Use xref for history list truncation.

[emacs.git] / lisp / emacs-lisp / unsafep.el

;;;; unsafep.el -- Determine whether a Lisp form is safe to evaluate

;; Copyright (C) Free Software Foundation, Inc.

;; Author: Jonathan Yavner <jyavner@engineer.com>
;; Maintainer: Jonathan Yavner <jyavner@engineer.com>
;; Keywords: safety lisp utility

;; This file is part of GNU Emacs.

;; GNU Emacs is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation; either version 2, or (at your option)
;; any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs; see the file COPYING.  If not, write to the
;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
;; Boston, MA 02111-1307, USA.

;;; Commentary:

;; This is a simplistic implementation that does not allow any modification of
;; buffers or global variables.  It does no dataflow analysis, so functions
;; like `funcall' and `setcar' are completely disallowed.  It is designed
;; for "pure Lisp" formulas, like those in spreadsheets, that don't make any
;; use of the text editing capabilities of Emacs.

;; A formula is safe if:
;;  1.  It's an atom.
;;  2.  It's a function call to a safe function and all arguments are safe
;;      formulas.
;;  3.  It's a special form whose arguments are like a function's (and,
;;      catch, if, or, prog1, prog2, progn, while, unwind-protect).
;;  4.  It's a special form or macro that creates safe temporary bindings
;;      (condition-case, dolist, dotimes, lambda, let, let*).
;;  4.  It's one of (cond, quote) that have special parsing.
;;  5.  It's one of (add-to-list, setq, push, pop) and the assignment variable
;;      is safe.
;;  6.  It's one of (apply, mapc, mapcar, mapconcat) and its first arg is a
;;      quoted safe function.
;;
;; A function is safe if:
;;  1.  It's a lambda containing safe formulas.
;;  2.  It's a member of list `safe-functions', so the user says it's safe.
;;  3.  It's a symbol with the `side-effect-free' property, defined by the
;;      byte compiler or function author.
;;  4.  It's a symbol with the `safe-function' property, defined here or by
;;      the function author.  Value t indicates a function that is safe but
;;      has innocuous side effects.  Other values will someday indicate
;;      functions with side effects that are not always safe.
;;  The `side-effect-free' and `safe-function' properties are provided for
;;  built-in functions and for functions and macros defined in subr.el.
;;
;; A temporary binding is unsafe if its symbol:
;;  1.  Has the `risky-local-variable' property.
;;  2.  Has a name that ends with -command, font-lock-keywords(-[0-9]+)?,
;;      font-lock-syntactic-keywords, -form, -forms, -frame-alist, -function,
;;       -functions, -history, -hook, -hooks, -map, -map-alist, -mode-alist,
;;       -predicate, or -program.
;;
;; An assignment variable is unsafe if:
;;   1. It would be unsafe as a temporary binding.
;;   2. It doesn't already have a temporary or buffer-local binding.

;; There are unsafe forms that `unsafep' cannot detect.  Beware of these:
;;   1. The form's result is a string with a display property containing a
;;      form to be evaluated later, and you insert this result into a
;;      buffer.  Always remove display properties before inserting!
;;   2. The form alters a risky variable that was recently added to Emacs and
;;      is not yet marked with the `risky-local-variable' property.
;;   3. The form uses undocumented features of built-in functions that have
;;      the `side-effect-free' property.  For example, in Emacs-20 if you
;;      passed a circular list to `assoc', Emacs would crash.  Historically,
;;      problems of this kind have been few and short-lived.

(provide 'unsafep)
(require 'byte-opt)  ;Set up the `side-effect-free' properties

(defcustom safe-functions nil
  "t to disable `unsafep', or a list of assumed-safe functions."
  :group 'lisp
  :type  '(choice (const :tag "No" nil) (const :tag "Yes" t) hook))

(defvar unsafep-vars nil
  "Dynamically-bound list of variables with lexical bindings at this point
in the parse.")
(put 'unsafep-vars 'risky-local-variable t)

;;Side-effect-free functions from subr.el
(dolist (x '(assoc-default assoc-ignore-case butlast last match-string
             match-string-no-properties member-ignore-case remove remq))
  (put x 'side-effect-free t))

;;Other safe functions
(dolist (x '(;;Special forms
             and catch if or prog1 prog2 progn while unwind-protect
             ;;Safe subrs that have some side-effects
             ding error message minibuffer-message random read-minibuffer
             signal sleep-for string-match throw y-or-n-p yes-or-no-p
             ;;Defsubst functions from subr.el
             caar cadr cdar cddr
             ;;Macros from subr.el
             save-match-data unless when with-temp-message
             ;;Functions from subr.el that have side effects
             read-passwd split-string replace-regexp-in-string
             play-sound-file))
  (put x 'safe-function t))

