dissect: make using a generic partition as root partition optional

In preparation for reusing the image dissector in the GPT auto-discovery
logic, only optionally fail the dissection when we can't identify a root
partition.

In the GPT auto-discovery we are completely fine with any kind of root,
given that we run when it is already mounted and all we do is find some
additional auxiliary partitions on the same disk.

src/dissect/dissect.c

@@ -35,7 +35,7 @@ static enum {
 } arg_action = ACTION_DISSECT;
 static const char *arg_image = NULL;
 static const char *arg_path = NULL;
-static DissectImageFlags arg_flags = DISSECT_IMAGE_DISCARD_ON_LOOP;
+static DissectImageFlags arg_flags = DISSECT_IMAGE_REQUIRE_ROOT|DISSECT_IMAGE_DISCARD_ON_LOOP;
 static void *arg_root_hash = NULL;
 static size_t arg_root_hash_size = 0;
 
@@ -191,7 +191,7 @@ int main(int argc, char *argv[]) {
                 goto finish;
         }
 
-        r = dissect_image(d->fd, arg_root_hash, arg_root_hash_size, 0, &m);
+        r = dissect_image(d->fd, arg_root_hash, arg_root_hash_size, arg_flags, &m);
         if (r == -ENOPKG) {
                 log_error_errno(r, "Couldn't identify a suitable partition table or file system in %s.", arg_image);
                 goto finish;

src/machine/image-dbus.c

@@ -336,7 +336,7 @@ static int raw_image_get_os_release(Image *image, char ***ret, sd_bus_error *err
         if (r < 0)
                 return sd_bus_error_set_errnof(error, r, "Failed to set up loop block device for %s: %m", image->path);
 
-        r = dissect_image(d->fd, NULL, 0, 0, &m);
+        r = dissect_image(d->fd, NULL, 0, DISSECT_IMAGE_REQUIRE_ROOT, &m);
         if (r == -ENOPKG)
                 return sd_bus_error_set_errnof(error, r, "Disk image %s not understood: %m", image->path);
         if (r < 0)

src/nspawn/nspawn.c

@@ -3743,7 +3743,11 @@ int main(int argc, char *argv[]) {
                         goto finish;
                 }
 
-                r = dissect_image(loop->fd, arg_root_hash, arg_root_hash_size, 0, &dissected_image);
+                r = dissect_image(
+                                loop->fd,
+                                arg_root_hash, arg_root_hash_size,
+                                DISSECT_IMAGE_REQUIRE_ROOT,
+                                &dissected_image);
                 if (r == -ENOPKG) {
                         log_error_errno(r, "Could not find a suitable file system or partition table in image: %s", arg_image);
 

src/shared/dissect-image.c

@@ -174,7 +174,8 @@ int dissect_image(int fd, const void *root_hash, size_t root_hash_size, DissectI
         if (!m)
                 return -ENOMEM;
 
-        if ((flags & DISSECT_IMAGE_GPT_ONLY) == 0) {
+        if (!(flags & DISSECT_IMAGE_GPT_ONLY) &&
+            (flags & DISSECT_IMAGE_REQUIRE_ROOT)) {
                 const char *usage = NULL;
 
                 (void) blkid_probe_lookup_value(b, "USAGE", &usage, NULL);
@@ -490,7 +491,7 @@ int dissect_image(int fd, const void *root_hash, size_t root_hash_size, DissectI
                  * either, then check if there's a single generic one, and use that. */
 
                 if (m->partitions[PARTITION_ROOT_VERITY].found)
-                        return -ENXIO;
+                        return -EADDRNOTAVAIL;
 
                 if (m->partitions[PARTITION_ROOT_SECONDARY].found) {
                         m->partitions[PARTITION_ROOT] = m->partitions[PARTITION_ROOT_SECONDARY];
@@ -499,8 +500,19 @@ int dissect_image(int fd, const void *root_hash, size_t root_hash_size, DissectI
                         m->partitions[PARTITION_ROOT_VERITY] = m->partitions[PARTITION_ROOT_SECONDARY_VERITY];
                         zero(m->partitions[PARTITION_ROOT_SECONDARY_VERITY]);
 
-                } else if (generic_node && !root_hash) {
+                } else if (flags & DISSECT_IMAGE_REQUIRE_ROOT) {
 
+                        /* If the root has was set, then we won't fallback to a generic node, because the root hash
+                         * decides */
+                        if (root_hash)
+                                return -EADDRNOTAVAIL;
+
+                        /* If we didn't find a generic node, then we can't fix this up either */
+                        if (!generic_node)
+                                return -ENXIO;
+
+                        /* If we didn't find a properly marked root partition, but we did find a single suitable
+                         * generic Linux partition, then use this as root partition, if the caller asked for it. */
                         if (multiple_generic)
                                 return -ENOTUNIQ;
 
@@ -514,14 +526,11 @@ int dissect_image(int fd, const void *root_hash, size_t root_hash_size, DissectI
                         };
 
                         generic_node = NULL;
-                } else
-                        return -ENXIO;
+                }
         }
 
-        assert(m->partitions[PARTITION_ROOT].found);
-
         if (root_hash) {
-                if (!m->partitions[PARTITION_ROOT_VERITY].found)
+                if (!m->partitions[PARTITION_ROOT_VERITY].found || !m->partitions[PARTITION_ROOT].found)
                         return -EADDRNOTAVAIL;
 
                 /* If we found the primary root with the hash, then we definitely want to suppress any secondary root

src/shared/dissect-image.h

@@ -69,6 +69,7 @@ typedef enum DissectImageFlags {
                                     DISSECT_IMAGE_DISCARD |
                                     DISSECT_IMAGE_DISCARD_ON_CRYPTO,
         DISSECT_IMAGE_GPT_ONLY = 16,         /* Only recognize images with GPT partition tables */
+        DISSECT_IMAGE_REQUIRE_ROOT = 32,     /* Don't accept disks without root partition */
 } DissectImageFlags;
 
 struct DissectedImage {

src/test/test-dissect-image.c

@@ -43,7 +43,7 @@ int main(int argc, char *argv[]) {
                 return EXIT_FAILURE;
         }
 
-        r = dissect_image(d->fd, NULL, 0, 0, &m);
+        r = dissect_image(d->fd, NULL, 0, DISSECT_IMAGE_REQUIRE_ROOT, &m);
         if (r < 0) {
                 log_error_errno(r, "Failed to dissect image: %m");
                 return EXIT_FAILURE;