Author Topic: Manual entry point example.  (Read 767 times)

hutch--

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Manual entry point example.
« on: December 31, 2017, 07:06:50 AM »
For those hardy souls who want to manually construct their stack frame, here is a simple example of why I prefer to use the ENTER / LEAVE pair instead of doing it manually. If you dis-assemble this simple code you will see why.

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    include \masm32\include64\masm64rt.inc

    .code

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 NOSTACKFRAME                       ; turn the stackframe off

 entry_point proc

    push rbp                        ; manually construct stack frame
    mov rbp, rsp                    ; store stack pointer in rbp
    sub rsp, 128                    ; allocate local space

    mov QWORD PTR [rsp-32], 0       ; write value to local

  ; ---------------------------
  ; call a stackframe procedure
  ; ---------------------------
    invoke showtext,"The time has come, the walrus said, to speak of many things"
    waitkey                         ; call a library procedure

  ; -----------------------------------------------
  ; call API with a manually defined local variable
  ; -----------------------------------------------
    invoke ExitProcess,QWORD PTR [rsp-32]

    mov rsp, rbp                    ; exit the stack frame
    pop rbp

    ret

 entry_point endp

 STACKFRAME                         ; turn the stack frame back on again

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 showtext proc text:QWORD

    conout text,lf,lf               ; display the text

    ret

 showtext endp

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    end
hutch at movsd dot com
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jj2007

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Re: Manual entry point example.
« Reply #1 on: December 31, 2017, 07:41:11 AM »
Works fine here. What exactly should go wrong? Can you post the exe?

hutch--

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Re: Manual entry point example.
« Reply #2 on: December 31, 2017, 07:51:21 AM »
Done.
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jj2007

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Re: Manual entry point example.
« Reply #3 on: December 31, 2017, 08:04:26 AM »
It works like a charm, even in the debugger. What is supposed to go wrong here?

felipe

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Re: Manual entry point example.
« Reply #4 on: December 31, 2017, 08:40:57 AM »
I don't know (almost nothing) of 64 bits yet, but looks pretty much (this code example) as 32 bits. This:

Code: [Select]
mov QWORD PTR [rsp-32], 0       ; write value to local
Shouldn't be this?:

Code: [Select]
mov QWORD PTR [rbp-32], 0       ; write value to local
And then this:

Code: [Select]
invoke ExitProcess,QWORD PTR [rsp-32]
this instead?:

Code: [Select]
invoke ExitProcess,QWORD PTR [rbp-32]
Felipe.

AW

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Re: Manual entry point example.
« Reply #5 on: December 31, 2017, 08:49:33 AM »
I don't know (almost nothing) of 64 bits yet, but looks pretty much (this code example) as 32 bits. This:
:biggrin: of course.

sinsi

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Re: Manual entry point example.
« Reply #6 on: December 31, 2017, 09:10:25 AM »
This is not local
Code: [Select]
    mov QWORD PTR [rsp-32], 0       ; write value to local
I can walk on water but stagger on beer.

felipe

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Re: Manual entry point example.
« Reply #7 on: December 31, 2017, 09:31:43 AM »
I don't know (almost nothing) of 64 bits yet, but looks pretty much (this code example) as 32 bits. This:
:biggrin: of course.

 :biggrin: Come on, at least tell me which of the statements above (or both) are you referring.  :biggrin:
Felipe.

jj2007

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Re: Manual entry point example.
« Reply #8 on: December 31, 2017, 09:34:34 AM »
This is not local
Code: [Select]
    mov QWORD PTR [rsp-32], 0       ; write value to local

Right, it's somewhere in the wild. But that should cause any trouble (and it doesn't, indeed), because it's below the stackpointer.
The code works just fine, so I really wonder what is Hutch' intention here ;)

sinsi

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Re: Manual entry point example.
« Reply #9 on: December 31, 2017, 09:47:53 AM »
Worst case (admittedly stretching things here)
Code: [Select]
;rsp = stackbase + 128 + 16
   sub rsp, 128                    ; allocate local space
;rsp = stackbase + 16
   mov QWORD PTR [rsp-32], 0       ; write value to local
;rsp-32 = stackbase - 16 = access violation?

No harm done usually, since if your stack is that low you have other issues, but not a good habit to form methinks.
I can walk on water but stagger on beer.

hutch--

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Re: Manual entry point example.
« Reply #10 on: December 31, 2017, 12:48:45 PM »
 :biggrin:

> so I really wonder what is Hutch' intention here

It was supposed to work OK but the answer was in the original post "why I prefer to use the ENTER / LEAVE pair instead of doing it manually".

Sinsi,
You are right but it was not the point of the example.
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sinsi

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Re: Manual entry point example.
« Reply #11 on: December 31, 2017, 01:43:25 PM »
I also think that [rsp-32] might be clobbered by the conout call, which would eventually end up calling a windows function.
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hutch--

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Re: Manual entry point example.
« Reply #12 on: December 31, 2017, 04:08:37 PM »
About the only problem  I can see with using RSP at the entry point instead of RBP is you have to use addresses for RSP based local values lower than with RBP.

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    include \masm32\include64\masm64rt.inc

    .code

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 NOSTACKFRAME

 entry_point proc

    push rbp                                ; align the stack
    mov rbp, rsp                            ; store stack pointer in rbp
    sub rsp, 128                            ; allocate local space

    mov QWORD PTR [rbp-16], 0
    mov QWORD PTR [rbp-24], 1234

    invoke showtext,"The time has come, the walrus said, to speak of many things"
    conout str$(QWORD PTR [rbp-16]),lf
    conout str$(QWORD PTR [rbp-24]),lf
    waitkey

    invoke ExitProcess,QWORD PTR [rbp-16]

    mov rsp, rbp                            ; never gets here
    pop rbp

    ret

 entry_point endp

 STACKFRAME

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 showtext proc text:QWORD

    LOCAL unused1   :QWORD
    LOCAL unused2   :QWORD

    mov unused1, 100
    mov unused2, 200

    conout text,lf                          ; display the text

    ret

 showtext endp

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    end
hutch at movsd dot com
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