glyph.c: fix a read beyond end of heap buffer

If compiled with -fsanitize=address this showed up when running startlxde:

==11551==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60d000018fbc at pc 0x7f270a9ed57b bp 0x7fff30ef3050 sp 0x7fff30ef2800
READ of size 204 at 0x60d000018fbc thread T0
    #0 0x7f270a9ed57a  (/usr/lib/x86_64-linux-gnu/libasan.so.5+0xb857a)
    #1 0x559dafcd5c93 in FindGlyphRef ../../render/glyph.c:179
    #2 0x559dafcd705d in AddGlyph /work/nx-libs/nx-X11/programs/Xserver/hw/nxagent/NXglyph.c:71
    #3 0x559dafccc0ff in ProcRenderAddGlyphs ../../mi/../render/render.c:1186
    #4 0x559dafcbd5a5 in ProcRenderDispatch /work/nx-libs/nx-X11/programs/Xserver/hw/nxagent/NXrender.c:1689
    #5 0x559dafcbc4ea in Dispatch /work/nx-libs/nx-X11/programs/Xserver/hw/nxagent/NXdispatch.c:476
    #6 0x559dafc4e9b0 in main /work/nx-libs/nx-X11/programs/Xserver/dix/main.c:353
    #7 0x7f2708e1d09a in __libc_start_main ../csu/libc-start.c:308
    #8 0x559dafc4f5d9 in _start (/work/nx-libs/nx-X11/programs/Xserver/nxagent+0x6e5d9)

0x60d000018fbc is located 0 bytes to the right of 140-byte region [0x60d000018f30,0x60d000018fbc)
allocated by thread T0 here:
    #0 0x7f270aa1e330 in __interceptor_malloc (/usr/lib/x86_64-linux-gnu/libasan.so.5+0xe9330)
    #1 0x559dafcd646c in AllocateGlyph ../../render/glyph.c:348

This happens when two glyphs are compared via memcmp and the smaller
one happens to be identical to the beginning of the bigger one.

Newer render implementations use a sha1 hash instead of memcmp so this
patch will (hopefully) be obsolete once render gets updated.

Author: uli42

nx-X11/programs/Xserver/hw/nxagent/NXglyph.c

@@ -59,6 +59,67 @@
 
 #endif
 
+GlyphRefPtr
+FindGlyphRef (GlyphHashPtr hash, CARD32 signature, Bool match, GlyphPtr compare)
+{
+    CARD32	elt, step, s;
+    GlyphPtr	glyph;
+    GlyphRefPtr	table, gr, del;
+    CARD32	tableSize = hash->hashSet->size;
+
+    table = hash->table;
+    elt = signature % tableSize;
+    step = 0;
+    del = 0;
+    for (;;)
+    {
+	gr = &table[elt];
+	s = gr->signature;
+	glyph = gr->glyph;
+	if (!glyph)
+	{
+	    if (del)
+		gr = del;
+	    break;
+	}
+	if (glyph == DeletedGlyph)
+	{
+	    if (!del)
+		del = gr;
+	    else if (gr == del)
+		break;
+	}
+#ifdef NXAGENT_SERVER
+	else if (s == signature && match && glyph->size != compare->size)
+        {
+          /*
+           * if the glyphsize is different there's no need to do a memcmp
+           * because it will surely report difference. And even worse:
+           * it will read beyond the end of glyph under some
+           * circumstances, which can be detected when compiling with
+           * -fsanitize=address.
+           */
+        }
+#endif
+	else if (s == signature &&
+		 (!match ||
+		  memcmp (&compare->info, &glyph->info, compare->size) == 0))
+	{
+	    break;
+	}
+	if (!step)
+	{
+	    step = signature % hash->hashSet->rehash;
+	    if (!step)
+		step = 1;
+	}
+	elt += step;
+	if (elt >= tableSize)
+	    elt -= tableSize;
+    }
+    return gr;
+}
+
 void
 AddGlyph (GlyphSetPtr glyphSet, GlyphPtr glyph, Glyph id)
 {

nx-X11/programs/Xserver/render/glyph.c

@@ -144,6 +144,7 @@ GlyphInit (ScreenPtr pScreen)
     return TRUE;
 }
 
+#ifndef NXAGENT_SERVER
 GlyphRefPtr
 FindGlyphRef (GlyphHashPtr hash, CARD32 signature, Bool match, GlyphPtr compare)
 {
@@ -192,6 +193,7 @@ FindGlyphRef (GlyphHashPtr hash, CARD32 signature, Bool match, GlyphPtr compare)
     }
     return gr;
 }
+#endif
 
 CARD32
 HashGlyph (GlyphPtr glyph)