Fix uses of RootFromContext.

RootFromContext can return a dirent with reference taken, or nil. We must call
DecRef if (and only if) a real dirent is returned.

PiperOrigin-RevId: 242965515
Change-Id: Ie2b7b4cb19ee09b6ccf788b71f3fd7efcdf35a11

pkg/sentry/fs/copy_up.go

@@ -188,11 +188,15 @@ func copyUpLocked(ctx context.Context, parent *Dirent, next *Dirent) error {
 
 	var childUpperInode *Inode
 	parentUpper := parent.Inode.overlay.upper
+	root := RootFromContext(ctx)
+	if root != nil {
+		defer root.DecRef()
+	}
 
 	// Create the file in the upper filesystem and get an Inode for it.
 	switch next.Inode.StableAttr.Type {
 	case RegularFile:
-		childFile, err := parentUpper.Create(ctx, RootFromContext(ctx), next.name, FileFlags{Read: true, Write: true}, attrs.Perms)
+		childFile, err := parentUpper.Create(ctx, root, next.name, FileFlags{Read: true, Write: true}, attrs.Perms)
 		if err != nil {
 			log.Warningf("copy up failed to create file: %v", err)
 			return syserror.EIO
@@ -201,7 +205,7 @@ func copyUpLocked(ctx context.Context, parent *Dirent, next *Dirent) error {
 		childUpperInode = childFile.Dirent.Inode
 
 	case Directory:
-		if err := parentUpper.CreateDirectory(ctx, RootFromContext(ctx), next.name, attrs.Perms); err != nil {
+		if err := parentUpper.CreateDirectory(ctx, root, next.name, attrs.Perms); err != nil {
 			log.Warningf("copy up failed to create directory: %v", err)
 			return syserror.EIO
 		}
@@ -221,7 +225,7 @@ func copyUpLocked(ctx context.Context, parent *Dirent, next *Dirent) error {
 			log.Warningf("copy up failed to read symlink value: %v", err)
 			return syserror.EIO
 		}
-		if err := parentUpper.CreateLink(ctx, RootFromContext(ctx), link, next.name); err != nil {
+		if err := parentUpper.CreateLink(ctx, root, link, next.name); err != nil {
 			log.Warningf("copy up failed to create symlink: %v", err)
 			return syserror.EIO
 		}

pkg/sentry/fs/file.go

@@ -410,7 +410,11 @@ func (f *File) ConfigureMMap(ctx context.Context, opts *memmap.MMapOpts) error {
 
 // MappedName implements memmap.MappingIdentity.MappedName.
 func (f *File) MappedName(ctx context.Context) string {
-	name, _ := f.Dirent.FullName(RootFromContext(ctx))
+	root := RootFromContext(ctx)
+	if root != nil {
+		defer root.DecRef()
+	}
+	name, _ := f.Dirent.FullName(root)
 	return name
 }
 

pkg/sentry/fs/file_overlay.go

@@ -169,7 +169,9 @@ func (f *overlayFileOperations) Seek(ctx context.Context, file *File, whence See
 // Readdir implements FileOperations.Readdir.
 func (f *overlayFileOperations) Readdir(ctx context.Context, file *File, serializer DentrySerializer) (int64, error) {
 	root := RootFromContext(ctx)
-	defer root.DecRef()
+	if root != nil {
+		defer root.DecRef()
+	}
 	dirCtx := &DirCtx{
 		Serializer: serializer,
 		DirCursor:  &f.dirCursor,
@@ -440,7 +442,11 @@ func (omi *overlayMappingIdentity) InodeID() uint64 {
 
 // MappedName implements MappingIdentity.MappedName.
 func (omi *overlayMappingIdentity) MappedName(ctx context.Context) string {
-	name, _ := omi.overlayFile.Dirent.FullName(RootFromContext(ctx))
+	root := RootFromContext(ctx)
+	if root != nil {
+		defer root.DecRef()
+	}
+	name, _ := omi.overlayFile.Dirent.FullName(root)
 	return name
 }
 

pkg/sentry/fs/fsutil/file.go

@@ -277,7 +277,9 @@ func (sdfo *StaticDirFileOperations) IterateDir(ctx context.Context, dirCtx *fs.
 // Readdir implements fs.FileOperations.Readdir.
 func (sdfo *StaticDirFileOperations) Readdir(ctx context.Context, file *fs.File, serializer fs.DentrySerializer) (int64, error) {
 	root := fs.RootFromContext(ctx)
-	defer root.DecRef()
+	if root != nil {
+		defer root.DecRef()
+	}
 	dirCtx := &fs.DirCtx{
 		Serializer: serializer,
 		DirCursor:  &sdfo.dirCursor,

