Fix some crashes on self-modifying Elisp code

Prompted by a problem report by Alex in:
http://lists.gnu.org/archive/html/emacs-devel/2017-08/msg00143.html
* src/eval.c (For, Fprogn, Fsetq, FletX, eval_sub):
Compute XCDR (x) near XCAR (x); although this doesn't fix any bugs,
it is likely to run a bit faster with typical hardware caches.
(Fif): Use Fcdr instead of XCDR, to avoid crashing on
self-modifying S-expressions.
(Fsetq, Flet, eval_sub): Count the number of arguments as we go
instead of trusting an Flength prepass, to avoid problems when the
code is self-modifying.
(Fquote, Ffunction, Fdefvar, Fdefconst): Prefer !NILP to CONSP
where either will do.  This is mostly to document the fact that
the value must be a proper list.  It's also a tiny bit faster on
typical machines nowadays.
(Fdefconst, FletX): Prefer XCAR+XCDR to Fcar+Fcdr when either will do.
(eval_sub): Check that the args are a list as opposed to some
other object that has a length. This prevents e.g. (if . "string")
from making Emacs dump core in some cases.
* test/src/eval-tests.el (eval-tests--if-dot-string)
(eval-tests--let-with-circular-defs, eval-tests--mutating-cond):
New tests.

diff --git a/src/eval.c b/src/eval.c
index e5900382dee95d658cb792a39c65fe800f7a2aff..fe2708b1bbc28a5f8386433b7771d61b0aed7e2a 100644 (file)
--- a/src/eval.c
+++ b/src/eval.c
@@ -354,10 +354,11 @@ usage: (or CONDITIONS...)  */)
 
   while (CONSP (args))
     {
-      val = eval_sub (XCAR (args));
+      Lisp_Object arg = XCAR (args);
+      args = XCDR (args);
+      val = eval_sub (arg);
       if (!NILP (val))
        break;
-      args = XCDR (args);
     }
 
   return val;
@@ -374,10 +375,11 @@ usage: (and CONDITIONS...)  */)
 
   while (CONSP (args))
     {
-      val = eval_sub (XCAR (args));
+      Lisp_Object arg = XCAR (args);
+      args = XCDR (args);
+      val = eval_sub (arg);
       if (NILP (val))
        break;
-      args = XCDR (args);
     }
 
   return val;
@@ -397,7 +399,7 @@ usage: (if COND THEN ELSE...)  */)
 
   if (!NILP (cond))
     return eval_sub (Fcar (XCDR (args)));
-  return Fprogn (XCDR (XCDR (args)));
+  return Fprogn (Fcdr (XCDR (args)));
 }
 
 DEFUN ("cond", Fcond, Scond, 0, UNEVALLED, 0,
@@ -439,8 +441,9 @@ usage: (progn BODY...)  */)
 
   while (CONSP (body))
     {
-      val = eval_sub (XCAR (body));
+      Lisp_Object form = XCAR (body);
       body = XCDR (body);
+      val = eval_sub (form);
     }
 
   return val;
@@ -488,35 +491,26 @@ The return value of the `setq' form is the value of the last VAL.
 usage: (setq [SYM VAL]...)  */)
   (Lisp_Object args)
 {
-  Lisp_Object val, sym, lex_binding;
+  Lisp_Object val = args, tail = args;
 
-  val = args;
-  if (CONSP (args))
+  for (EMACS_INT nargs = 0; CONSP (tail); nargs += 2)
     {
-      Lisp_Object args_left = args;
-      Lisp_Object numargs = Flength (args);
-
-      if (XINT (numargs) & 1)
-        xsignal2 (Qwrong_number_of_arguments, Qsetq, numargs);
-
-      do
-       {
-         val = eval_sub (Fcar (XCDR (args_left)));
-         sym = XCAR (args_left);
-
-         /* Like for eval_sub, we do not check declared_special here since
-            it's been done when let-binding.  */
-         if (!NILP (Vinternal_interpreter_environment) /* Mere optimization!  */
-             && SYMBOLP (sym)
-             && !NILP (lex_binding
-                       = Fassq (sym, Vinternal_interpreter_environment)))
-           XSETCDR (lex_binding, val); /* SYM is lexically bound.  */
-         else
-           Fset (sym, val);    /* SYM is dynamically bound.  */
-
-         args_left = Fcdr (XCDR (args_left));
-       }
-      while (CONSP (args_left));
+      Lisp_Object sym = XCAR (tail), lex_binding;
+      tail = XCDR (tail);
+      if (!CONSP (tail))
+       xsignal2 (Qwrong_number_of_arguments, Qsetq, make_number (nargs + 1));
+      Lisp_Object arg = XCAR (tail);
+      tail = XCDR (tail);
+      val = eval_sub (arg);
+      /* Like for eval_sub, we do not check declared_special here since
+        it's been done when let-binding.  */
+      if (!NILP (Vinternal_interpreter_environment) /* Mere optimization!  */
+         && SYMBOLP (sym)
+         && !NILP (lex_binding
+                   = Fassq (sym, Vinternal_interpreter_environment)))
+       XSETCDR (lex_binding, val); /* SYM is lexically bound.  */
+      else
+       Fset (sym, val);        /* SYM is dynamically bound.  */
     }
 
