Subject: bug#8254: race condition in dired.c's scmp function

From: Paul Eggert <[email-masked]>
Date: Mon, 14 Mar 2011 23:16:26 -0700

The following code in the Emacs trunk src/dired.c's scmp function has
undefined behavior:

      while (l
	     && (DOWNCASE ((unsigned char) *s1++)
		 == DOWNCASE ((unsigned char) *s2++)))
	l--;

Because the DOWNCASE macro assigns to the global variables case_temp1
and case_temp2, (DOWNCASE (x) == DOWNCASE (y)) is not valid, as the
assignments can collide and lead to a race condition.

This bug was found by gcc -Wsequence-point (GCC 4.5.2), which warns:

  dired.c:799:7: error: operation on 'case_temp2' may be undefined [-Wsequence-point]
  dired.c:799:7: error: operation on 'case_temp1' may be undefined [-Wsequence-point]

I plan to work around the problem with something like the following
patch.

----

Fix a race condition diagnosed by gcc -Wsequence-point.
The expression (DOWNCASE ((unsigned char) *s1++) ==
DOWNCASE ((unsigned char) *s2++)), found in dired.c's scmp
function, had undefined behavior.
* lisp.h (DOWNCASE_TABLE, UPCASE_TABLE, DOWNCASE, UPPERCASEP):
(NOCASEP, LOWERCASEP, UPCASE, UPCASE1): Move from here ...
* buffer.h: ... to here, because these macros use current_buffer,
and the new implementation with inline functions needs to have
current_buffer in scope now, rather than later when the macros
are used.
(downcase, upcase1): New static inline functions.
(DOWNCASE, UPCASE1): Reimplement using these functions.
This avoids undefined behavior in expressions like
DOWNCASE (x) == DOWNCASE (y), which previously suffered
from race conditions in accessing the global variables
case_temp1 and case_temp2.
* casetab.c (case_temp1, case_temp2): Remove; no longer needed.
* lisp.h (case_temp1, case_temp2): Remove their decls.
* character.h (ASCII_CHAR_P): Move from here ...
* lisp.h: ... to here, so that the inline functions mentioned
above can use them.
=== modified file 'src/buffer.h'
--- src/buffer.h	2011-03-13 22:25:16 +0000
+++ src/buffer.h	2011-03-15 05:52:48 +0000
@@ -1026,4 +1026,47 @@

 #define PER_BUFFER_VALUE(BUFFER, OFFSET) \
       (*(Lisp_Object *)((OFFSET) + (char *) (BUFFER)))
-
+
+/* Current buffer's map from characters to lower-case characters.  */
+
+#define DOWNCASE_TABLE BVAR (current_buffer, downcase_table)
+
+/* Current buffer's map from characters to upper-case characters.  */
+
+#define UPCASE_TABLE BVAR (current_buffer, upcase_table)
+
+/* Downcase a character, or make no change if that cannot be done.  */
+
+static inline EMACS_INT
+downcase (int ch)
+{
+  Lisp_Object down = CHAR_TABLE_REF (DOWNCASE_TABLE, ch);
+  return NATNUMP (down) ? XFASTINT (down) : ch;
+}
+#define DOWNCASE(CH) downcase (CH)
+
+/* 1 if CH is upper case.  */
+
+#define UPPERCASEP(CH) (DOWNCASE (CH) != (CH))
+
+/* 1 if CH is neither upper nor lower case.  */
+
+#define NOCASEP(CH) (UPCASE1 (CH) == (CH))
+
+/* 1 if CH is lower case.  */
+
+#define LOWERCASEP(CH) (!UPPERCASEP (CH) && !NOCASEP(CH))
+
+/* Upcase a character, or make no change if that cannot be done.  */
+
+#define UPCASE(CH) (!UPPERCASEP (CH) ? UPCASE1 (CH) : (CH))
+
+/* Upcase a character known to be not upper case.  */
+
+static inline EMACS_INT
+upcase1 (int ch)
+{
+  Lisp_Object up = CHAR_TABLE_REF (UPCASE_TABLE, ch);
+  return NATNUMP (up) ? XFASTINT (up) : ch;
+}
+#define UPCASE1(CH) upcase1 (CH)

