5 files changed, 107 insertions, 1 deletions
diff --git a/pkg/sentry/fs/ext/BUILD b/pkg/sentry/fs/ext/BUILD
index 7028b3f78..e660e545d 100644
--- a/pkg/sentry/fs/ext/BUILD
+++ b/pkg/sentry/fs/ext/BUILD
@@ -44,6 +44,7 @@ go_test(
         "//pkg/sentry/context",
         "//pkg/sentry/context/contexttest",
         "//pkg/sentry/fs/ext/disklayout",
+        "//pkg/sentry/kernel/auth",
         "//pkg/sentry/vfs",
         "//runsc/test/testutil",
         "@com_github_google_go-cmp//cmp:go_default_library",
diff --git a/pkg/sentry/fs/ext/dentry.go b/pkg/sentry/fs/ext/dentry.go
index 07be99a84..054fb42b6 100644
--- a/pkg/sentry/fs/ext/dentry.go
+++ b/pkg/sentry/fs/ext/dentry.go
@@ -31,6 +31,15 @@ type dentry struct {
 // Compiles only if dentry implements vfs.DentryImpl.
 var _ vfs.DentryImpl = (*dentry)(nil)
 
+// newDentry is the dentry constructor.
+func newDentry(in *inode) *dentry {
+	d := &dentry{
+		inode: in,
+	}
+	d.vfsd.Init(d)
+	return d
+}
+
 // IncRef implements vfs.DentryImpl.IncRef.
 func (d *dentry) IncRef(vfsfs *vfs.Filesystem) {
 	d.inode.incRef()
diff --git a/pkg/sentry/fs/ext/disklayout/inode.go b/pkg/sentry/fs/ext/disklayout/inode.go
index 9ab9a4988..88ae913f5 100644
--- a/pkg/sentry/fs/ext/disklayout/inode.go
+++ b/pkg/sentry/fs/ext/disklayout/inode.go
@@ -20,6 +20,12 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/kernel/time"
 )
 
+// Special inodes. See https://www.kernel.org/doc/html/latest/filesystems/ext4/overview.html#special-inodes.
+const (
+	// RootDirInode is the inode number of the root directory inode.
+	RootDirInode = 2
+)
+
 // The Inode interface must be implemented by structs representing ext inodes.
 // The inode stores all the metadata pertaining to the file (except for the
 // file name which is held by the directory entry). It does NOT expose all
diff --git a/pkg/sentry/fs/ext/ext.go b/pkg/sentry/fs/ext/ext.go
index c8b554f4d..2604169de 100644
--- a/pkg/sentry/fs/ext/ext.go
+++ b/pkg/sentry/fs/ext/ext.go
@@ -131,7 +131,12 @@ func (fstype filesystemType) NewFilesystem(ctx context.Context, creds *auth.Cred
 		return nil, nil, err
 	}
 
-	return &fs.vfsfs, nil, nil
+	rootInode, err := fs.getOrCreateInode(disklayout.RootDirInode)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return &fs.vfsfs, &newDentry(rootInode).vfsd, nil
 }
 
 // getOrCreateInode gets the inode corresponding to the inode number passed in.
diff --git a/pkg/sentry/fs/ext/ext_test.go b/pkg/sentry/fs/ext/ext_test.go
index f92e77450..ee7f7907c 100644
--- a/pkg/sentry/fs/ext/ext_test.go
+++ b/pkg/sentry/fs/ext/ext_test.go
@@ -25,6 +25,7 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/context"
 	"gvisor.dev/gvisor/pkg/sentry/context/contexttest"
 	"gvisor.dev/gvisor/pkg/sentry/fs/ext/disklayout"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 
 	"gvisor.dev/gvisor/runsc/test/testutil"
@@ -82,6 +83,90 @@ func setUp(t *testing.T, imagePath string) (context.Context, *vfs.Filesystem, *v
 	return mockCtx, fs, d, tearDown, nil
 }
 
+// TestRootDir tests that the root directory inode is correctly initialized and
+// returned from setUp.
+func TestRootDir(t *testing.T) {
+	type inodeProps struct {
+		Mode      linux.FileMode
+		UID       auth.KUID
+		GID       auth.KGID
+		Size      uint64
+		InodeSize uint16
+		Links     uint16
+		Flags     disklayout.InodeFlags
+	}
+
+	type rootDirTest struct {
+		name      string
+		image     string
+		wantInode inodeProps
+	}
+
+	tests := []rootDirTest{
+		{
+			name:  "ext4 root dir",
+			image: ext4ImagePath,
+			wantInode: inodeProps{
+				Mode:      linux.ModeDirectory | 0755,
+				Size:      0x400,
+				InodeSize: 0x80,
+				Links:     3,
+				Flags:     disklayout.InodeFlags{Extents: true},
+			},
+		},
+		{
+			name:  "ext3 root dir",
+			image: ext3ImagePath,
+			wantInode: inodeProps{
+				Mode:      linux.ModeDirectory | 0755,
+				Size:      0x400,
+				InodeSize: 0x80,
+				Links:     3,
+			},
+		},
+		{
+			name:  "ext2 root dir",
+			image: ext2ImagePath,
+			wantInode: inodeProps{
+				Mode:      linux.ModeDirectory | 0755,
+				Size:      0x400,
+				InodeSize: 0x80,
+				Links:     3,
+			},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			_, _, vfsd, tearDown, err := setUp(t, test.image)
+			if err != nil {
+				t.Fatalf("setUp failed: %v", err)
+			}
+			defer tearDown()
+
+			d, ok := vfsd.Impl().(*dentry)
+			if !ok {
+				t.Fatalf("ext dentry of incorrect type: %T", vfsd.Impl())
+			}
+
+			// Offload inode contents into local structs for comparison.
+			gotInode := inodeProps{
+				Mode:      d.inode.diskInode.Mode(),
+				UID:       d.inode.diskInode.UID(),
+				GID:       d.inode.diskInode.GID(),
+				Size:      d.inode.diskInode.Size(),
+				InodeSize: d.inode.diskInode.InodeSize(),
+				Links:     d.inode.diskInode.LinksCount(),
+				Flags:     d.inode.diskInode.Flags(),
+			}
+
+			if diff := cmp.Diff(gotInode, test.wantInode); diff != "" {
+				t.Errorf("inode mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
 // TestFilesystemInit tests that the filesystem superblock and block group
 // descriptors are correctly read in and initialized.
 func TestFilesystemInit(t *testing.T) {