Close diagnostic and restore coverage gaps

internal/fileops/confined.go

@@ -156,13 +156,17 @@ func setOpenedDirectoryMode(root *os.Root, mode os.FileMode) error {
 }
 
 func createSiblingTemp(parent *os.Root, base string) (*os.File, string, error) {
+	return createSiblingTempWithFlags(parent, base, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0o600)
+}
+
+func createSiblingTempWithFlags(parent *os.Root, base string, flags int, mode os.FileMode) (*os.File, string, error) {
 	for attempt := 0; attempt < 100; attempt++ {
 		var randomBytes [16]byte
 		if _, err := rand.Read(randomBytes[:]); err != nil {
 			return nil, "", fmt.Errorf("generate temporary file name: %w", err)
 		}
 		name := "." + base + ".tmp-" + hex.EncodeToString(randomBytes[:])
-		file, err := parent.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0o600)
+		file, err := parent.OpenFile(name, flags|os.O_CREATE|os.O_EXCL, mode)
 		if errors.Is(err, os.ErrExist) {
 			continue
 		}
@@ -184,28 +188,128 @@ func syncOpenedDirectory(parent *os.Root) error {
 }
 
 // OpenFileConfined opens a file after verifying its parent hierarchy without
-// following symbolic links. Existing symbolic-link and non-regular leaves are
+// mutating through symbolic links. Truncating opens atomically replace the leaf
+// with a newly created sibling, while other creating opens never carry O_CREATE
+// into an existing-leaf open. Existing symbolic-link and non-regular leaves are
 // rejected.
 func OpenFileConfined(path string, flags int, mode os.FileMode) (*os.File, error) {
+	return openFileConfined(path, flags, mode, nil)
+}
+
+func openFileConfined(path string, flags int, mode os.FileMode, beforeOpen func() error) (*os.File, error) {
 	parent, name, err := openConfinedParent(path, false, 0)
 	if err != nil {
 		return nil, err
 	}
 	defer func() { _ = parent.Close() }()
-	if info, err := parent.Lstat(name); err == nil {
-		if info.Mode()&os.ModeSymlink != 0 {
-			return nil, fmt.Errorf("destination file %q is a symbolic link", name)
+
+	if flags&(os.O_CREATE|os.O_EXCL) == os.O_CREATE|os.O_EXCL {
+		if err := runBeforeConfinedOpen(beforeOpen); err != nil {
+			return nil, err
 		}
-		if !info.Mode().IsRegular() {
-			return nil, fmt.Errorf("destination file %q is not a regular file", name)
+		file, err := parent.OpenFile(name, flags, mode)
+		if err != nil {
+			return nil, err
 		}
-	} else if err != nil && !errors.Is(err, os.ErrNotExist) {
-		return nil, fmt.Errorf("inspect destination file %q: %w", name, err)
+		return validateOpenedConfinedFile(parent, name, file)
 	}
-	file, err := parent.OpenFile(name, flags, mode)
+
+	exists, err := validateConfinedLeaf(parent, name)
 	if err != nil {
 		return nil, err
 	}
+	if flags&os.O_TRUNC != 0 {
+		if !exists && flags&os.O_CREATE == 0 {
+			return nil, fmt.Errorf("open destination file %q: %w", name, os.ErrNotExist)
+		}
+		return replaceAndOpenConfinedFile(parent, name, flags, mode, beforeOpen)
+	}
+
+	if err := runBeforeConfinedOpen(beforeOpen); err != nil {
+		return nil, err
+	}
+	if !exists && flags&os.O_CREATE != 0 {
+		file, err := parent.OpenFile(name, flags|os.O_EXCL, mode)
+		if err == nil {
+			return validateOpenedConfinedFile(parent, name, file)
+		}
+		if !errors.Is(err, os.ErrExist) {
+			return nil, err
+		}
+		if _, validationErr := validateConfinedLeaf(parent, name); validationErr != nil {
+			return nil, validationErr
+		}
+	}
+
+	// The leaf now exists, so removing O_CREATE prevents a raced dangling
+	// symlink from creating its target. Opening an existing symlink can expose a
+	// handle, but no caller can mutate through it before the identity check below.
+	file, err := parent.OpenFile(name, flags&^os.O_CREATE, mode)
+	if err != nil {
+		return nil, err
+	}
+	return validateOpenedConfinedFile(parent, name, file)
+}
+
+func validateConfinedLeaf(parent *os.Root, name string) (bool, error) {
+	info, err := parent.Lstat(name)
+	if errors.Is(err, os.ErrNotExist) {
+		return false, nil
+	}
+	if err != nil {
+		return false, fmt.Errorf("inspect destination file %q: %w", name, err)
+	}
+	if info.Mode()&os.ModeSymlink != 0 {
+		return false, fmt.Errorf("destination file %q is a symbolic link", name)
+	}
+	if !info.Mode().IsRegular() {
+		return false, fmt.Errorf("destination file %q is not a regular file", name)
+	}
+	return true, nil
+}
+
+func replaceAndOpenConfinedFile(parent *os.Root, name string, flags int, mode os.FileMode, beforeOpen func() error) (_ *os.File, resultErr error) {
+	temporary, temporaryName, err := createSiblingTempWithFlags(parent, name, flags&^os.O_TRUNC, mode)
+	if err != nil {
+		return nil, fmt.Errorf("create replacement for destination file %q: %w", name, err)
+	}
+	installed := false
+	defer func() {
+		if installed {
+			return
+		}
+		closeErr := temporary.Close()
+		removeErr := parent.Remove(temporaryName)
+		if errors.Is(removeErr, os.ErrNotExist) {
+			removeErr = nil
+		}
+		resultErr = errors.Join(resultErr, closeErr, removeErr)
+	}()
+
+	if err := runBeforeConfinedOpen(beforeOpen); err != nil {
+		return nil, err
+	}
+	// Rename replaces the directory entry itself rather than following it. A
+	// leaf changed to a symlink after validation therefore cannot redirect the
+	// truncation to its target.
+	if err := parent.Rename(temporaryName, name); err != nil {
+		return nil, fmt.Errorf("install replacement for destination file %q: %w", name, err)
+	}
+	installed = true
+	return validateOpenedConfinedFile(parent, name, temporary)
+}
+
+func runBeforeConfinedOpen(beforeOpen func() error) error {
+	if beforeOpen == nil {
+		return nil
+	}
+	if err := beforeOpen(); err != nil {
+		return fmt.Errorf("check before confined file open: %w", err)
+	}
+	return nil
+}
+
+func validateOpenedConfinedFile(parent *os.Root, name string, file *os.File) (*os.File, error) {
 	opened, err := file.Stat()
 	if err != nil {
 		_ = file.Close()