System Calls in windows & Native API?

Question

Recently I've been using lot of Assembly language in *NIX operating systems. I was wondering about the windows domain.

---

Calling convention in linux:

mov $SYS_Call_NUM, %eax
mov $param1 , %ebx
mov $param2 , %ecx
int $0x80

Thats it. That is how we should make a system call in linux.

Reference of all system calls in linux:

Regarding which $SYS_Call_NUM & which parameters we can use this reference : http://docs.cs.up.ac.za/programming/asm/derick_tut/syscalls.html

OFFICIAL Reference : http://kernel.org/doc/man-pages/online/dir_section_2.html

---

Calling convention in Windows:

???

Reference of all system calls in Windows:

???

Unofficial : http://www.metasploit.com/users/opcode/syscalls.html , but how do I use these in assembly unless I know the calling convention.

OFFICIAL : ???

• If you say, they didn't documented it. Then how is one going to write libc for windows without knowing system calls? How is one gonna do Windows Assembly programming? Atleast in the driver programming one needs to know these. right?

---

Now, whats up with the so called Native API? Is Native API & System calls for windows both are different terms referring to same thing? In order to confirm I compared these from two UNOFFICIAL Sources

System Calls: http://www.metasploit.com/users/opcode/syscalls.html

Native API: http://undocumented.ntinternals.net/aindex.html

My observations:

1. All system calls are beginning with letters Nt where as Native API is consisting of lot of functions which are not beginning with letters Nt.
2. System Call of windows are subset of Native API. System calls are just part of Native API.

Can any one confirm this and explain.

EDIT:

There was another answer. It was a 2nd answer. I really liked it but I don't know why answerer has deleted it. I request him to repost his answer.

Answer 1

If you're doing assembly programming under Windows you don't do manual syscalls. You use NTDLL and the Native API to do that for you.

The Native API is simply a wrapper around the kernelmode side of things. All it does is perform a syscall for the correct API.

You should NEVER need to manually syscall so your entire question is redundant.

Linux syscall codes do not change, Windows's do, that's why you need to work through an extra abstraction layer (aka NTDLL).

EDIT:

Also, even if you're working at the assembly level, you still have full access to the Win32 API, there's no reason to be using the NT API to begin with! Imports, exports, etc all work just fine in assembly programs.

EDIT2:

If you REALLY want to do manual syscalls, you're going to need to reverse NTDLL for each relevant Windows version, add version detection (via the PEB), and perform a syscall lookup for each call.

However, that would be silly. NTDLL is there for a reason.

People have already done the reverse-engineering part: see https://j00ru.vexillium.org/syscalls/nt/64/ for a table of system-call numbers for each Windows kernel. (Note that the later rows do change even between versions of Windows 10.) Again, this is a bad idea outside of personal-use-only experiments on your own machine to learn more about asm and/or Windows internals. Don't inline system calls into code that you distribute to anyone else.

Answer 2

The other thing you need to know about the windows syscall convention is that as I understand it the syscall tables are generated as part of the build process. This means that they can simply change - no one tracks them. If someone adds a new one at the top of the list, it doesn't matter. NTDLL still works, so everyone else who calls NTDLL still works.

Even the mechanism used to perform syscalls (which int, or sysenter) is not fixed in stone and has changed in the past, and I think that once upon a time the same version of windows used different DLLs which used different entry mechanisms depending on the CPU in the machine.

Answer 3

Windows system calls are performed by calling into system DLLs such as kernel32.dll or gdi32.dll, which is done with ordinary subroutine calls. The mechanisms for trapping into the OS privileged layer is undocumented, but that is okay because DLLs like kernel32.dll do this for you.

And by system calls, I'm referring to documented Windows API entry points like CreateProcess() or GetWindowText(). Device drivers will generally use a different API from the Windows DDK.

