Over a million developers have joined DZone.
{{announcement.body}}
{{announcement.title}}

P/Invoke Stack Imbalance MDA

DZone's Guide to

P/Invoke Stack Imbalance MDA

·
Free Resource

More than a year after writing my first post touching on the subject of Managed Debugging Assistants (MDA) through the “Callback on Garbage Collected Delegate” case study, it’s time for a brief mention of another useful MDA – “P/Invoke Stack Imbalance”.

This MDA fires whenever a P/Invoke call causes an imbalance on the stack. What does a stack imbalance mean? The top of the thread’s stack is pointed to by the ESP register (RSP on x64), and a stack imbalance occurs if the ESP value before making a function call is not the same as the ESP value after the function returns.

Why would a P/Invoke call cause a stack imbalance? Well, recall that parameters are typically passed on the stack when doing P/Invoke calls, and there is no single calling convention that every API in the world adheres to. For more information on x86 calling conventions, here’s a great (and lengthy) read [less details]; the two most popular calling conventions are stdcall and cdecl, which differ on a very important aspect: who is responsible for removing the function parameters from the stack after the function call completes.

In the cdecl calling convention (which is the default for C/C++ functions to this day), the caller is responsible for removing parameters from the stack. In other words, whenever the compiler encounters a function call and the function uses the cdecl calling convention, it will emit assembly instructions to remove the parameters from the stack (e.g. “add esp, 8” to remove two integer parameters from the stack).

On the other hand, in the stdcall calling convention (which is used almost exclusively by Win32 APIs), the callee is responsible for removing parameters from the stack. When the compiler emits code for an stdcall function call, it does not include instructions for removing parameters from the stack, and relies instead on the called function to do so.

Observe that two things can go wrong here, even if we assume only two calling conventions. Suppose there is a function void f(int) using the cdecl calling convention and you call it (mistakenly!) with the stdcall calling convention. After calling the function, the stack contains an extra four bytes of garbage – which is undesired, but will not crash the program. Here is a conceptual view of the stack before the call:

local var of caller
local var of caller
saved EBP of caller
ret address of caller
param of caller
param of caller

During the call:

local var of f
saved EBP of f
ret address of f
param of f
local var of caller
local var of caller
saved EBP of caller
ret address of caller
param of caller
param of caller

After the call:

param of f (garbage!)
local var of caller
local var of caller
saved EBP of caller
ret address of caller
param of caller
param of caller

Now suppose that there is a function void g(int) using the stdcall calling convention and you call it with the cdecl calling convention. The function cleans up the four byte parameter from the stack, and so does the caller – now, if additional functions are called, the stack space previously reserved to the calling function may be reused for other data (e.g. return address for a function).

When using P/Invoke, there’s ample chance to create a stack imbalance, typically because of a calling convention mismatch. The [DllImport] attribute has an explicit property called CallingConvention, which you must set to the appropriate value (the default is Stdcall!).

The “P/Invoke Stack Imbalance” MDA detects the condition where the stack becomes unbalanced after a P/Invoke call, and raises an exception in the debugger (as any MDA does). This gives you an immediate opportunity to fix the problematic P/Invoke signature.

image

To fix the signature, the best bet is to look up the documentation and the header files for the appropriate calling convention. If you can’t figure out the problem, try to call the native function from native code and look at the assembly generated at the call site. Perhaps you’ll be able to see a parameter size mismatch or an unfamiliar calling convention being used.

Topics:

Published at DZone with permission of Sasha Goldshtein, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

THE DZONE NEWSLETTER

Dev Resources & Solutions Straight to Your Inbox

Thanks for subscribing!

Awesome! Check your inbox to verify your email so you can start receiving the latest in tech news and resources.

X

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}