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1 ;;;; unsafep.el -- Determine whether a Lisp form is safe to evaluate
3 ;; Copyright (C) 2002-2015 Free Software Foundation, Inc.
5 ;; Author: Jonathan Yavner <jyavner@member.fsf.org>
6 ;; Maintainer: Jonathan Yavner <jyavner@member.fsf.org>
7 ;; Keywords: safety lisp utility
9 ;; This file is part of GNU Emacs.
11 ;; GNU Emacs is free software: you can redistribute it and/or modify
12 ;; it under the terms of the GNU General Public License as published by
13 ;; the Free Software Foundation, either version 3 of the License, or
14 ;; (at your option) any later version.
16 ;; GNU Emacs is distributed in the hope that it will be useful,
17 ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
18 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
19 ;; GNU General Public License for more details.
21 ;; You should have received a copy of the GNU General Public License
22 ;; along with GNU Emacs. If not, see <http://www.gnu.org/licenses/>.
24 ;;; Commentary:
26 ;; This is a simplistic implementation that does not allow any modification of
27 ;; buffers or global variables. It does no dataflow analysis, so functions
28 ;; like `funcall' and `setcar' are completely disallowed. It is designed
29 ;; for "pure Lisp" formulas, like those in spreadsheets, that don't make any
30 ;; use of the text editing capabilities of Emacs.
32 ;; A formula is safe if:
33 ;; 1. It's an atom.
34 ;; 2. It's a function call to a safe function and all arguments are safe
35 ;; formulas.
36 ;; 3. It's a special form whose arguments are like a function's (and,
37 ;; catch, if, or, prog1, prog2, progn, while, unwind-protect).
38 ;; 4. It's a special form or macro that creates safe temporary bindings
39 ;; (condition-case, dolist, dotimes, lambda, let, let*).
40 ;; 4. It's one of (cond, quote) that have special parsing.
41 ;; 5. It's one of (add-to-list, setq, push, pop) and the assignment variable
42 ;; is safe.
43 ;; 6. It's one of (apply, mapc, mapcar, mapconcat) and its first arg is a
44 ;; quoted safe function.
45 ;;
46 ;; A function is safe if:
47 ;; 1. It's a lambda containing safe formulas.
48 ;; 2. It's a member of list `safe-functions', so the user says it's safe.
49 ;; 3. It's a symbol with the `side-effect-free' property, defined by the
50 ;; byte compiler or function author.
51 ;; 4. It's a symbol with the `safe-function' property, defined here or by
52 ;; the function author. Value t indicates a function that is safe but
53 ;; has innocuous side effects. Other values will someday indicate
54 ;; functions with side effects that are not always safe.
55 ;; The `side-effect-free' and `safe-function' properties are provided for
56 ;; built-in functions and for functions and macros defined in subr.el.
57 ;;
58 ;; A temporary binding is unsafe if its symbol:
59 ;; 1. Has the `risky-local-variable' property.
60 ;; 2. Has a name that ends with -command, font-lock-keywords(-[0-9]+)?,
61 ;; font-lock-syntactic-keywords, -form, -forms, -frame-alist, -function,
62 ;; -functions, -history, -hook, -hooks, -map, -map-alist, -mode-alist,
63 ;; -predicate, or -program.
64 ;;
65 ;; An assignment variable is unsafe if:
66 ;; 1. It would be unsafe as a temporary binding.
67 ;; 2. It doesn't already have a temporary or buffer-local binding.
69 ;; There are unsafe forms that `unsafep' cannot detect. Beware of these:
70 ;; 1. The form's result is a string with a display property containing a
71 ;; form to be evaluated later, and you insert this result into a
72 ;; buffer. Always remove display properties before inserting!
73 ;; 2. The form alters a risky variable that was recently added to Emacs and
74 ;; is not yet marked with the `risky-local-variable' property.
75 ;; 3. The form uses undocumented features of built-in functions that have
76 ;; the `side-effect-free' property. For example, in Emacs-20 if you
77 ;; passed a circular list to `assoc', Emacs would crash. Historically,
78 ;; problems of this kind have been few and short-lived.
