[PATCH libX11] MakeBigReq: don't move the last word, already handled by Data32

[Peter Hutterer]

From: Karl Tomlinson <xmail at karlt.net>

MakeBigReq inserts a length field after the first 4 bytes of the request
(after req->length), pushing everything else back by 4 bytes.

The current memmove moves everything but the first 4 bytes back.
If a request aligns to the end of the buffer pointer when MakeBigReq is
invoked for that request, this runs over the buffer.
Instead, we need to memmove minus the first 4 bytes (which aren't moved),
minus the last 4 bytes (so we still align to the previous tail).

The 4 bytes that fell out are already handled with Data32, which will handle
the buffermax correctly.

The case where req->length = 1 was already not functional.

Reported by Abhishek Arya <inferno at chromium.org>.

https://bugzilla.mozilla.org/show_bug.cgi?id=803762

Reviewed-by: Jeff Muizelaar <jmuizelaar at mozilla.com>
Reviewed-by: Peter Hutterer <peter.hutterer at who-t.net>
---
 include/X11/Xlibint.h | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/include/X11/Xlibint.h b/include/X11/Xlibint.h
index 80edeec..29c1598 100644
--- a/include/X11/Xlibint.h
+++ b/include/X11/Xlibint.h
@@ -502,6 +502,14 @@ extern void *_XGetRequest(Display *dpy, CARD8 type, size_t len);
 	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) \
     { \
@@ -509,7 +517,7 @@ extern void *_XGetRequest(Display *dpy, CARD8 type, size_t len);
     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); \
     }
@@ -521,7 +529,7 @@ extern void *_XGetRequest(Display *dpy, CARD8 type, size_t len);
     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); \
     }
@@ -532,13 +540,20 @@ extern void *_XGetRequest(Display *dpy, CARD8 type, size_t len);
     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) { \
-- 
1.8.1.2