What can go wrong when you mismatch the calling convention?

Raymond Chen


Believe it or not, calling conventions is one of the things that
programs frequently get wrong. The compiler yells at you when
you mismatch a calling convention, but lazy programmers will just
stick a cast in there to get the compiler to “shut up already”.

And then Windows is stuck having to support your buggy code forever.

The window procedure

So many people misdeclare their window procedures (usually by
declaring them as __cdecl instead of __stdcall),
that the function that dispatches messages to window procedures
contains extra protection
to detect incorrectly-declared window procedures and perform the
appropriate fixup. This is the source of the mysterious 0xdcbaabcd
on the stack. The function that dispatches messages to window
procedures checks whether this value is on the stack in the correct
place. If not, then it checks whether the window procedure popped
one dword too much off the stack (if so, it fixes up the stack;
I have no idea how this messed up a window procedure could have existed),
or whether the window procedure was mistakenly declared as __cdecl
instead of __stdcall (if so, it pops the parameters off the stack that
the window procedure was supposed to do).

DirectX callbacks

Many DirectX functions use callbacks, and people once again misdeclared
their callbacks as __cdecl instead of __stdcall, so the DirectX
enumerators have to do special stack cleanup for those bad functions.


I remember there was one program that decided to declare their
CreateViewWindow function incorrectly, and somehow they managed
to trick the compiler into accepting it!

class BuggyFolder : public IShellFolder ... {
 // wrong function signature!
 HRESULT CreateViewObject(HWND hwnd) { return S_OK; }

Not only did they get the function signature wrong, they
returned S_OK even though they failed to do anything!
I had to add extra code to clean up the stack after calling this
function, as well as verify that the return value wasn’t a lie.

Rundll32.exe entry points

The function signature required for functions called by rundll32.exe
is documented in this Knowledge Base article
That hasn’t stopped people from using rundll32 to call random functions
that weren’t designed to be called by rundll32,

like user32 LockWorkStation

user32 ExitWindowsEx

Let’s walk through what happens when you try to use rundll32.exe
to call a function like ExitWindowsEx:

The rundll32.exe program parses its command line and calls the
ExitWindowsEx function on the assumption that the function is written
like this:

void CALLBACK ExitWindowsEx(HWND hwnd, HINSTANCE hinst,
       LPSTR pszCmdLine, int nCmdShow);

But it isn’t. The actual function signature for ExitWindowsEx is

BOOL WINAPI ExitWindowsEx(UINT uFlags, DWORD dwReserved);

What happens? Well, on entry to ExitWindowsEx, the stack
looks like this:

.. rest of stack ..
return address<- ESP

However, the function is expecting to see

.. rest of stack ..
return address<- ESP

What happens? The hwnd passed by rundll32.exe gets misinterpreted
as uFlags and the hinst gets misinterpreted as dwReserved.
Since window handles are pseudorandom, you end up passing random
flags to ExitWindowsEx. Maybe today it’s EWX_LOGOFF, tomorrow it’s
EWX_FORCE, the next time it might be EWX_POWEROFF.

Now suppose that the function manages to return. (For example,
the exit fails.) The ExitWindowsEx function cleans two parameters
off the stack, unaware that it was passed four. The resulting stack

.. rest of stack ..
nCmdShow(garbage not cleaned up)
pszCmdLine<- ESP (garbage not cleaned up)

Now the stack is corrupted and really fun things happen.
For example, suppose the thing at “.. rest of the stack ..” is
a return address. Well, the original code is going to execute
a “return” instruction to return through that return address,
but with this corrupted stack, the “return” instruction will
instead return to a command line and attempt to execute it as if
it were code.

Random custom functions

An anonymous commenter exported a function as __cdecl but treated it as if
it were __stdcall
. This will seem to work, but on return, the stack
will be corrupted (because the caller is expecting a __stdcall function
that cleans the stack, but what it gets is a __cdecl funcion that doesn’t),
and bad things will happen as a result.

Okay, enough with the examples; I think you get the point.
Here are some questions I’m sure you’re asking:

Why doesn’t the compiler catch all these errors?

It does. (Well, not the rundll32 one.)
But people have gotten into the habit of just inserting the
function cast to get the compiler to shut up.

a random example I found

   WPARAM wParam, LPARAM lParam);

This is the incorrect function signature for a dialog procedure.
correct signature

INT_PTR CALLBACK DialogProc(HWND hwndDlg, UINT uMsg,
    WPARAM wParam, LPARAM lParam);

You start with

          hWnd, DlgProc);

but the compiler rightly spits out the error message

error C2664: 'DialogBoxParamA' : cannot convert parameter 4

so you fix it by slapping a cast in to make the compiler
shut up:

          hWnd, reinterpret_cast<DLGPROC>(DlgProc));

“Aw, come on, who would be so stupid as to insert a cast to
make an error go away without actually fixing the error?”

Apparently everyone.

I stumbled across

this page that does exactly the same

and this one in German which gets not only the return value wrong,
but also misdeclares the third and fourth parameters

and this one in Japanese

It’s as easy to fix (incorrectly) as 1-2-3

How did programs with these bugs ever work at all?
Certainly these programs worked to some degree or people would have
noticed and fixed the bug.
How can the program survive a corrupted stack?

I’ll answer this question tomorrow.

Raymond Chen
Raymond Chen

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