sysupdate: Support volatile-root for finding the root partition

The existing logic can't find the root device in scenarios where
the root has been replaced with an overlay. We support looking
at "/run/systemd/volatile-root" to find the original root, similar
to what systemd-repart and gpt-auto-generator do.

diff --git a/src/sysupdate/sysupdate-resource.c b/src/sysupdate/sysupdate-resource.c
index bf521980e26638bd4e8731241eed06dc0777f1bd..2814fdb6fa307a77eb52c602f3e3cb2f76bbc041 100644 (file)
--- a/src/sysupdate/sysupdate-resource.c
+++ b/src/sysupdate/sysupdate-resource.c
@@ -8,6 +8,7 @@
 #include "blockdev-util.h"
 #include "chase-symlinks.h"
 #include "device-util.h"
+#include "devnum-util.h"
 #include "dirent-util.h"
 #include "env-util.h"
 #include "fd-util.h"
@@ -525,10 +526,14 @@ int resource_resolve_path(
         assert(rr);
 
         if (rr->path_auto) {
+                struct stat orig_root_stats;
 
-                /* NB: we don't actually check the backing device of the root fs "/", but of "/usr", in order
-                 * to support environments where the root fs is a tmpfs, and the OS itself placed exclusively
-                 * in /usr/. */
+                /* NB: If the root mount has been replaced by some form of volatile file system (overlayfs),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
+                 * follow that link here. If that doesn't exist, we check the backing device of "/usr". We
+                 * don't actually check the backing device of the root fs "/", in order to support
+                 * environments where the root fs is a tmpfs, and the OS itself placed exclusively in
+                 * /usr/. */
 
                 if (rr->type != RESOURCE_PARTITION)
                         return log_error_errno(SYNTHETIC_ERRNO(EOPNOTSUPP),
@@ -546,7 +551,16 @@ int resource_resolve_path(
                         return log_error_errno(SYNTHETIC_ERRNO(EPERM),
                                                "Block device is not allowed when using --root= mode.");
 
-                r = get_block_device_harder("/usr/", &d);
+                r = stat("/run/systemd/volatile-root", &orig_root_stats);
+                if (r < 0) {
+                        if (errno == -ENOENT) /* volatile-root not found */
+                                r = get_block_device_harder("/usr/", &d);
+                        else
+                                return log_error_errno(r, "Failed to stat /run/systemd/volatile-root: %m");
+                } else if (!S_ISBLK(orig_root_stats.st_mode)) /* symlink was present but not block device */
+                        return log_error_errno(SYNTHETIC_ERRNO(ENOTBLK), "/run/systemd/volatile-root is not linked to a block device.");
+                else /* symlink was present and a block device */
+                        d = orig_root_stats.st_rdev;
 
         } else if (rr->type == RESOURCE_PARTITION) {
                 _cleanup_close_ int fd = -1, real_fd = -1;