overlay: drop relLowers workaround

commit 7c5964d changed the way the
mountFrom program works by always using the /proc/self/fd/%d path for
the lower dirs.  Hence there is no gain to pass a path relative to the
storage, but we can just pass the absolute path as the resulting file
path length is the same and won't affect how many layers we can
specify.

This fixes idmapped mounts when the total length of lower layers is >
4096 and it also solves the problem of using layers from additional
stores.

Closes: #1410

Signed-off-by: Giuseppe Scrivano <[email protected]>

drivers/overlay/overlay.go

@@ -1378,28 +1378,9 @@ func (d *Driver) get(id string, disableShifting bool, options graphdriver.MountO
 		return "", errors.New("max depth exceeded")
 	}
 
-	// absLowers is the list of lowers as absolute paths, which works well with additional stores.
+	// absLowers is the list of lowers as absolute paths.
 	absLowers := []string{}
-	// relLowers is the list of lowers as paths relative to the driver's home directory.  If
-	// additional stores are being used, some of these will still be absolute paths.
-	relLowers := []string{}
 
-	// Check if $link/../diff{1-*} exist.  If they do, add them, in order, as the front of the lowers
-	// lists that we're building.  "diff" itself is the upper, so it won't be in the lists.
-	link, err := os.ReadFile(path.Join(dir, "link"))
-	if err != nil {
-		if !os.IsNotExist(err) {
-			return "", err
-		}
-		logrus.Warnf("Can't read parent link %q because it does not exist. Going through storage to recreate the missing links.", path.Join(dir, "link"))
-		if err := d.recreateSymlinks(); err != nil {
-			return "", fmt.Errorf("recreating the links: %w", err)
-		}
-		link, err = os.ReadFile(path.Join(dir, "link"))
-		if err != nil {
-			return "", err
-		}
-	}
 	diffN := 1
 	perms := defaultPerms
 	if d.options.forceMask != nil {
@@ -1413,7 +1394,6 @@ func (d *Driver) get(id string, disableShifting bool, options graphdriver.MountO
 	}
 	for err == nil {
 		absLowers = append(absLowers, filepath.Join(dir, nameWithSuffix("diff", diffN)))
-		relLowers = append(relLowers, dumbJoin(linkDir, string(link), "..", nameWithSuffix("diff", diffN)))
 		diffN++
 		st, err = os.Stat(filepath.Join(dir, nameWithSuffix("diff", diffN)))
 		if err == nil && !permsKnown {
@@ -1463,20 +1443,17 @@ func (d *Driver) get(id string, disableShifting bool, options graphdriver.MountO
 			lower = newpath
 		}
 		absLowers = append(absLowers, lower)
-		relLowers = append(relLowers, l)
 		diffN = 1
 		_, err = os.Stat(dumbJoin(lower, "..", nameWithSuffix("diff", diffN)))
 		for err == nil {
 			absLowers = append(absLowers, dumbJoin(lower, "..", nameWithSuffix("diff", diffN)))
-			relLowers = append(relLowers, dumbJoin(l, "..", nameWithSuffix("diff", diffN)))
 			diffN++
 			_, err = os.Stat(dumbJoin(lower, "..", nameWithSuffix("diff", diffN)))
 		}
 	}
 
 	if len(absLowers) == 0 {
 		absLowers = append(absLowers, path.Join(dir, "empty"))
-		relLowers = append(relLowers, path.Join(id, "empty"))
 	}
 	// user namespace requires this to move a directory from lower to upper.
 	rootUID, rootGID, err := idtools.GetRootUIDGID(d.uidMaps, d.gidMaps)
@@ -1610,17 +1587,16 @@ func (d *Driver) get(id string, disableShifting bool, options graphdriver.MountO
 			return nil
 		}
 	} else if len(mountData) >= pageSize {
-		// Use (possibly) relative paths and mountFrom when the mount data has exceeded
-		// the page size. The mount syscall fails if the mount data cannot
-		// fit within a page and relative links make the mount data much
+		// Use mountFrom when the mount data has exceeded the page size. The mount syscall fails if
+		// the mount data cannot fit within a page and relative links make the mount data much
 		// smaller at the expense of requiring a fork exec to chdir().
 
 		workdir = path.Join(id, "work")
 		if readWrite {
 			diffDir := path.Join(id, "diff")
-			opts = fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", strings.Join(relLowers, ":"), diffDir, workdir)
+			opts = fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", strings.Join(absLowers, ":"), diffDir, workdir)
 		} else {
-			opts = fmt.Sprintf("lowerdir=%s:%s", diffDir, strings.Join(relLowers, ":"))
+			opts = fmt.Sprintf("lowerdir=%s:%s", diffDir, strings.Join(absLowers, ":"))
 		}
 		if len(optsList) > 0 {
 			opts = fmt.Sprintf("%s,%s", opts, strings.Join(optsList, ","))