dissect: optionally allow attaching images that do not qualify as DDIs with --attach

It's nice to use --attach since it validates our images. So far, it strictly insisted on checking that the image was a DDI however, which limits its use for certain usecases. Add "--relax" to address this limitation.
2026-09-22 16:04:26 +02:00
parent efa16c7f31
commit b0edc60661
4 changed files with 58 additions and 19 deletions
@@ -209,7 +209,8 @@
        of <command>losetup --find --show --partscan</command>, but will validate the image as DDI before
        attaching, and derive the correct sector size to use automatically. Moreover, it ensures the
        per-partition block devices are created before returning and attempts to enable direct IO mode, if
-        possible. Takes a path to a disk image file.</para>
+        possible. Takes a path to a disk image file. Use <option>--relax</option> to attach images that do
+        not qualify as DDI.</para>

        <para>If combined with <option>--quiet</option> output of the block device name is suppressed.</para>

@@ -541,6 +542,18 @@
        <xi:include href="version-info.xml" xpointer="v258"/></listitem>
      </varlistentry>

+      <varlistentry>
+        <term><option>--relax</option></term>
+
+        <listitem><para>If combined with <option>--attach</option>, skips validation of the image and
+        attaches it to the loopback block device as it is, even if it does not qualify as DDI. If this
+        switch is used, no image dissection takes place, and thus it is not guaranteed that the
+        per-partition block devices are created before the command returns. May not be combined with any
+        other command than <option>--attach</option>.</para>
+
+        <xi:include href="version-info.xml" xpointer="v263"/></listitem>
+      </varlistentry>
+
      <varlistentry>
        <term><option>--mtree-hash=no</option></term>

@@ -37,7 +37,8 @@ _systemd_dissect() {
                     -r --read-only
                     --mkdir
                     --rmdir
-                     --in-memory'
+                     --in-memory
+                     --relax'
        [ARG]='-m --mount -M
               -u --umount -U
               --attach
@@ -109,6 +109,7 @@ static RuntimeScope arg_runtime_scope = _RUNTIME_SCOPE_INVALID;
 static bool arg_all = false;
 static uid_t arg_uid_base = UID_INVALID;
 static bool arg_quiet = false;
+static bool arg_relax = false;
 static ImageFilter *arg_image_filter = NULL;
 static int arg_copy_ownership = -1;

@@ -355,6 +356,10 @@ static int parse_argv(int argc, char *argv[]) {
                        arg_loop_ref_auto = true;
                        break;

+                OPTION_LONG("relax", NULL, "Skip DDI validation, attach image as it is"):
+                        arg_relax = true;
+                        break;
+
                OPTION_LONG("image-policy", "POLICY", "Specify image dissection policy"):
                        r = parse_image_policy_argument(opts.arg, &arg_image_policy);
                        if (r < 0)
@@ -707,6 +712,10 @@ static int parse_argv(int argc, char *argv[]) {
                        return r;
        }

+        if (arg_relax && arg_action != ACTION_ATTACH)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                       "The --relax switch is only supported in combination with --attach.");
+
        SET_FLAG(arg_flags, DISSECT_IMAGE_ALLOW_INTERACTIVE_AUTH, isatty_safe(STDIN_FILENO));

        return 1;
@@ -1809,19 +1818,16 @@ static int action_discover(void) {
        return table_print_with_pager(t, arg_json_format_flags, arg_pager_flags, arg_legend);
 }

-static int action_attach(DissectedImage *m, LoopDevice *d) {
+static int action_attach(LoopDevice *d) {
        int r;

-        assert(m);
        assert(d);

        r = loop_device_set_autoclear(d, false);
        if (r < 0)
                return log_error_errno(r, "Failed to disable auto-clear logic on loopback device: %m");

-        r = dissected_image_relinquish(m);
-        if (r < 0)
-                return log_error_errno(r, "Failed to relinquish DM and loopback block devices: %m");
+        loop_device_relinquish(d);

        if (!arg_quiet)
                puts(d->node);
@@ -1997,7 +2003,7 @@ static int run(int argc, char *argv[]) {
                ;
        }

-        if (arg_image) {
+        if (arg_image && !arg_relax) {
                r = verity_settings_load(
                                &arg_verity_settings,
                                arg_image,
@@ -2053,6 +2059,12 @@ static int run(int argc, char *argv[]) {
                                                log_warning_errno(r, "Failed to set loop reference string to '%s', ignoring: %m", arg_loop_ref);
                                }

+                                /* Unless --relax is specified we insist that the image qualifies as DDI:
+                                 * dissecting it validates it against the image policy and waits until the
+                                 * kernel created the per-partition block devices, so that callers may
+                                 * access them immediately after we return. With --relax we skip dissection
+                                 * entirely, and attach the image as it is. */
+                                if (!arg_relax) {
                                        r = dissect_loop_device_and_warn(
                                                        d,
                                                        &arg_verity_settings,
@@ -2063,9 +2075,10 @@ static int run(int argc, char *argv[]) {
                                                        &m);
                                        if (r < 0)
                                                return r;
+                                }

                                if (arg_action == ACTION_ATTACH)
-                                        return action_attach(m, d);
+                                        return action_attach(d);

                                r = dissected_image_load_verity_sig_partition(
                                                m,
@@ -1066,6 +1066,18 @@ test ! -e "/dev/disk/by-loop-ref/$name"
 systemd-dissect --detach "$MINIMAL_IMAGE.raw"
 (! systemd-dissect --detach "$MINIMAL_IMAGE.raw")

+# --attach --relax also works on images that do not qualify as DDI, e.g. an entirely blank one
+BLANK_IMAGE="$(mktemp /var/tmp/blankXXX.raw)"
+truncate -s 16M "$BLANK_IMAGE"
+(! systemd-dissect --attach "$BLANK_IMAGE")
+LOOP="$(systemd-dissect --attach --relax "$BLANK_IMAGE")"
+test -b "$LOOP"
+systemd-dissect --detach "$LOOP"
+rm -f "$BLANK_IMAGE"
+
+# --relax makes no sense for the other verbs
+(! systemd-dissect --relax "$MINIMAL_IMAGE.raw")
+
 # check for confext functionality
 mkdir -p /run/confexts/test/etc/extension-release.d
 echo "ID=_any" >/run/confexts/test/etc/extension-release.d/extension-release.test