From: Karl Tomlinson <xmail@karlt.net>
Date: Wed, 24 Oct 2012 01:05:25 +0000
Subject: MakeBigReq: don't move the last word, already handled by Data32

The case where req->length = 1 was already not functional.
Reported by Abhishek Arya <inferno@chromium.org>.
https://bugzilla.mozilla.org/show_bug.cgi?id=803762

Reviewed-by: Jeff Muizelaar <jmuizelaar@mozilla.com>
--- libX11-1.5.0/include/X11/Xlibint.h	2012-10-24 12:36:48.768190632 +1300
+++ libX11-1.5.0/include/X11/Xlibint.h	2012-10-26 10:49:05.498523215 +1300
@@ -498,51 +498,66 @@ extern void *_XGetRequest(Display *dpy,
 #define GetEmptyReq(name, req) \
 	req = (xReq *) _XGetRequest(dpy, X_##name, SIZEOF(xReq))
 #else
 #define GetEmptyReq(name, req) \
 	req = (xReq *) _XGetRequest(dpy, X_/**/name, SIZEOF(xReq))
 #endif
 
+/*
+ * MakeBigReq sets the CARD16 "req->length" to 0 and inserts a new CARD32
+ * length, after req->length, before the data in the request.  The new length
+ * includes the "n" extra 32-bit words.
+ *
+ * Do not use MakeBigReq if there is no data already in the request.
+ * req->length must already be >= 2.
+ */
 #ifdef WORD64
 #define MakeBigReq(req,n) \
     { \
     char _BRdat[4]; \
     unsigned long _BRlen = req->length - 1; \
     req->length = 0; \
     memcpy(_BRdat, ((char *)req) + (_BRlen << 2), 4); \
-    memmove(((char *)req) + 8, ((char *)req) + 4, _BRlen << 2); \
+    memmove(((char *)req) + 8, ((char *)req) + 4, (_BRlen - 1) << 2); \
     memcpy(((char *)req) + 4, _BRdat, 4); \
     Data32(dpy, (long *)&_BRdat, 4); \
     }
 #else
 #ifdef LONG64
 #define MakeBigReq(req,n) \
     { \
     CARD64 _BRdat; \
     CARD32 _BRlen = req->length - 1; \
     req->length = 0; \
     _BRdat = ((CARD32 *)req)[_BRlen]; \
-    memmove(((char *)req) + 8, ((char *)req) + 4, _BRlen << 2); \
+    memmove(((char *)req) + 8, ((char *)req) + 4, (_BRlen - 1) << 2); \
     ((CARD32 *)req)[1] = _BRlen + n + 2; \
     Data32(dpy, &_BRdat, 4); \
     }
 #else
 #define MakeBigReq(req,n) \
     { \
     CARD32 _BRdat; \
     CARD32 _BRlen = req->length - 1; \
     req->length = 0; \
     _BRdat = ((CARD32 *)req)[_BRlen]; \
-    memmove(((char *)req) + 8, ((char *)req) + 4, _BRlen << 2); \
+    memmove(((char *)req) + 8, ((char *)req) + 4, (_BRlen - 1) << 2); \
     ((CARD32 *)req)[1] = _BRlen + n + 2; \
     Data32(dpy, &_BRdat, 4); \
     }
 #endif
 #endif
 
+/*
+ * SetReqLen increases the count of 32-bit words in the request by "n",
+ * or by "badlen" if "n" is too large.
+ *
+ * Do not use SetReqLen if "req" does not already have data after the
+ * xReq header.  req->length must already be >= 2.
+ */
 #ifndef __clang_analyzer__
 #define SetReqLen(req,n,badlen) \
     if ((req->length + n) > (unsigned)65535) { \
 	if (dpy->bigreq_size) { \
 	    MakeBigReq(req,n) \
 	} else { \
 	    n = badlen; \