From 5e25f36e5e23fdd759cbc87fb3dad61e7742da10 Mon Sep 17 00:00:00 2001
From: AprilNEA <github@sku.moe>
Date: Fri, 17 Jul 2026 13:37:54 +0800
Subject: [PATCH] overlay: don't override a configured index mount option
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NewSnapshotter auto-appends "index=off" unless the configured
mount_options already carry an index option — but the check used
hasOption(..., "index", false), which compares against the literal
string "index". No valid mount option ever equals "index" (it is
always valued, "index=on"/"index=off"), so the append happened
unconditionally whenever the kernel exposes the overlay index
parameter.

Mount options are last-wins in the kernel, so a user-configured
"index=on" was silently overridden. Worse, configuring
mount_options = ["index=on", "nfs_export=on"] (the documented way to
make overlay mounts NFS-exportable) produced
"index=on,nfs_export=on,index=off", which the kernel rejects with
EINVAL (nfs_export=on conflicts with an explicit index=off) — breaking
every snapshot mount including image unpack.

Make hasOption match the key of a "key[=value]" option so callers need
not care whether the option takes a value, and add a regression test.

Signed-off-by: AprilNEA <github@sku.moe>
---
 plugins/snapshots/overlay/overlay.go      | 19 +++++++++++--------
 plugins/snapshots/overlay/overlay_test.go | 23 +++++++++++++++++++++++
 2 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/plugins/snapshots/overlay/overlay.go b/plugins/snapshots/overlay/overlay.go
index 80535ba823adb..c56ffe5bab4ba 100644
--- a/plugins/snapshots/overlay/overlay.go
+++ b/plugins/snapshots/overlay/overlay.go
@@ -147,7 +147,7 @@ func NewSnapshotter(root string, opts ...Opt) (snapshots.Snapshotter, error) {
 		return nil, err
 	}
 
-	if !hasOption(config.mountOptions, "userxattr", false) {
+	if !hasOption(config.mountOptions, "userxattr") {
 		// figure out whether "userxattr" option is recognized by the kernel && needed
 		userxattr, err := overlayutils.NeedsUserXAttr(root)
 		if err != nil {
@@ -158,7 +158,9 @@ func NewSnapshotter(root string, opts ...Opt) (snapshots.Snapshotter, error) {
 		}
 	}
 
-	if !hasOption(config.mountOptions, "index", false) && supportsIndex() {
+	// Mount options are last-wins in the kernel, so appending "index=off"
+	// after a configured "index=on" would silently override it.
+	if !hasOption(config.mountOptions, "index") && supportsIndex() {
 		config.mountOptions = append(config.mountOptions, "index=off")
 	}
 
@@ -173,13 +175,14 @@ func NewSnapshotter(root string, opts ...Opt) (snapshots.Snapshotter, error) {
 	}, nil
 }
 
-func hasOption(options []string, key string, hasValue bool) bool {
+// hasOption reports whether the "key" option is present in options, either
+// as a bare flag ("userxattr") or in "key=value" form ("index=on").
+func hasOption(options []string, key string) bool {
 	for _, option := range options {
-		if hasValue {
-			if strings.HasPrefix(option, key) && len(option) > len(key) && option[len(key)] == '=' {
-				return true
-			}
-		} else if option == key {
+		if option == key {
+			return true
+		}
+		if optionKey, _, ok := strings.Cut(option, "="); ok && optionKey == key {
 			return true
 		}
 	}
diff --git a/plugins/snapshots/overlay/overlay_test.go b/plugins/snapshots/overlay/overlay_test.go
index d5fe82b3905bd..440dff90e99bf 100644
--- a/plugins/snapshots/overlay/overlay_test.go
+++ b/plugins/snapshots/overlay/overlay_test.go
@@ -48,6 +48,29 @@ func newSnapshotterWithOpts(opts ...Opt) testsuite.SnapshotterFunc {
 	}
 }
 
+// NewSnapshotter must not auto-append "index=off" when the configuration
+// already carries an index option: mount options are last-wins in the
+// kernel, so the appended "index=off" would silently override a configured
+// "index=on" — and combined with "nfs_export=on" it makes every overlay
+// mount fail with EINVAL (nfs_export=on conflicts with an explicit
+// index=off).
+func TestOverlayConfiguredIndexNotOverridden(t *testing.T) {
+	if !supportsIndex() {
+		t.Skip("kernel does not expose the overlay index parameter")
+	}
+	sn, err := NewSnapshotter(t.TempDir(), WithMountOptions([]string{"index=on", "nfs_export=on"}))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer sn.Close()
+	options := sn.(*snapshotter).options
+	for _, opt := range options {
+		if opt == "index=off" {
+			t.Fatalf("configured index option overridden by auto-appended index=off (options: %v)", options)
+		}
+	}
+}
+
 func TestOverlay(t *testing.T) {
 	testutil.RequiresRoot(t)
 	optTestCases := map[string][]Opt{