[greatlrd] 26985: irc : Myrria (from linux xbox project) desc a bug in KiUserExceptionDispatcher and how to fix it. See issue #2279 for more details.

Author: greatlrd
Date: Mon Jun  4 09:50:39 2007
New Revision: 26985

URL: http://svn.reactos.org/svn/reactos?rev=26985&view=rev
Log:
irc : Myrria (from linux xbox project) desc a bug in KiUserExceptionDispatcher and how to
fix it.

See issue #2279 for more details.

Modified:
    trunk/reactos/dll/ntdll/main/i386/dispatch.S

Modified: trunk/reactos/dll/ntdll/main/i386/dispatch.S
URL:
http://svn.reactos.org/svn/reactos/trunk/reactos/dll/ntdll/main/i386/dispat…
==============================================================================
--- trunk/reactos/dll/ntdll/main/i386/dispatch.S (original)
+++ trunk/reactos/dll/ntdll/main/i386/dispatch.S Mon Jun  4 09:50:39 2007
@@ -183,6 +183,30 @@
 .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]