dissect: Don't count RPMB and boot partitions

Filter-out RPMB partitions and boot partitions from MMC devices when
comparing the size of the kernel and blkid partition lists.

This complement the previous fix to the problem reported in
systemd/systemd#5806

https://phabricator.endlessm.com/T21744

src/shared/dissect-image.c

@@ -277,8 +277,22 @@ int dissect_image(int fd, const void *root_hash, size_t root_hash_size, DissectI
                 /* Count the partitions enumerated by the kernel */
                 n = 0;
                 first = udev_enumerate_get_list_entry(e);
-                udev_list_entry_foreach(item, first)
+                udev_list_entry_foreach(item, first) {
+                        _cleanup_udev_device_unref_ struct udev_device *q;
+                        const char *node, *sysname;
+
+                        /* Filter out weird MMC RPMB partitions, which cannot reasonably be read, see
+                         * https://github.com/systemd/systemd/issues/5806 */
+                        q = udev_device_new_from_syspath(udev, udev_list_entry_get_name(item));
+                        if (!q)
+                                return -errno;
+                        sysname = udev_device_get_sysname(q);
+                        if (sysname && startswith(sysname, "mmcblk") &&
+                                (endswith(sysname, "rpmb") || endswith(sysname, "boot0" ) || endswith(sysname, "boot1")))
+                                continue;
+
                         n++;
+                }
 
                 /* Count the partitions enumerated by blkid */
                 z = blkid_partlist_numof_partitions(pl);