GNU bug report logs - #8318
int overflow problem in SAFE_ALLOCA, SAFE_ALLOCA_LISP

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: bug-gnu-emacs <at> gnu.org
Subject: int overflow problem in SAFE_ALLOCA, SAFE_ALLOCA_LISP
Date: Tue, 22 Mar 2011 02:12:52 -0700

The SAFE_ALLOCA macro assumes that adding 1 to the integer
variable sa_must_free cannot overflow, but this assumption
is incorrect in some cases.  I plan to commit the following
patch to fix this.

I found this bug using gcc 4.5.2 -O2 -Wstrict-overflow.

* lisp.h (SAFE_ALLOCA, SAFE_ALLOCA_LISP): Avoid 'int' overflow
leading to a memory leak, possible in functions like
load_charset_map_from_file that can allocate an unbounded number
of objects.
=== modified file 'src/lisp.h'
--- src/lisp.h	2011-03-18 04:58:44 +0000
+++ src/lisp.h	2011-03-22 09:04:53 +0000
@@ -3602,7 +3602,7 @@
     else						  \
       {							  \
 	buf = (type) xmalloc (size);			  \
-	sa_must_free++;					  \
+	sa_must_free = 1;				  \
 	record_unwind_protect (safe_alloca_unwind,	  \
 			       make_save_value (buf, 0)); \
       }							  \
@@ -3632,7 +3632,7 @@
 	buf = (Lisp_Object *) xmalloc (size_);		  \
 	arg_ = make_save_value (buf, nelt);		  \
 	XSAVE_VALUE (arg_)->dogc = 1;			  \
-	sa_must_free++;					  \
+	sa_must_free = 1;				  \
 	record_unwind_protect (safe_alloca_unwind, arg_); \
       }							  \
   } while (0)

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