[sfnt] Fix broken pointer overflow checks.

Many compilers such as gcc and clang optimize away pointer overflow
checks `p + n < p', because pointer overflow is undefined behavior.
Use a safe form `n > p_limit - p' instead.

Also avoid possible integer overflow issues, for example, using
`num_glyphs > ( p_limit - p ) / 2' rather than `num_glyphs * 2'
given a large `num_glyphs'.

* src/sfnt/ttsbit0.c (tt_sbit_decoder_load_image): Implement it.

ChangeLog

@@ -1,6 +1,20 @@
+2013-01-25  Xi Wang  <xi.wang@gmail.com>
+
+	[sfnt] Fix broken pointer overflow checks.
+
+	Many compilers such as gcc and clang optimize away pointer overflow
+	checks `p + n < p', because pointer overflow is undefined behavior. 
+	Use a safe form `n > p_limit - p' instead.
+
+	Also avoid possible integer overflow issues, for example, using
+	`num_glyphs > ( p_limit - p ) / 2' rather than `num_glyphs * 2'
+	given a large `num_glyphs'.
+
+	* src/sfnt/ttsbit0.c (tt_sbit_decoder_load_image): Implement it.
+
 2013-01-25  Werner Lemberg  <wl@gnu.org>
 
-	Fix `make multi'
+	[base] Fix `make multi'.
 
 	* src/base/ftoutln.c, src/base/fttrigon.c: Include
 	FT_INTERNAL_CALC_H.

src/sfnt/ttsbit0.c

@@ -823,11 +823,11 @@
     image_offset = FT_NEXT_ULONG( p );
 
     /* overflow check */
-    if ( decoder->eblc_base + decoder->strike_index_array + image_offset <
-           decoder->eblc_base )
+    p = decoder->eblc_base + decoder->strike_index_array;
+    if ( image_offset > (FT_ULong)( p_limit - p ) )
       goto Failure;
 
-    p = decoder->eblc_base + decoder->strike_index_array + image_offset;
+    p += image_offset;
     if ( p + 8 > p_limit )
       goto NoBitmap;
 
@@ -894,11 +894,8 @@
 
         num_glyphs = FT_NEXT_ULONG( p );
 
-        /* overflow check */
-        if ( p + ( num_glyphs + 1 ) * 4 < p )
-          goto Failure;
-
-        if ( p + ( num_glyphs + 1 ) * 4 > p_limit )
+        /* overflow check for p + ( num_glyphs + 1 ) * 4 */
+        if ( num_glyphs > (FT_ULong)( ( ( p_limit - p ) >> 2 ) - 1 ) )
           goto NoBitmap;
 
         for ( mm = 0; mm < num_glyphs; mm++ )
@@ -936,11 +933,8 @@
 
         num_glyphs = FT_NEXT_ULONG( p );
 
-        /* overflow check */
-        if ( p + 2 * num_glyphs < p )
-          goto Failure;
-
-        if ( p + 2 * num_glyphs > p_limit )
+        /* overflow check for p + 2 * num_glyphs */
+        if ( num_glyphs > (FT_ULong)( ( p_limit - p ) >> 1 ) )
           goto NoBitmap;
 
         for ( mm = 0; mm < num_glyphs; mm++ )