   return val;
@@ -535,7 +529,7 @@ of unexpected results when a quoted object is modified.
 usage: (quote ARG)  */)
   (Lisp_Object args)
 {
-  if (CONSP (XCDR (args)))
+  if (!NILP (XCDR (args)))
     xsignal2 (Qwrong_number_of_arguments, Qquote, Flength (args));
   return XCAR (args);
 }
@@ -549,7 +543,7 @@ usage: (function ARG)  */)
 {
   Lisp_Object quoted = XCAR (args);
 
-  if (CONSP (XCDR (args)))
+  if (!NILP (XCDR (args)))
     xsignal2 (Qwrong_number_of_arguments, Qfunction, Flength (args));
 
   if (!NILP (Vinternal_interpreter_environment)
@@ -734,9 +728,9 @@ usage: (defvar SYMBOL &optional INITVALUE DOCSTRING)  */)
   sym = XCAR (args);
   tail = XCDR (args);
 
-  if (CONSP (tail))
+  if (!NILP (tail))
     {
-      if (CONSP (XCDR (tail)) && CONSP (XCDR (XCDR (tail))))
+      if (!NILP (XCDR (tail)) && !NILP (XCDR (XCDR (tail))))
        error ("Too many arguments");
 
       tem = Fdefault_boundp (sym);
@@ -803,20 +797,24 @@ usage: (defconst SYMBOL INITVALUE [DOCSTRING])  */)
   Lisp_Object sym, tem;
 
   sym = XCAR (args);
-  if (CONSP (Fcdr (XCDR (XCDR (args)))))
-    error ("Too many arguments");
+  Lisp_Object docstring = Qnil;
+  if (!NILP (XCDR (XCDR (args))))
+    {
+      if (!NILP (XCDR (XCDR (XCDR (args)))))
+       error ("Too many arguments");
+      docstring = XCAR (XCDR (XCDR (args)));
+    }
 
-  tem = eval_sub (Fcar (XCDR (args)));
+  tem = eval_sub (XCAR (XCDR (args)));
   if (!NILP (Vpurify_flag))
     tem = Fpurecopy (tem);
   Fset_default (sym, tem);
   XSYMBOL (sym)->declared_special = 1;
-  tem = Fcar (XCDR (XCDR (args)));
-  if (!NILP (tem))
+  if (!NILP (docstring))
     {
       if (!NILP (Vpurify_flag))
-       tem = Fpurecopy (tem);
-      Fput (sym, Qvariable_documentation, tem);
+       docstring = Fpurecopy (docstring);
+      Fput (sym, Qvariable_documentation, docstring);
     }
   Fput (sym, Qrisky_local_variable, Qt);
   LOADHIST_ATTACH (sym);
@@ -844,27 +842,29 @@ Each VALUEFORM can refer to the symbols already bound by this VARLIST.
 usage: (let* VARLIST BODY...)  */)
   (Lisp_Object args)
 {
-  Lisp_Object varlist, var, val, elt, lexenv;
+  Lisp_Object var, val, elt, lexenv;
   ptrdiff_t count = SPECPDL_INDEX ();
 
   lexenv = Vinternal_interpreter_environment;
 
-  for (varlist = XCAR (args); CONSP (varlist); varlist = XCDR (varlist))
+  Lisp_Object varlist = XCAR (args);
+  while (CONSP (varlist))
     {
       maybe_quit ();
 
       elt = XCAR (varlist);
+      varlist = XCDR (varlist);
       if (SYMBOLP (elt))
        {
          var = elt;
          val = Qnil;
        }
-      else if (! NILP (Fcdr (Fcdr (elt))))
-       signal_error ("`let' bindings can have only one value-form", elt);
       else
        {
          var = Fcar (elt);
-         val = eval_sub (Fcar (Fcdr (elt)));
+         if (! NILP (Fcdr (XCDR (elt))))
+           signal_error ("`let' bindings can have only one value-form", elt);
+         val = eval_sub (Fcar (XCDR (elt)));
        }
 
       if (!NILP (lexenv) && SYMBOLP (var)
@@ -911,33 +911,37 @@ usage: (let VARLIST BODY...)  */)
   CHECK_LIST (varlist);
 
