libfstools/mount: Support latest overlayfs versions

The latest overlayfs revisions (v23 and above) require an additional directory
to be given in the options.

Since we need to support both pre-v23 and post-v23 versions, take a lazy
approach and try to first mount without a workdir, and if it fails, try with
it.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

diff --git a/libfstools/mount.c b/libfstools/mount.c
index efcfcd8bcf15283a958e83f2cf3caa0e072af316..903043f940d5b938d0ada1c539b842ddddff7dd9 100644 (file)
--- a/libfstools/mount.c
+++ b/libfstools/mount.c
@@ -90,11 +90,35 @@ fopivot(char *rw_root, char *ro_root)
        }
 
        snprintf(overlay, sizeof(overlay), "overlayfs:%s", rw_root);
-       snprintf(lowerdir, sizeof(lowerdir), "lowerdir=/,upperdir=%s", rw_root);
 
+       /*
+        * First, try to mount without a workdir, for overlayfs v22 and before.
+        * If it fails, it means that we are probably using a v23 and
+        * later versions that require a workdir
+        */
+       snprintf(lowerdir, sizeof(lowerdir), "lowerdir=/,upperdir=%s", rw_root);
        if (mount(overlay, "/mnt", "overlayfs", MS_NOATIME, lowerdir)) {
-               fprintf(stderr, "mount failed: %s\n", strerror(errno));
-               return -1;
+               char upperdir[64], workdir[64];
+
+               snprintf(upperdir, sizeof(upperdir), "%s/upper", rw_root);
+               snprintf(workdir, sizeof(workdir), "%s/work", rw_root);
+               snprintf(lowerdir, sizeof(lowerdir), "lowerdir=/,upperdir=%s,workdir=%s",
+                        upperdir, workdir);
+
+               /*
+                * Overlay FS v23 and later requires both a upper and
+                * a work directory, both on the same filesystem, but
+                * not part of the same subtree.
+                * We can't really deal with these constraints without
+                * creating two new subdirectories in /overlay.
+                */
+               mkdir(upperdir, 0755);
+               mkdir(workdir, 0755);
+
+               if (mount(overlay, "/mnt", "overlayfs", MS_NOATIME, lowerdir)) {
+                       fprintf(stderr, "mount failed: %s, options %s\n", strerror(errno), lowerdir);
+                       return -1;
+               }
        }
 
        return pivot("/mnt", ro_root);