80 ;;; Code:
82 (provide 'unsafep)
83 (require 'byte-opt) ;Set up the `side-effect-free' properties
85 (defcustom safe-functions nil
86 "A list of assumed-safe functions, or t to disable `unsafep'."
87 :group 'lisp
88 :type '(choice (const :tag "No" nil) (const :tag "Yes" t) hook))
90 (defvar unsafep-vars nil
91 "Dynamically-bound list of variables with lexical bindings at this point
92 in the parse.")
93 (put 'unsafep-vars 'risky-local-variable t)
95 ;;Side-effect-free functions from subr.el
96 (dolist (x '(assoc-default assoc-ignore-case butlast last match-string
97 match-string-no-properties member-ignore-case remove remq))
98 (put x 'side-effect-free t))
100 ;;Other safe functions
101 (dolist (x '(;;Special forms
102 and catch if or prog1 prog2 progn while unwind-protect
103 ;;Safe subrs that have some side-effects
104 ding error random signal sleep-for string-match throw
105 ;;Defsubst functions from subr.el
106 caar cadr cdar cddr
107 ;;Macros from subr.el
108 save-match-data unless when
109 ;;Functions from subr.el that have side effects
110 split-string replace-regexp-in-string play-sound-file))
111 (put x 'safe-function t))
113 ;;;###autoload
114 (defun unsafep (form &optional unsafep-vars)
115 "Return nil if evaluating FORM couldn't possibly do any harm.
116 Otherwise result is a reason why FORM is unsafe.
117 UNSAFEP-VARS is a list of symbols with local bindings."
118 (catch 'unsafep
119 (if (or (eq safe-functions t) ;User turned off safety-checking
120 (atom form)) ;Atoms are never unsafe
121 (throw 'unsafep nil))
122 (let* ((fun (car form))
123 (reason (unsafep-function fun))
124 arg)
125 (cond
126 ((not reason)
127 ;;It's a normal function - unsafe if any arg is
128 (unsafep-progn (cdr form)))
129 ((eq fun 'quote)
130 ;;Never unsafe
131 nil)
132 ((memq fun '(apply mapc mapcar mapconcat))
133 ;;Unsafe if 1st arg isn't a quoted lambda
134 (setq arg (cadr form))
135 (cond
136 ((memq (car-safe arg) '(quote function))
137 (setq reason (unsafep-function (cadr arg))))
138 ((eq (car-safe arg) 'lambda)
139 ;;Self-quoting lambda
140 (setq reason (unsafep arg unsafep-vars)))
141 (t
142 (setq reason `(unquoted ,arg))))
143 (or reason (unsafep-progn (cddr form))))
144 ((eq fun 'lambda)
145 ;;First arg is temporary bindings
146 (mapc #'(lambda (x)
147 (or (memq x '(&optional &rest))
148 (let ((y (unsafep-variable x t)))
149 (if y (throw 'unsafep y))
150 (push x unsafep-vars))))
151 (cadr form))
152 (unsafep-progn (cddr form)))
153 ((eq fun 'let)
154 ;;Creates temporary bindings in one step
155 (setq unsafep-vars (nconc (mapcar #'unsafep-let (cadr form))
156 unsafep-vars))
157 (unsafep-progn (cddr form)))
158 ((eq fun 'let*)
159 ;;Creates temporary bindings iteratively
160 (dolist (x (cadr form))
161 (push (unsafep-let x) unsafep-vars))
162 (unsafep-progn (cddr form)))
163 ((eq fun 'setq)
164 ;;Safe if odd arguments are local-var syms, evens are safe exprs
165 (setq arg (cdr form))
166 (while arg
167 (setq reason (or (unsafep-variable (car arg) nil)
168 (unsafep (cadr arg) unsafep-vars)))
169 (if reason (throw 'unsafep reason))
170 (setq arg (cddr arg))))
171 ((eq fun 'pop)
172 ;;safe if arg is local-var sym
173 (unsafep-variable (cadr form) nil))
174 ((eq fun 'push)
175 ;;Safe if 2nd arg is a local-var sym
176 (or (unsafep (cadr form) unsafep-vars)
177 (unsafep-variable (nth 2 form) nil)))
178 ((eq fun 'add-to-list)
179 ;;Safe if first arg is a quoted local-var sym
180 (setq arg (cadr form))
181 (if (not (eq (car-safe arg) 'quote))
182 `(unquoted ,arg)
183 (or (unsafep-variable (cadr arg) nil)
184 (unsafep-progn (cddr form)))))
185 ((eq fun 'cond)
186 ;;Special form with unusual syntax - safe if all args are
187 (dolist (x (cdr form))
188 (setq reason (unsafep-progn x))
189 (if reason (throw 'unsafep reason))))
190 ((memq fun '(dolist dotimes))
191 ;;Safe if COUNT and RESULT are safe. VAR is bound while checking BODY.
