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Re: Custom stack for function invocation (any signature)

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About an hour ago, I wrote:
> Half an hour ago, I posted:
> > 
> > 
> > // Step 4: Invoke the function 
> > " call rax \n" // --- Invoke the function!
> Ah crap, I realised just now that I'm editing the stack here. The return address is pushed onto the stack when I invoke the 'call' instruction. This hasn't caused a malfunction because none of the function arguments are on the stack. The code would malfunction if the target function were to have more than 6 parameters, as the 7th would be pushed onto the stack. 
> 
> Somehow I need to have a 'jmp' instruction instead of a 'call' instruction here. . .
>  and yet somehow have control over where it jumps back to. I need to make a
> change to the caller of the caller.


I have an idea. When the assembler function is entered, the return address is at the top of the stack. So the first thing I will do is copy the return address to a global thread_local variable. Next I will edit the return address at the top of the stack. Then I will change the 'call' to a 'jmp', and then later when it 'ret' back to my own location, I will then jump back to the address that I stored in the thread_local variable.

Furthermore, if the target function returns a very very big struct by value, and therefore pushes a portion of the return value onto the stack, then I can compare the stack pointer before and after the call, something like:

    old = get_stack_ptr();
    InvokeTargetFunction();
    new = get_stack_ptr();
    if ( new < old )
    {
        bytes_to_copy = old - new;
        memcpy( original_stack -= bytes_to_copy, custom_stack, bytes_to_copy );
    }

I'll try code this when I'm home later.

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Custom stack for function invocation (any signature) Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2023-07-13 07:38 -0700
  Re: Custom stack for function invocation (any signature) Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2023-07-13 08:09 -0700
    Re: Custom stack for function invocation (any signature) Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2023-07-13 09:42 -0700
      Re: Custom stack for function invocation (any signature) Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2023-07-13 14:50 -0700
        Re: Custom stack for function invocation (any signature) Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2023-07-14 02:19 -0700
          Re: Custom stack for function invocation (any signature) Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2023-07-14 05:01 -0700
            Re: Custom stack for function invocation (any signature) Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2023-07-14 18:55 -0700
              Re: Custom stack for function invocation (any signature) Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2023-07-15 06:42 -0700

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