pkg/sentry/fs/gofer/file.go

@@ -120,7 +120,9 @@ func (f *fileOperations) Release() {
 // Readdir implements fs.FileOperations.Readdir.
 func (f *fileOperations) Readdir(ctx context.Context, file *fs.File, serializer fs.DentrySerializer) (int64, error) {
 	root := fs.RootFromContext(ctx)
-	defer root.DecRef()
+	if root != nil {
+		defer root.DecRef()
+	}
 
 	dirCtx := &fs.DirCtx{
 		Serializer: serializer,

pkg/sentry/fs/host/file.go

@@ -166,7 +166,9 @@ func (f *fileOperations) Readiness(mask waiter.EventMask) waiter.EventMask {
 // Readdir implements fs.FileOperations.Readdir.
 func (f *fileOperations) Readdir(ctx context.Context, file *fs.File, serializer fs.DentrySerializer) (int64, error) {
 	root := fs.RootFromContext(ctx)
-	defer root.DecRef()
+	if root != nil {
+		defer root.DecRef()
+	}
 	dirCtx := &fs.DirCtx{
 		Serializer: serializer,
 		DirCursor:  &f.dirCursor,

pkg/sentry/fs/proc/fds.go

@@ -119,7 +119,9 @@ func (f *fd) GetFile(context.Context, *fs.Dirent, fs.FileFlags) (*fs.File, error
 // Readlink returns the current target.
 func (f *fd) Readlink(ctx context.Context, _ *fs.Inode) (string, error) {
 	root := fs.RootFromContext(ctx)
-	defer root.DecRef()
+	if root != nil {
+		defer root.DecRef()
+	}
 	n, _ := f.Dirent.FullName(root)
 	return n, nil
 }

pkg/sentry/fs/proc/proc.go

@@ -210,7 +210,9 @@ func (rpf *rootProcFile) Readdir(ctx context.Context, file *fs.File, ser fs.Dent
 
 	// Add dot and dotdot.
 	root := fs.RootFromContext(ctx)
-	defer root.DecRef()
+	if root != nil {
+		defer root.DecRef()
+	}
 	dot, dotdot := file.Dirent.GetDotAttrs(root)
 	names = append(names, ".", "..")
 	m["."] = dot

pkg/sentry/fs/proc/task.go

@@ -162,7 +162,9 @@ func (f *subtasksFile) Readdir(ctx context.Context, file *fs.File, ser fs.Dentry
 	if offset == 0 {
 		// Serialize "." and "..".
 		root := fs.RootFromContext(ctx)
-		defer root.DecRef()
+		if root != nil {
+			defer root.DecRef()
+		}
 		dot, dotdot := file.Dirent.GetDotAttrs(root)
 		if err := dirCtx.DirEmit(".", dot); err != nil {
 			return offset, err

pkg/sentry/fs/ramfs/dir.go

@@ -417,7 +417,9 @@ func (dfo *dirFileOperations) IterateDir(ctx context.Context, dirCtx *fs.DirCtx,
 // Readdir implements FileOperations.Readdir.
 func (dfo *dirFileOperations) Readdir(ctx context.Context, file *fs.File, serializer fs.DentrySerializer) (int64, error) {
 	root := fs.RootFromContext(ctx)
-	defer root.DecRef()
+	if root != nil {
+		defer root.DecRef()
+	}
 	dirCtx := &fs.DirCtx{
 		Serializer: serializer,
 		DirCursor:  &dfo.dirCursor,

pkg/sentry/fs/tty/dir.go

@@ -315,7 +315,9 @@ func (df *dirFileOperations) IterateDir(ctx context.Context, dirCtx *fs.DirCtx,
 // Readdir implements FileOperations.Readdir.
 func (df *dirFileOperations) Readdir(ctx context.Context, file *fs.File, serializer fs.DentrySerializer) (int64, error) {
 	root := fs.RootFromContext(ctx)
-	defer root.DecRef()
+	if root != nil {
+		defer root.DecRef()
+	}
 	dirCtx := &fs.DirCtx{
 		Serializer: serializer,
 		DirCursor:  &df.dirCursor,

pkg/sentry/kernel/kernel.go

@@ -676,6 +676,10 @@ func (k *Kernel) CreateProcess(args CreateProcessArgs) (*ThreadGroup, ThreadID,
 	root := args.Root
 	if root == nil {
 		root = fs.RootFromContext(ctx)
+		// Is the root STILL nil?
+		if root == nil {
+			return nil, 0, fmt.Errorf("CreateProcessArgs.Root was not provided, and failed to get root from context")
+		}
 	}
 	defer root.DecRef()
 	args.Root = nil