[cff] Rework the stream limit checks.

The old stream limit checks, before 6986ddac1e, were good but
pointless for the crafted t2_strings.  Checking limits there is
not necessary as they are created to hold all data.  By using two
conditions, we can detect the actual crossing of the stream boundary
as appropriate for the stream pointer only.  The t2_strings parsing
will not be triggering these checks.

* src/cff/cffparse.c (cff_parser_within_limits): Removed.
(cff_parse_real, cff_parse_integer): Redesign the stream limit check.
(cff_parse_num, do fixed, cff_parse_fixed_dynamic): Update callers.

src/cff/cffparse.c

@@ -110,55 +110,14 @@
   }
 
 
-  /* Assuming `first >= last'. */
-
-  static FT_Error
-  cff_parser_within_limits( CFF_Parser  parser,
-                            FT_Byte*    first,
-                            FT_Byte*    last )
-  {
-#ifndef CFF_CONFIG_OPTION_OLD_ENGINE
-
-    /* Fast path for regular FreeType builds with the "new" engine; */
-    /*   `first >= parser->start' can be assumed.                   */
-
-    FT_UNUSED( first );
-
-    return last < parser->limit ? FT_Err_Ok : FT_THROW( Invalid_Argument );
-
-#else /* CFF_CONFIG_OPTION_OLD_ENGINE */
-
-    FT_ListNode  node;
-
-
-    if ( first >= parser->start &&
-         last  <  parser->limit )
-      return FT_Err_Ok;
-
-    node = parser->t2_strings.head;
-
-    while ( node )
-    {
-      CFF_T2_String  t2 = (CFF_T2_String)node->data;
-
-
-      if ( first >= t2->start &&
-           last  <  t2->limit )
-        return FT_Err_Ok;
-
-      node = node->next;
-    }
-
-    return FT_THROW( Invalid_Argument );
-
-#endif /* CFF_CONFIG_OPTION_OLD_ENGINE */
-  }
-
+  /* The parser limit checks in the next two functions are supposed */
+  /* to detect the immediate crossing of the stream boundary.  They */
+  /* shall not be triggered from the distant t2_strings buffers.    */
 
   /* read an integer */
   static FT_Long
-  cff_parse_integer( CFF_Parser  parser,
-                     FT_Byte*    start )
+  cff_parse_integer( FT_Byte*  start,
+                     FT_Byte*  limit )
   {
     FT_Byte*  p   = start;
     FT_Int    v   = *p++;
@@ -167,14 +126,14 @@
 
     if ( v == 28 )
     {
-      if ( cff_parser_within_limits( parser, p, p + 1 ) )
+      if ( p + 2 > limit && limit >= p )
         goto Bad;
 
       val = (FT_Short)( ( (FT_UShort)p[0] << 8 ) | p[1] );
     }
     else if ( v == 29 )
     {
-      if ( cff_parser_within_limits( parser, p, p + 3 ) )
+      if ( p + 4 > limit && limit >= p )
         goto Bad;
 
       val = (FT_Long)( ( (FT_ULong)p[0] << 24 ) |
@@ -188,14 +147,14 @@
     }
     else if ( v < 251 )
     {
-      if ( cff_parser_within_limits( parser, p, p ) )
+      if ( p + 1 > limit && limit >= p )
         goto Bad;
 
       val = ( v - 247 ) * 256 + p[0] + 108;
     }
     else
     {
-      if ( cff_parser_within_limits( parser, p, p ) )
+      if ( p + 1 > limit && limit >= p )
         goto Bad;
 
       val = -( v - 251 ) * 256 - p[0] - 108;
@@ -244,10 +203,10 @@
 
   /* read a real */
   static FT_Fixed
-  cff_parse_real( CFF_Parser  parser,
-                  FT_Byte*    start,
-                  FT_Long     power_ten,
-                  FT_Long*    scaling )
+  cff_parse_real( FT_Byte*  start,
+                  FT_Byte*  limit,
+                  FT_Long   power_ten,
+                  FT_Long*  scaling )
   {
     FT_Byte*  p = start;
     FT_Int    nib;
@@ -282,7 +241,7 @@
         p++;
 
         /* Make sure we don't read past the end. */
-        if ( cff_parser_within_limits( parser, p, p ) )
+        if ( p + 1 > limit && limit >= p )
           goto Bad;
       }
 
@@ -319,7 +278,7 @@
           p++;
 
           /* Make sure we don't read past the end. */
-          if ( cff_parser_within_limits( parser, p, p ) )
+          if ( p + 1 > limit && limit >= p )
             goto Bad;
         }
 
@@ -358,7 +317,7 @@
           p++;
 
           /* Make sure we don't read past the end. */
-          if ( cff_parser_within_limits( parser, p, p ) )
+          if ( p + 1 > limit && limit >= p )
             goto Bad;
         }
 
@@ -525,7 +484,7 @@
     if ( **d == 30 )
     {
       /* binary-coded decimal is truncated to integer */
-      return cff_parse_real( parser, *d, 0, NULL ) >> 16;
+      return cff_parse_real( *d, parser->limit, 0, NULL ) >> 16;
     }
 
     else if ( **d == 255 )
@@ -551,7 +510,7 @@
     }
 
     else
-      return cff_parse_integer( parser, *d );
+      return cff_parse_integer( *d, parser->limit );
   }
 
 
@@ -562,10 +521,10 @@
             FT_Long     scaling )
   {
     if ( **d == 30 )
-      return cff_parse_real( parser, *d, scaling, NULL );
+      return cff_parse_real( *d, parser->limit, scaling, NULL );
     else
     {
-      FT_Long  val = cff_parse_integer( parser, *d );
+      FT_Long  val = cff_parse_integer( *d, parser->limit );
 
 
       if ( scaling )
@@ -630,14 +589,14 @@
     FT_ASSERT( scaling );
 
     if ( **d == 30 )
-      return cff_parse_real( parser, *d, 0, scaling );
+      return cff_parse_real( *d, parser->limit, 0, scaling );
     else
     {
       FT_Long  number;
       FT_Int   integer_length;
 
 
-      number = cff_parse_integer( parser, d[0] );
+      number = cff_parse_integer( *d, parser->limit );
 
       if ( number > 0x7FFFL )
       {