Answer 4

I was interested in doing a windows API call in assembly with no imports (as an educational exercise), so I wrote the following FASM assembly to do what NtDll!NtCreateFile does. It's a rough demonstration on my 64-bit version of Windows (Win10 1803 Version 10.0.17134), and it crashes out after the call, but the return value of the syscall is zero so it is successful. Everything is set up per the Windows x64 calling convention, then the system call number is loaded into RAX, and then it's the syscall assembly instruction to run the call. My example creates the file c:\HelloWorldFile_FASM, so it has to be run "as administrator".

format PE64 GUI 4.0


entry start


section '.text' code readable executable


 start: 
 ;puting the first four parameters into the right registers

                            mov rcx, _Handle
                            mov rdx, [_access_mask]
                            mov r8, objectAttributes
                            mov r9, ioStatusBlock

 ;I think we need 1 stack word of padding:

                            push 0x0DF0AD8B


 ;pushing the other params in reverse order:

                            push [_eaLength]
                            push [_eaBuffer]
                            push [_createOptions]
                            push [_createDisposition]
                            push [_shareAcceses]
                            push [_fileAttributes]
                            push [_pLargeInterger]

 ;adding the shadow space (4x8)

 ;                               push 0x0
 ;                               push 0x0
 ;                               push 0x0
 ;                               push 0x0

 ;pushing the 4 register params into the shadow space for ease of debugging

                            push r9
                            push r8
                            push rdx
                            push rcx

 ;now pushing the return address to the stack:

                            push endOfProgram

                            mov r10, rcx ;copied from ntdll!NtCreateFile, not sure of the reason for this
                            mov eax, 0x55
                            syscall

 endOfProgram:
                            retn




 section '.data' data readable writeable

 ;parameters------------------------------------------------------------------------------------------------

 _Handle                         dq      0x0
 _access_mask                    dq      0x00000000c0100080
 _pObjectAttributes              dq      objectAttributes        ; at 00402058
 _pIoStatusBlock                 dq           ioStatusBlock
 _pLargeInterger                 dq      0x0
 _fileAttributes                 dq      0x0000000000000080
 _shareAcceses                   dq      0x0000000000000002
 _createDisposition              dq      0x0000000000000005
 _createOptions                  dq      0x0000000000000060
 _eaBuffer                       dq      0x0000000000000000       ; "optional" param
 _eaLength                       dq      0x0000000000000000

 ;----------------------------------------------------------------------------------------------------------


                            align   16
 objectAttributes:
 _oalength                       dq      0x30
 _rootDirectory                  dq      0x0
 _objectName                     dq           unicodeString
 _attributes                     dq      0x40
 _pSecurityDescriptor            dq      0x0
 _pSecurityQualityOfService      dq      securityQualityOfService


 unicodeString:
 _unicodeStringLength            dw      0x34
 _unicodeStringMaxumiumLength    dw      0x34, 0x0, 0x0
 _pUnicodeStringBuffer           dq      _unicodeStringBuffer


 _unicodeStringBuffer            du      '\??\c:\HelloWorldFile_FASM'       ; may need to "run as adinistrator" for the file create to work.



 ioStatusBlock:
 _status_pointer                 dq      0x0
 _information                    dq      0x0


 securityQualityOfService:
 _sqlength                       dd      0xC
 _impersonationLevel             dd      0x2
 _contextTrackingMode            db      0x1
 _effectiveOnly                  db      0x1, 0x0, 0x0

I used the documentation for Ntdll!NtCreateFile, and I also used the kernel debugger to look at and copy a lot of the params.

__kernel_entry NTSTATUS NtCreateFile(
  OUT PHANDLE                      FileHandle,
  IN ACCESS_MASK                   DesiredAccess,
  IN POBJECT_ATTRIBUTES            ObjectAttributes,
  OUT PIO_STATUS_BLOCK             IoStatusBlock,
  IN PLARGE_INTEGER AllocationSize OPTIONAL,
  IN ULONG                         FileAttributes,
  IN ULONG                         ShareAccess,
  IN ULONG                         CreateDisposition,
  IN ULONG                         CreateOptions,
  IN PVOID EaBuffer                OPTIONAL,
  IN ULONG                         EaLength
);

Answer 5

OFFICIAL Calling convention in Windows: http://msdn.microsoft.com/en-us/library/7kcdt6fy.aspx

(hope this link survives in the future; if it doesn't, just search for "x64 Software Conventions" on MSDN).

The function calling convention differs in Linux & Windows x86_64. In both ABIs, parameters are preferably passed via registers, but the registers used differ. More on the Linux ABI can be found at http://www.x86-64.org/documentation/abi.pdf