gpt-auto-generator: Use volatile-root by default and automatic logic …

…as fallback

Previously volatile-root was only checked if "/" wasn't backed by a
block device, but the block device isn't necessarily original root block
device (ex: if the rootfs is copied to a ext4 fs backed by zram in the
initramfs), so we always want volatile-root checked.

So shuffle the code around so volatile-root is checked first and
fallback to the automatic logic.

Fix #20557

man/systemd-gpt-auto-generator.xml

@@ -228,6 +228,11 @@
       For more information, see <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
     </para>
 
+    <para>The root partition can be specified by symlinking <filename>/run/systemd/volatile-root</filename>
+    to <filename>/dev/block/$major:$minor</filename>. This is especially useful if the root mount has been
+    replaced by some form of volatile file system (overlayfs).
+    </para>
+
     <para>Mount and automount units for the EFI System Partition (ESP) are generated on EFI systems. The ESP
     is mounted to <filename>/boot/</filename> (except if an Extended Boot Loader partition exists, see
     below), unless a mount point directory <filename>/efi/</filename> exists, in which case it is mounted

src/gpt-auto-generator/gpt-auto-generator.c

@@ -764,41 +764,36 @@ static int enumerate_partitions(dev_t devnum) {
 }
 
 static int add_mounts(void) {
-        dev_t devno;
+        _cleanup_free_ char *p = NULL;
         int r;
+        dev_t devno;
 
-        r = get_block_device_harder("/", &devno);
-        if (r == -EUCLEAN)
-                return btrfs_log_dev_root(LOG_ERR, r, "root file system");
-        if (r < 0)
-                return log_error_errno(r, "Failed to determine block device of root file system: %m");
-        if (r == 0) { /* Not backed by block device */
-                r = get_block_device_harder("/usr", &devno);
+        /* If the root mount has been replaced by some form of volatile file system (overlayfs), the
+         * original root block device node is symlinked in /run/systemd/volatile-root. Let's read that
+         * here. */
+        r = readlink_malloc("/run/systemd/volatile-root", &p);
+        if (r == -ENOENT) { /* volatile-root not found */
+                r = get_block_device_harder("/", &devno);
                 if (r == -EUCLEAN)
-                        return btrfs_log_dev_root(LOG_ERR, r, "/usr");
+                        return btrfs_log_dev_root(LOG_ERR, r, "root file system");
                 if (r < 0)
-                        return log_error_errno(r, "Failed to determine block device of /usr file system: %m");
-                if (r == 0) {
-                        _cleanup_free_ char *p = NULL;
-                        mode_t m;
-
-                        /* If the root mount has been replaced by some form of volatile file system (overlayfs), the
-                         * original root block device node is symlinked in /run/systemd/volatile-root. Let's read that
-                         * here. */
-                        r = readlink_malloc("/run/systemd/volatile-root", &p);
-                        if (r == -ENOENT) {
-                                log_debug("Neither root nor /usr file system are on a (single) block device.");
-                                return 0;
-                        }
+                        return log_error_errno(r, "Failed to determine block device of root file system: %m");
+                if (r == 0) { /* Not backed by block device */
+                        r = get_block_device_harder("/usr", &devno);
+                        if (r == -EUCLEAN)
+                                return btrfs_log_dev_root(LOG_ERR, r, "/usr");
                         if (r < 0)
-                                return log_error_errno(r, "Failed to read symlink /run/systemd/volatile-root: %m");
-
-                        r = device_path_parse_major_minor(p, &m, &devno);
-                        if (r < 0)
-                                return log_error_errno(r, "Failed to parse major/minor device node: %m");
-                        if (!S_ISBLK(m))
-                                return log_error_errno(SYNTHETIC_ERRNO(ENOTBLK), "Volatile root device is of wrong type.");
+                                return log_error_errno(r, "Failed to determine block device of /usr file system: %m");
                 }
+        { else if (r < 0)
+                return log_error_errno(r, "Failed to read symlink /run/systemd/volatile-root: %m");
+        else {
+                mode_t m;
+                r = device_path_parse_major_minor(p, &m, &devno);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to parse major/minor device node: %m");
+                if (!S_ISBLK(m))
+                        return log_error_errno(SYNTHETIC_ERRNO(ENOTBLK), "Volatile root device is of wrong type.");
         }
 
         return enumerate_partitions(devno);