192 (setq arg (cadr form))
193 (or (unsafep-progn (cdr arg))
194 (let ((unsafep-vars (cons (car arg) unsafep-vars)))
195 (unsafep-progn (cddr form)))))
196 ((eq fun 'condition-case)
197 ;;Special form with unusual syntax - safe if all args are
198 (or (unsafep-variable (cadr form) t)
199 (unsafep (nth 2 form) unsafep-vars)
200 (let ((unsafep-vars (cons (cadr form) unsafep-vars)))
201 ;;var is bound only during handlers
202 (dolist (x (nthcdr 3 form))
203 (setq reason (unsafep-progn (cdr x)))
204 (if reason (throw 'unsafep reason))))))
205 ((eq fun '\`)
206 ;; Backquoted form - safe if its expansion is.
207 (unsafep (cdr (backquote-process (cadr form)))))
208 (t
209 ;;First unsafep-function call above wasn't nil, no special case applies
210 reason)))))
213 (defun unsafep-function (fun)
214 "Return nil if FUN is a safe function.
215 \(Either a safe lambda or a symbol that names a safe function).
216 Otherwise result is a reason code."
217 (cond
218 ((eq (car-safe fun) 'lambda)
219 (unsafep fun unsafep-vars))
220 ((not (and (symbolp fun)
221 (or (get fun 'side-effect-free)
222 (eq (get fun 'safe-function) t)
223 (eq safe-functions t)
224 (memq fun safe-functions))))
225 `(function ,fun))))
227 (defun unsafep-progn (list)
228 "Return nil if all forms in LIST are safe.
229 Else, return the reason for the first unsafe form."
230 (catch 'unsafep-progn
231 (let (reason)
232 (dolist (x list)
233 (setq reason (unsafep x unsafep-vars))
234 (if reason (throw 'unsafep-progn reason))))))
236 (defun unsafep-let (clause)
237 "Check the safety of a let binding.
238 CLAUSE is a let-binding, either SYM or (SYM) or (SYM VAL).
239 Check VAL and throw a reason to `unsafep' if unsafe.
240 Return SYM."
241 (let (reason sym)
242 (if (atom clause)
243 (setq sym clause)
244 (setq sym (car clause)
245 reason (unsafep (cadr clause) unsafep-vars)))
246 (setq reason (or (unsafep-variable sym t) reason))
247 (if reason (throw 'unsafep reason))
248 sym))
250 (defun unsafep-variable (sym to-bind)
251 "Return nil if SYM is safe to set or bind, or a reason why not.
252 If TO-BIND is nil, check whether SYM is safe to set.
253 If TO-BIND is t, check whether SYM is safe to bind."
254 (cond
255 ((not (symbolp sym))
256 `(variable ,sym))
257 ((risky-local-variable-p sym nil)
258 `(risky-local-variable ,sym))
259 ((not (or to-bind
260 (memq sym unsafep-vars)
261 (local-variable-p sym)))
262 `(global-variable ,sym))))
264 ;;; unsafep.el ends here