diff --git a/src/os/error.go b/src/os/error.go
index e26ce279706..2612f58bd1b 100644
--- a/src/os/error.go
+++ b/src/os/error.go
@@ -63,3 +63,16 @@ func IsNotExist(err error) bool {
 func IsPermission(err error) bool {
 	return isPermission(err)
 }
+
+// underlyingError returns the underlying error for known os error types.
+func underlyingError(err error) error {
+	switch err := err.(type) {
+	case *PathError:
+		return err.Err
+	case *LinkError:
+		return err.Err
+	case *SyscallError:
+		return err.Err
+	}
+	return err
+}
diff --git a/src/os/error_plan9.go b/src/os/error_plan9.go
index 2dc6b39c395..a67343981e2 100644
--- a/src/os/error_plan9.go
+++ b/src/os/error_plan9.go
@@ -5,46 +5,30 @@
 package os
 
 func isExist(err error) bool {
-	switch pe := err.(type) {
-	case nil:
-		return false
-	case *PathError:
-		err = pe.Err
-	case *LinkError:
-		err = pe.Err
-	case *SyscallError:
-		err = pe.Err
-	}
-	return contains(err.Error(), " exists")
+	return checkErrMessageContent(err, " exists")
 }
 
 func isNotExist(err error) bool {
-	switch pe := err.(type) {
-	case nil:
-		return false
-	case *PathError:
-		err = pe.Err
-	case *LinkError:
-		err = pe.Err
-	case *SyscallError:
-		err = pe.Err
-	}
-	return contains(err.Error(), "does not exist") || contains(err.Error(), "not found") ||
-		contains(err.Error(), "has been removed") || contains(err.Error(), "no parent")
+	return checkErrMessageContent(err, "does not exist", "not found",
+		"has been removed", "no parent")
 }
 
 func isPermission(err error) bool {
-	switch pe := err.(type) {
-	case nil:
+	return checkErrMessageContent(err, "permission denied")
+}
+
+// checkErrMessageContent checks if err message contains one of msgs.
+func checkErrMessageContent(err error, msgs ...string) bool {
+	if err == nil {
 		return false
-	case *PathError:
-		err = pe.Err
-	case *LinkError:
-		err = pe.Err
-	case *SyscallError:
-		err = pe.Err
 	}
-	return contains(err.Error(), "permission denied")
+	err = underlyingError(err)
+	for _, msg := range msgs {
+		if contains(err.Error(), msg) {
+			return true
+		}
+	}
+	return false
 }
 
 // contains is a local version of strings.Contains. It knows len(sep) > 1.
diff --git a/src/os/error_unix.go b/src/os/error_unix.go
index 3c78eb4dd2f..be1440cacba 100644
--- a/src/os/error_unix.go
+++ b/src/os/error_unix.go
@@ -9,43 +9,16 @@ package os
 import "syscall"
 
 func isExist(err error) bool {
-	switch pe := err.(type) {
-	case nil:
-		return false
-	case *PathError:
-		err = pe.Err
-	case *LinkError:
-		err = pe.Err
-	case *SyscallError:
-		err = pe.Err
-	}
+	err = underlyingError(err)
 	return err == syscall.EEXIST || err == syscall.ENOTEMPTY || err == ErrExist
 }
 
 func isNotExist(err error) bool {
-	switch pe := err.(type) {
-	case nil:
-		return false
-	case *PathError:
-		err = pe.Err
-	case *LinkError:
-		err = pe.Err
-	case *SyscallError:
-		err = pe.Err
-	}
+	err = underlyingError(err)
 	return err == syscall.ENOENT || err == ErrNotExist
 }
 
 func isPermission(err error) bool {
-	switch pe := err.(type) {
-	case nil:
-		return false
-	case *PathError:
-		err = pe.Err
-	case *LinkError:
-		err = pe.Err
-	case *SyscallError:
-		err = pe.Err
-	}
+	err = underlyingError(err)
 	return err == syscall.EACCES || err == syscall.EPERM || err == ErrPermission
 }
diff --git a/src/os/error_windows.go b/src/os/error_windows.go
index 2c1c39c414b..5a57c0d1e68 100644
--- a/src/os/error_windows.go
+++ b/src/os/error_windows.go
@@ -7,16 +7,7 @@ package os
 import "syscall"
 
 func isExist(err error) bool {
-	switch pe := err.(type) {
-	case nil:
-		return false
-	case *PathError:
-		err = pe.Err
-	case *LinkError:
-		err = pe.Err
-	case *SyscallError:
-		err = pe.Err
-	}
+	err = underlyingError(err)
 	return err == syscall.ERROR_ALREADY_EXISTS ||
 		err == syscall.ERROR_FILE_EXISTS || err == ErrExist
 }
@@ -24,31 +15,13 @@ func isExist(err error) bool {
 const _ERROR_BAD_NETPATH = syscall.Errno(53)
 
 func isNotExist(err error) bool {
-	switch pe := err.(type) {
-	case nil:
-		return false
-	case *PathError:
-		err = pe.Err
-	case *LinkError:
-		err = pe.Err
-	case *SyscallError:
-		err = pe.Err
-	}
+	err = underlyingError(err)
 	return err == syscall.ERROR_FILE_NOT_FOUND ||
 		err == _ERROR_BAD_NETPATH ||
 		err == syscall.ERROR_PATH_NOT_FOUND || err == ErrNotExist
 }
 
 func isPermission(err error) bool {
-	switch pe := err.(type) {
-	case nil:
-		return false
-	case *PathError:
-		err = pe.Err
-	case *LinkError:
-		err = pe.Err
-	case *SyscallError:
-		err = pe.Err
-	}
+	err = underlyingError(err)
 	return err == syscall.ERROR_ACCESS_DENIED || err == ErrPermission
 }