irc : Myrria (from linux xbox project) desc a bug in KiUserExceptionDispatcher and how to fix it.

See issue #2279 for more details.

svn path=/trunk/; revision=26985

reactos/dll/ntdll/main/i386/dispatch.S

@@ -183,6 +183,30 @@ _KiRaiseUserExceptionDispatcher@0:
 .globl _KiUserExceptionDispatcher@8
 _KiUserExceptionDispatcher@8:
 
+    /* clear the direct flag 
+     * text from bug 2279
+     * if it not clear it means that if an exception occurs while
+     * the direction flag is set (typically inside memmove), the 
+     * exception handlers will be called with the direction flag still
+     * set.  The Windows x86-32 and x86-64 ABI requires that the 
+     * direction flag be Calling memset() with a compile-time constant 
+     * size on both GCC and MSVC will result in inlining a "rep stosd" 
+     * instruction.  Because of the ABI, they will assume that the 
+     * direction flag is clear and not emit a "cld" instruction. 
+     * Using memset() in an exception handler therefore will 
+     * corrupt memory if the exception occurred during a reverse copy 
+     * such as a forward overlapping memmove().
+     *
+     * For reliability and ease of debugging, please add "cld" to the beginning of
+     * KiUserExceptionDispatcher.  Note that the same will be true of x86-64 whenever
+     * that happens.  This does not affect continuing execution; the CONTEXT of the
+     * exception has the direction flag set and will be restored upon NtContinue. 
+     * KiUserApcDispatcher and KiUserCallbackDispatcher need to be evaluated for this
+     * issue.
+     */
+     
+    cld
+
     /* Save the Context and Exception Records */
     mov ecx, [esp+4]
     mov ebx, [esp]