=== modified file 'src/casetab.c'
--- src/casetab.c	2011-02-16 15:02:50 +0000
+++ src/casetab.c	2011-03-15 04:01:35 +0000
@@ -28,11 +28,6 @@
 Lisp_Object Vascii_downcase_table, Vascii_upcase_table;
 Lisp_Object Vascii_canon_table, Vascii_eqv_table;

-/* Used as a temporary in DOWNCASE and other macros in lisp.h.  No
-   need to mark it, since it is used only very temporarily.  */
-int case_temp1;
-Lisp_Object case_temp2;
-
 static void set_canon (Lisp_Object case_table, Lisp_Object range, Lisp_Object elt);
 static void set_identity (Lisp_Object table, Lisp_Object c, Lisp_Object elt);
 static void shuffle (Lisp_Object table, Lisp_Object c, Lisp_Object elt);
@@ -302,4 +297,3 @@
   defsubr (&Sset_case_table);
   defsubr (&Sset_standard_case_table);
 }
-

=== modified file 'src/character.h'
--- src/character.h	2011-03-08 04:37:19 +0000
+++ src/character.h	2011-03-15 05:52:52 +0000
@@ -128,9 +128,6 @@
     XSETCDR ((x), tmp);			\
   } while (0)

-/* Nonzero iff C is an ASCII character.  */
-#define ASCII_CHAR_P(c) ((unsigned) (c) < 0x80)
-
 /* Nonzero iff C is a character of code less than 0x100.  */
 #define SINGLE_BYTE_CHAR_P(c) ((unsigned) (c) < 0x100)

=== modified file 'src/lisp.h'
--- src/lisp.h	2011-03-15 01:32:33 +0000
+++ src/lisp.h	2011-03-15 04:23:51 +0000
@@ -841,6 +841,9 @@

 #endif	/* not __GNUC__ */

+/* Nonzero iff C is an ASCII character.  */
+#define ASCII_CHAR_P(c) ((unsigned) (c) < 0x80)
+
 /* Almost equivalent to Faref (CT, IDX) with optimization for ASCII
    characters.  Do not check validity of CT.  */
 #define CHAR_TABLE_REF(CT, IDX)					\
@@ -2041,50 +2044,6 @@

 #define QUITP (!NILP (Vquit_flag) && NILP (Vinhibit_quit))
 
-/* Variables used locally in the following case handling macros.  */
-extern int case_temp1;
-extern Lisp_Object case_temp2;
-
-/* Current buffer's map from characters to lower-case characters.  */
-
-#define DOWNCASE_TABLE BVAR (current_buffer, downcase_table)
-
-/* Current buffer's map from characters to upper-case characters.  */
-
-#define UPCASE_TABLE BVAR (current_buffer, upcase_table)
-
-/* Downcase a character, or make no change if that cannot be done.  */
-
-#define DOWNCASE(CH)						\
-  ((case_temp1 = (CH),						\
-    case_temp2 = CHAR_TABLE_REF (DOWNCASE_TABLE, case_temp1),	\
-    NATNUMP (case_temp2))					\
-   ? XFASTINT (case_temp2) : case_temp1)
-
-/* 1 if CH is upper case.  */
-
-#define UPPERCASEP(CH) (DOWNCASE (CH) != (CH))
-
-/* 1 if CH is neither upper nor lower case.  */
-
-#define NOCASEP(CH) (UPCASE1 (CH) == (CH))
-
-/* 1 if CH is lower case.  */
-
-#define LOWERCASEP(CH) (!UPPERCASEP (CH) && !NOCASEP(CH))
-
-/* Upcase a character, or make no change if that cannot be done.  */
-
-#define UPCASE(CH) (!UPPERCASEP (CH) ? UPCASE1 (CH) : (CH))
-
-/* Upcase a character known to be not upper case.  */
-
-#define UPCASE1(CH)						\
-  ((case_temp1 = (CH),						\
-    case_temp2 = CHAR_TABLE_REF (UPCASE_TABLE, case_temp1),	\
-    NATNUMP (case_temp2))					\
-   ? XFASTINT (case_temp2) : case_temp1)
-
 extern Lisp_Object Vascii_downcase_table, Vascii_upcase_table;
 extern Lisp_Object Vascii_canon_table, Vascii_eqv_table;