[ros-kernel] csrss explained

[Aliberti E. <ea...@us...>]

CSRSS Explained
---------------

This is very basic overview of the csrss.exe internals.

A thing to keep in mind is that, at the origins of NT, there was a 
single user-mode server that made environment subsystems live. That 
is why the meta-server is still called "client/server runtime" (CSR) 
even if actually now is the runtime exclusively for the Win32 server.

Today you can see its command line in

HKLM\SYSTEM\CurrentControlSet\Control\
  Session Manager\Subsystems\Windows

I write it down here (from an NT 4.0 system):

%SystemRoot%\system32\csrss.exe
  ObjectDirectory=\Windows
  SharedSection=1024,3072
  Windows=On
  SubSystemType=Windows
  ServerDll=basesrv,1
  ServerDll=winsrv:UserServerDllInitialization,3
  ServerDll=winsrv:ConServerDllInitialization,2
  ProfileControl=Off
  MaxRequestThreads=16

In those days, if I was going to write a new environment subsystem, I 
just needed to write a DLL and add one more parameter to that key

ServerDll={module}[:{entrypoint}],{subsystem_id}

where

{module} is the server DLL file name without ".dll" suffix;

{entrypoint} is optional and needs to be specified only if the DLL 
does not export the symbol "ServerDllInitialization" (winsrv.dll 
contains two servers);

{subsystem_id} is a nonnegative integer to be used in 
NTDLL.CsrClientCallServer to specify the subsystem to the which the 
LPC message is sent to:

The key element in this entry is the subsystem_id, because it allows 
the client side DLLs to communicate with a server side DLL loaded by 
csrss.exe.

Id 0 is only used in NTDLL.DLL itself to call service code in 
CSRSS.EXE;

Id 1 and 2 are used in KERNEL32.DLL;

Id 3 is used in USER32.DLL and probably other libraries.

To call service number n in a server DLL loaded with server number m 
you just need to fill a CSR_CCS object and pass it (by value) as the 
3rd parameter of NTDLL.CsrClientCallServer().

/* CSRSS basics */

typedef struct _CSR_CCS
{
    WORD  Index;	   // API number 
    WORD  ServerId;  // Server's ID

} CSR_CCS, * PCSR_CCS;

#define CSR_SERVER_INTERNAL         0
#define CSR_SERVER_WINDOWS_BASE     1
#define CSR_SERVER_WINDOWS_CONSOLE  2
#define CSR_SERVER_WINDOWS_USER     3

NTSTATUS STDCALL
CsrClientCallServer(
    PVOID   RequestData,
    PVOID   Unknown OPTIONAL, // Csr capture facility related
    CSR_CCS SvrApi,
    ULONG   SizeOfRequestData
    );

/* Pseudo code for a call:

CSR_CCS svrapi;
NTSTATUS Status;
DWORD data[16];

svrapi.ServerId = CSR_SERVER_WINDOWS_CONSOLE;
svrapi.Index = 0x0208; // GetConsoleMode()

Status = CsrClientCallServer (data, NULL, svrapi, sizeof data);

*/

Just like in ReactOS, in NT, NTDLL.CsrClientCallServer() uses the LPC 
facility.