[systemd-devel] [PATCH] cryptsetup: Support key-slot option

[Christian Seiler]

Debian recently introduced the option key-slot to /etc/crypttab to
specify the LUKS key slot to be used for decrypting the device. On
systems where a keyfile is used and the key is not in the first slot,
this can speed up the boot process quite a bit, since cryptsetup does
not need to try all of the slots sequentially. (Unsuccessfully testing
a key slot typically takes up to about 1 second.)

This patch makes systemd aware of this option.

Debian bug that introduced the feature:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=704470
---
 man/crypttab.xml            | 14 ++++++++++++++
 src/cryptsetup/cryptsetup.c | 13 +++++++++++--
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/man/crypttab.xml b/man/crypttab.xml
index 90d8ce9..5f386e5 100644
--- a/man/crypttab.xml
+++ b/man/crypttab.xml
@@ -164,6 +164,20 @@
                         </varlistentry>
 
                         <varlistentry>
+                                <term><varname>key-slot=</varname></term>
+
+                                <listitem><para>Specifies the key slot to
+                                compare the passphrase or key against.
+                                If the key slot does not match the given
+                                passphrase or key, but another would, the
+                                setup of the device will fail regardless.
+                                This implies <varname>luks</varname>. See
+                                <citerefentry><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+                                for possible values. The default is to try
+                                all key slots in sequential order.</para></listitem>
+                        </varlistentry>
+
+                        <varlistentry>
                                 <term><varname>luks</varname></term>
 
                                 <listitem><para>Force LUKS mode. When this mode
diff --git a/src/cryptsetup/cryptsetup.c b/src/cryptsetup/cryptsetup.c
index 0a15b50..033c0cd 100644
--- a/src/cryptsetup/cryptsetup.c
+++ b/src/cryptsetup/cryptsetup.c
@@ -39,6 +39,7 @@
 static const char *opt_type = NULL; /* CRYPT_LUKS1, CRYPT_TCRYPT or CRYPT_PLAIN */
 static char *opt_cipher = NULL;
 static unsigned opt_key_size = 0;
+static int opt_key_slot = CRYPT_ANY_SLOT;
 static unsigned opt_keyfile_size = 0;
 static unsigned opt_keyfile_offset = 0;
 static char *opt_hash = NULL;
@@ -87,6 +88,14 @@ static int parse_one_option(const char *option) {
                         return 0;
                 }
 
+        } else if (startswith(option, "key-slot=")) {
+
+                opt_type = CRYPT_LUKS1;
+                if (safe_atoi(option+9, &opt_key_slot) < 0) {
+                        log_error("key-slot= parse failure, ignoring.");
+                        return 0;
+                }
+
         } else if (startswith(option, "tcrypt-keyfile=")) {
 
                 opt_type = CRYPT_TCRYPT;
@@ -425,7 +434,7 @@ static int attach_luks_or_plain(struct crypt_device *cd,
                  crypt_get_device_name(cd));
 
         if (key_file) {
-                r = crypt_activate_by_keyfile_offset(cd, name, CRYPT_ANY_SLOT,
+                r = crypt_activate_by_keyfile_offset(cd, name, opt_key_slot,
                                                      key_file, opt_keyfile_size,
                                                      opt_keyfile_offset, flags);
                 if (r < 0) {
@@ -439,7 +448,7 @@ static int attach_luks_or_plain(struct crypt_device *cd,
                         if (pass_volume_key)
                                 r = crypt_activate_by_volume_key(cd, name, *p, opt_key_size, flags);
                         else
-                                r = crypt_activate_by_passphrase(cd, name, CRYPT_ANY_SLOT, *p, strlen(*p), flags);
+                                r = crypt_activate_by_passphrase(cd, name, opt_key_slot, *p, strlen(*p), flags);
 
                         if (r >= 0)
                                 break;
-- 
1.8.3.1