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emacs/lisp/emacs-lisp/unsafep.el
2012-01-05 01:46:05 -08:00

265 lines
9.9 KiB
EmacsLisp

;;;; unsafep.el -- Determine whether a Lisp form is safe to evaluate
;; Copyright (C) 2002-2012 Free Software Foundation, Inc.
;; Author: Jonathan Yavner <jyavner@member.fsf.org>
;; Maintainer: Jonathan Yavner <jyavner@member.fsf.org>
;; 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 3 of the License, 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. If not, see <http://www.gnu.org/licenses/>.
;;; 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.
;;; Code:
(provide 'unsafep)
(require 'byte-opt) ;Set up the `side-effect-free' properties
(defcustom safe-functions nil
"A list of assumed-safe functions, or t to disable `unsafep'."
: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 random signal sleep-for string-match throw
;;Defsubst functions from subr.el
caar cadr cdar cddr
;;Macros from subr.el
save-match-data unless when
;;Functions from subr.el that have side effects
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)
(or (memq x '(&optional &rest))
(let ((y (unsafep-variable x t)))
(if y (throw 'unsafep y))
(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))))))
((eq fun '\`)
;; Backquoted form - safe if its expansion is.
(unsafep (cdr (backquote-process (cadr form)))))
(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.
Else, return 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)
"Check the safety of a let binding.
CLAUSE is a let-binding, either SYM or (SYM) or (SYM VAL).
Check VAL and throw a reason to `unsafep' if unsafe.
Return 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 to-bind)
"Return nil if SYM is safe to set or bind, or a reason why not.
If TO-BIND is nil, check whether SYM is safe to set.
If TO-BIND is t, check whether SYM is safe to bind."
(cond
((not (symbolp sym))
`(variable ,sym))
((risky-local-variable-p sym nil)
`(risky-local-variable ,sym))
((not (or to-bind
(memq sym unsafep-vars)
(local-variable-p sym)))
`(global-variable ,sym))))
;;; unsafep.el ends here