From aed9d8ce9a80f35dda2e44290d14e6ed5a627e30 Mon Sep 17 00:00:00 2001
From: Nicolas Lacasse <nlacasse@google.com>
Date: Thu, 4 Feb 2021 11:45:02 -0800
Subject: Call kernfs.syntheticDir.InitRefs() on creation.

PiperOrigin-RevId: 355675900
---
 pkg/sentry/fsimpl/kernfs/synthetic_directory.go | 11 +++------
 test/e2e/integration_test.go                    | 33 +++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 7 deletions(-)

diff --git a/pkg/sentry/fsimpl/kernfs/synthetic_directory.go b/pkg/sentry/fsimpl/kernfs/synthetic_directory.go
index 463d77d79..11694c392 100644
--- a/pkg/sentry/fsimpl/kernfs/synthetic_directory.go
+++ b/pkg/sentry/fsimpl/kernfs/synthetic_directory.go
@@ -42,19 +42,16 @@ type syntheticDirectory struct {
 var _ Inode = (*syntheticDirectory)(nil)
 
 func newSyntheticDirectory(ctx context.Context, creds *auth.Credentials, perm linux.FileMode) Inode {
-	inode := &syntheticDirectory{}
-	inode.Init(ctx, creds, 0 /* devMajor */, 0 /* devMinor */, 0 /* ino */, perm)
-	return inode
-}
-
-func (dir *syntheticDirectory) Init(ctx context.Context, creds *auth.Credentials, devMajor, devMinor uint32, ino uint64, perm linux.FileMode) {
 	if perm&^linux.PermissionsMask != 0 {
 		panic(fmt.Sprintf("perm contains non-permission bits: %#o", perm))
 	}
-	dir.InodeAttrs.Init(ctx, creds, devMajor, devMinor, ino, linux.S_IFDIR|perm)
+	dir := &syntheticDirectory{}
+	dir.InitRefs()
+	dir.InodeAttrs.Init(ctx, creds, 0 /* devMajor */, 0 /* devMinor */, 0 /* ino */, linux.S_IFDIR|perm)
 	dir.OrderedChildren.Init(OrderedChildrenOptions{
 		Writable: true,
 	})
+	return dir
 }
 
 // Open implements Inode.Open.
diff --git a/test/e2e/integration_test.go b/test/e2e/integration_test.go
index b798cf132..49cd74887 100644
--- a/test/e2e/integration_test.go
+++ b/test/e2e/integration_test.go
@@ -430,6 +430,39 @@ func TestTmpMount(t *testing.T) {
 	}
 }
 
+// TestSyntheticDirs checks that submounts can be created inside a readonly
+// mount even if the target path does not exist.
+func TestSyntheticDirs(t *testing.T) {
+	ctx := context.Background()
+	d := dockerutil.MakeContainer(ctx, t)
+	defer d.CleanUp(ctx)
+
+	opts := dockerutil.RunOpts{
+		Image: "basic/alpine",
+		// Make the root read-only to force use of synthetic dirs
+		// inside the root gofer mount.
+		ReadOnly: true,
+		Mounts: []mount.Mount{
+			// Mount inside read-only gofer-backed root.
+			{
+				Type:   mount.TypeTmpfs,
+				Target: "/foo/bar/baz",
+			},
+			// Mount inside sysfs, which always uses synthetic dirs
+			// for submounts.
+			{
+				Type:   mount.TypeTmpfs,
+				Target: "/sys/foo/bar/baz",
+			},
+		},
+	}
+	// Make sure the directories exist.
+	if _, err := d.Run(ctx, opts, "ls", "/foo/bar/baz", "/sys/foo/bar/baz"); err != nil {
+		t.Fatalf("docker run failed: %v", err)
+	}
+
+}
+
 // TestHostOverlayfsCopyUp tests that the --overlayfs-stale-read option causes
 // runsc to hide the incoherence of FDs opened before and after overlayfs
 // copy-up on the host.
-- 
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