;;;###autoload
(defun unsafep (form &optional unsafep-vars)
  "Return nil if evaluating FORM couldn't possibly do any harm;
otherwise result is a reason why FORM is unsafe.  UNSAFEP-VARS is a list
of symbols with local bindings."
  (catch 'unsafep
    (if (or (eq safe-functions t)           ;User turned off safety-checking
            (atom form))                    ;Atoms are never unsafe
        (throw 'unsafep nil))
    (let* ((fun    (car form))
           (reason (unsafep-function fun))
           arg)
      (cond
       ((not reason)
        ;;It's a normal function - unsafe if any arg is
        (unsafep-progn (cdr form)))
       ((eq fun 'quote)
        ;;Never unsafe
        nil)
       ((memq fun '(apply mapc mapcar mapconcat))
        ;;Unsafe if 1st arg isn't a quoted lambda
        (setq arg (cadr form))
        (cond
         ((memq (car-safe arg) '(quote function))
          (setq reason (unsafep-function (cadr arg))))
         ((eq (car-safe arg) 'lambda)
          ;;Self-quoting lambda
          (setq reason (unsafep arg unsafep-vars)))
         (t
          (setq reason `(unquoted ,arg))))
        (or reason (unsafep-progn (cddr form))))
       ((eq fun 'lambda)
        ;;First arg is temporary bindings
        (mapc #'(lambda (x)
                  (let ((y (unsafep-variable x t)))
                    (if y (throw 'unsafep y)))
                  (or (memq x '(&optional &rest))
                      (push x unsafep-vars)))
              (cadr form))
        (unsafep-progn (cddr form)))
       ((eq fun 'let)
        ;;Creates temporary bindings in one step
        (setq unsafep-vars (nconc (mapcar #'unsafep-let (cadr form))
                                  unsafep-vars))
        (unsafep-progn (cddr form)))
       ((eq fun 'let*)
        ;;Creates temporary bindings iteratively
        (dolist (x (cadr form))
          (push (unsafep-let x) unsafep-vars))
        (unsafep-progn (cddr form)))
       ((eq fun 'setq)
        ;;Safe if odd arguments are local-var syms, evens are safe exprs
        (setq arg (cdr form))
        (while arg
          (setq reason (or (unsafep-variable (car arg) nil)
                           (unsafep (cadr arg) unsafep-vars)))
          (if reason (throw 'unsafep reason))
          (setq arg (cddr arg))))
       ((eq fun 'pop)
        ;;safe if arg is local-var sym
        (unsafep-variable (cadr form) nil))
       ((eq fun 'push)
        ;;Safe if 2nd arg is a local-var sym
        (or (unsafep (cadr form) unsafep-vars)
            (unsafep-variable (nth 2 form) nil)))
       ((eq fun 'add-to-list)
        ;;Safe if first arg is a quoted local-var sym
        (setq arg (cadr form))
        (if (not (eq (car-safe arg) 'quote))
            `(unquoted ,arg)
          (or (unsafep-variable (cadr arg) nil)
              (unsafep-progn (cddr form)))))
       ((eq fun 'cond)
        ;;Special form with unusual syntax - safe if all args are
        (dolist (x (cdr form))
          (setq reason (unsafep-progn x))
          (if reason (throw 'unsafep reason))))
       ((memq fun '(dolist dotimes))
        ;;Safe if COUNT and RESULT are safe.  VAR is bound while checking BODY.
        (setq arg (cadr form))
        (or (unsafep-progn (cdr arg))
            (let ((unsafep-vars (cons (car arg) unsafep-vars)))
              (unsafep-progn (cddr form)))))
       ((eq fun 'condition-case)
        ;;Special form with unusual syntax - safe if all args are
        (or (unsafep-variable (cadr form) t)
            (unsafep (nth 2 form) unsafep-vars)
            (let ((unsafep-vars (cons (cadr form) unsafep-vars)))
              ;;var is bound only during handlers
              (dolist (x (nthcdr 3 form))
                (setq reason (unsafep-progn (cdr x)))
                (if reason (throw 'unsafep reason))))))
       (t
        ;;First unsafep-function call above wasn't nil, no special case applies
        reason)))))


(defun unsafep-function (fun)
  "Return nil if FUN is a safe function
\(either a safe lambda or a symbol that names a safe function).  Otherwise
result is a reason code." 
  (cond
   ((eq (car-safe fun) 'lambda)
    (unsafep fun unsafep-vars))
   ((not (and (symbolp fun)
              (or (get fun 'side-effect-free)
                  (eq (get fun 'safe-function) t)
                  (eq safe-functions t)
                  (memq fun safe-functions))))
    `(function ,fun))))

(defun unsafep-progn (list)
  "Return nil if all forms in LIST are safe, or the reason
for the first unsafe form."
  (catch 'unsafep-progn
    (let (reason)
      (dolist (x list)
        (setq reason (unsafep x unsafep-vars))
        (if reason (throw 'unsafep-progn reason))))))

(defun unsafep-let (clause)
  "CLAUSE is a let-binding, either SYM or (SYM) or (SYM VAL).  Checks VAL
and throws a reason to `unsafep' if unsafe.  Returns SYM."
  (let (reason sym)
    (if (atom clause)
        (setq sym clause)
      (setq sym    (car clause)
            reason (unsafep (cadr clause) unsafep-vars)))
    (setq reason (or (unsafep-variable sym t) reason))
    (if reason (throw 'unsafep reason))
    sym))

(defun unsafep-variable (sym global-okay)
  "Returns nil if SYM is safe as a let-binding sym
\(because it already has a temporary binding or is a non-risky buffer-local
variable), otherwise a reason why it is unsafe.  Failing to be locally bound
is okay if GLOBAL-OKAY is non-nil."
  (cond
   ((not (symbolp sym))
    `(variable ,sym))
   ((risky-local-variable-p sym nil)
    `(risky-local-variable ,sym))
   ((not (or global-okay
             (memq sym unsafep-vars)
             (local-variable-p sym)))
    `(global-variable ,sym))))

;; unsafep.el ends here.