understanding system call dispatcher for windows?

Question

I am trying to do some reversing to find out a function call behind the scene.

While debugging using windbg I came across a call,

mov     edx,offset SharedUserData!SystemCallStub
call    dword ptr [edx]

call leads to code below,

ntdll!KiFastSystemCall:

8bd4        mov     edx,esp
0f34        sysenter

According to this documentation, eax contains the system call ordinal. and the value in eax is 11CC.

I am trying to figure out, what actually is this function which will be called. Does anyone has any idea how can I proceed further?

Answer 1

Basically you need a way of dumping the SSDT - on x32 this can be done easily. Probably the easiest way is do look for a utility which would dump the SSDT along the necessary indexes and you will see what corresponds to this particular index. Basically eax would store an index in a function table so the system disaptcher would at some point do call FunctionTable[eax] A up-to-date listing of call tables can be found here