   /* Make space to hold the values to give the bound variables.  */
-  elt = Flength (varlist);
-  SAFE_ALLOCA_LISP (temps, XFASTINT (elt));
+  EMACS_INT varlist_len = XFASTINT (Flength (varlist));
+  SAFE_ALLOCA_LISP (temps, varlist_len);
+  ptrdiff_t nvars = varlist_len;
 
   /* Compute the values and store them in `temps'.  */
 
-  for (argnum = 0; CONSP (varlist); varlist = XCDR (varlist))
+  for (argnum = 0; argnum < nvars && CONSP (varlist); argnum++)
     {
       maybe_quit ();
       elt = XCAR (varlist);
+      varlist = XCDR (varlist);
       if (SYMBOLP (elt))
-       temps [argnum++] = Qnil;
+       temps[argnum] = Qnil;
       else if (! NILP (Fcdr (Fcdr (elt))))
        signal_error ("`let' bindings can have only one value-form", elt);
       else
-       temps [argnum++] = eval_sub (Fcar (Fcdr (elt)));
+       temps[argnum] = eval_sub (Fcar (Fcdr (elt)));
     }
+  nvars = argnum;
 
   lexenv = Vinternal_interpreter_environment;
 
   varlist = XCAR (args);
-  for (argnum = 0; CONSP (varlist); varlist = XCDR (varlist))
+  for (argnum = 0; argnum < nvars && CONSP (varlist); argnum++)
     {
       Lisp_Object var;
 
       elt = XCAR (varlist);
+      varlist = XCDR (varlist);
       var = SYMBOLP (elt) ? elt : Fcar (elt);
-      tem = temps[argnum++];
+      tem = temps[argnum];
 
       if (!NILP (lexenv) && SYMBOLP (var)
          && !XSYMBOL (var)->declared_special
@@ -2135,6 +2139,7 @@ eval_sub (Lisp_Object form)
 
   original_fun = XCAR (form);
   original_args = XCDR (form);
+  CHECK_LIST (original_args);
 
   /* This also protects them from gc.  */
   count = record_in_backtrace (original_fun, &original_args, UNEVALLED);
@@ -2176,15 +2181,16 @@ eval_sub (Lisp_Object form)
 
          SAFE_ALLOCA_LISP (vals, XINT (numargs));
 
-         while (!NILP (args_left))
+         while (CONSP (args_left) && argnum < XINT (numargs))
            {
-             vals[argnum++] = eval_sub (Fcar (args_left));
-             args_left = Fcdr (args_left);
+             Lisp_Object arg = XCAR (args_left);
+             args_left = XCDR (args_left);
+             vals[argnum++] = eval_sub (arg);
            }
 
-         set_backtrace_args (specpdl + count, vals, XINT (numargs));
+         set_backtrace_args (specpdl + count, vals, argnum);
 
-         val = (XSUBR (fun)->function.aMANY) (XINT (numargs), vals);
+         val = XSUBR (fun)->function.aMANY (argnum, vals);
 
          check_cons_list ();
          lisp_eval_depth--;

diff --git a/test/src/eval-tests.el b/test/src/eval-tests.el
index 03f408716b102919e458800f61bc37173f434b2f..b98de0aa65e739730c8857db8ed07eb27911ee2e 100644 (file)
--- a/test/src/eval-tests.el
+++ b/test/src/eval-tests.el
@@ -59,4 +59,24 @@ Bug#24912 and Bug#24913."
         (should-error (,form ,arg) :type 'wrong-type-argument))
      t)))
 
+(ert-deftest eval-tests--if-dot-string ()
+  "Check that Emacs rejects (if . \"string\")."
+  (should-error (eval '(if . "abc")) :type 'wrong-type-argument)
+  (let ((if-tail (list '(setcdr if-tail "abc") t)))
+    (should-error (eval (cons 'if if-tail))))
+  (let ((if-tail (list '(progn (setcdr if-tail "abc") nil) t)))
+    (should-error (eval (cons 'if if-tail)))))
+
+(ert-deftest eval-tests--let-with-circular-defs ()
+  "Check that Emacs reports an error for (let VARS ...) when VARS is circular."
+  (let ((vars (list 'v)))
+    (setcdr vars vars)
+    (dolist (let-sym '(let let*))
+      (should-error (eval (list let-sym vars))))))
+
+(ert-deftest eval-tests--mutating-cond ()
+  "Check that Emacs doesn't crash on a cond clause that mutates during eval."
+  (let ((clauses (list '((progn (setcdr clauses "ouch") nil)))))
+    (should-error (eval (cons 'cond clauses)))))
+
 ;;; eval-tests.el ends here