Tuesday, May 5, 2015

Getting an object type on Windows 10 Technical Preview Build 10074

   Windows 10 Technical Preview Build 10074 came with a surprise. A bit of history - Windows 7 introduced a new way for retrieving an object type by object address, the object type pointer Type in OBJECT_HEADER was replaced with the TypeIndex which is an index in ObTypeIndexTable, this saved 3 ( 32 bit) or 7 (on 64 bit) bytes compared to a pointer. Windows 10 Build 10074 added a new feature, the TypeIndex value is not an index but a result of a binary operation between an index in ObTypeIndexTable, the second lowest byte of the object address and a value from ObHeaderCookie. The actual reason of this is not yet clear for me but it looks like an attempt to reduce an inter CPU cache coherency traffic by spreading the ObTypeIndexTable to contain copies of the object types and multiplexing access based on the object address. The exported ObGetObjectType function can be used to retrieve an object type address. Lets take a look on ObGetObjectType.

lea         rax,[rcx-30h]
movzx   ecx,byte ptr [rcx-18h]
shr        rax,8
movzx   eax,al
xor        rax,rcx
movzx   ecx,byte ptr [nt!ObHeaderCookie (fffff802`eae3d42c)]
xor        rax,rcx
lea         rcx,[nt!ObTypeIndexTable (fffff802`eae3d8e0)]
mov       rax,qword ptr [rcx+rax*8]

which can be written in C as ( where XOR(a,b) is a^b )

ObGetObjectType( __in PVOID Object )
UCHAR    Index = XOR( Header->TypeIndex, (UCHAR)(Header>>8) );
       UCHAR    Cookie= *(PUCHAR)ObHeaderCookie;

        return  ObTypeIndexTable[ XOR(Index, Cookie) ];

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