sector numbers used in initialization vector
+        (IV) calculation. Using <option>--offset</option> will shift the IV
+        calculation by the same negative amount.  Hence, if <option>--offset n</option>,
+        sector n will  get a sector number of 0 for the IV calculation.
+        Using <option>--skip</option> causes sector n to also be the first
+        sector of the mapped device, but with its number for IV generation is n.</para>
+
+        <para>This option is only relevant for plain devices.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
         <term><option>keyfile-offset=</option></term>
 
         <listitem><para>Specifies the number of bytes to skip at the
diff --git a/src/cryptsetup/cryptsetup.c b/src/cryptsetup/cryptsetup.c
index ba0ef72d06..a5018f13ed 100644
--- a/src/cryptsetup/cryptsetup.c
+++ b/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;