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Index > Windows > Why is MSVCRT not found?

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NanoBytes



Joined: 02 Jun 2011
Posts: 57
Location: Iowa, United States
NanoBytes
Code:
format PE console
include 'win32ax.inc'

.code
start:
        invoke AllocConsole


        invoke GetProcAddress, <invoke GetModuleHandle,"MSVCRT.dll">, "getchar"
        stdcall EAX
        invoke ExitProcess,0
.end start
    

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He is no fool who gives what he cannot
keep to gain what he cannot loose.
Post 22 May 2012, 03:56
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17271
Location: In your JS exploiting you and your system
revolution
MSVCRT is not loaded by default. Before you can get a handle to MSVCRT you have to use LoadLibrary to initialise and bind to it.
Post 22 May 2012, 04:43
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NanoBytes



Joined: 02 Jun 2011
Posts: 57
Location: Iowa, United States
NanoBytes
Thank you, it works correctly until I try to put it in a function
Code:
format PE console
include 'win32ax.inc'

Library DD ?

proc halt
        invoke GetProcAddress, [Library], "getchar"
        stdcall EAX
        ret
endp

.code
start:
        invoke AllocConsole
        invoke GetModuleHandle,"MSVCRT.dll"
        mov [Library],EAX


        stdcall halt
        invoke ExitProcess,0
.end start  
    

_________________
He is no fool who gives what he cannot
keep to gain what he cannot loose.
Post 22 May 2012, 16:01
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AsmGuru62



Joined: 28 Jan 2004
Posts: 1409
Location: Toronto, Canada
AsmGuru62
Where is "LoadLibrary"?

Also, what is a calling convention for getchar()?
Are you sure it is 'stdcall'? Maybe it is 'cinvoke'?
Post 22 May 2012, 16:29
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NanoBytes



Joined: 02 Jun 2011
Posts: 57
Location: Iowa, United States
NanoBytes
Never mind, i got it
Post 22 May 2012, 16:50
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NanoBytes



Joined: 02 Jun 2011
Posts: 57
Location: Iowa, United States
NanoBytes
Nope, i spoke too soon
Code:
format PE console
include 'win32ax.inc'

Input   DD ?
Memory  DD ?
Library DD ?

String  DD ?

proc halt
        invoke GetProcAddress, [Library], "getchar"
        stdcall EAX
        ret
endp

proc ftoa
        local Float:QWORD
        fstp [Float]
        invoke GetProcAddress, [Library], "sprintf"
        stdcall EAX,String,"%g",double[Float]
        cinvoke wsprintf,String,'%s',String
        ret
endp

.code
start:
        invoke AllocConsole
        invoke GetProcessHeap
        mov  [Memory],eax
        invoke LoadLibrary,"MSVCRT.dll"
        mov [Library],EAX

        fld1
        mov dword[esp],89
        fidiv dword[esp]
        stdcall ftoa

        invoke GetProcAddress, [Library], "printf"
        stdcall EAX,String

        stdcall halt
        invoke ExitProcess,0
.end start   
    

_________________
He is no fool who gives what he cannot
keep to gain what he cannot loose.
Post 22 May 2012, 16:54
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NanoBytes



Joined: 02 Jun 2011
Posts: 57
Location: Iowa, United States
NanoBytes
Apparently the problem comes from ftoa
Post 22 May 2012, 17:01
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typedef



Joined: 25 Jul 2010
Posts: 2913
Location: 0x77760000
typedef
sprintf / printf functions are __cdecl not __stdcall Wink

It's fucking up your frame.

String is declared as DWORD and your format specifier is %s which will crash your app, unless your string is less than or equal to 3 bytes the 4th byte being the null terminator.

Another thing,

Code:
        fld1
        mov dword[esp],89
        fidiv dword[esp] 
        stdcall ftoa
    


How certain are you that the value at ESP is yours / unwanted?

You might want to push a dummy value then work on that space instead of messing around with the unknown.
Post 22 May 2012, 17:47
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NanoBytes



Joined: 02 Jun 2011
Posts: 57
Location: Iowa, United States
NanoBytes
how do you use __cdecl, could you give me an eample
Post 22 May 2012, 18:44
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AsmGuru62



Joined: 28 Jan 2004
Posts: 1409
Location: Toronto, Canada
AsmGuru62
Use cinvoke instead of stdcall:
Code:
ftoa DD 0
...
GetProcAddress, [hDLL], "ftoa"
mov [ftoa], eax
cinvoke ftoa, ...
    
Post 22 May 2012, 18:48
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NanoBytes



Joined: 02 Jun 2011
Posts: 57
Location: Iowa, United States
NanoBytes
See, it works correctly when i use the code directly
Code:
...
        invoke GetProcAddress, [Library], "printf"
        mov [printf],EAX
        cinvoke printf,String

        invoke GetProcAddress, [Library], "getchar"
        mov [getchar],EAX
        cinvoke getchar
...

    

But when i call the function with the exact same code in it it screws up
Code:
..
proc halt
        invoke GetProcAddress, [Library], "getchar"
        mov [getchar],EAX
        cinvoke getchar
        ret
endp 
...
        invoke GetProcAddress, [Library], "printf"
        mov [printf],EAX
        cinvoke printf,String

        stdcall halt 
...
    

I dont know why it wont work when it is used as a function
Post 22 May 2012, 19:00
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AsmGuru62



Joined: 28 Jan 2004
Posts: 1409
Location: Toronto, Canada
AsmGuru62
Use a debugger and see how this call is made.
Post 22 May 2012, 19:33
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typedef



Joined: 25 Jul 2010
Posts: 2913
Location: 0x77760000
typedef
NanoBytes wrote:
I dont know why it wont work when it is used as a function


Did you change your code to what I told you.
Post 23 May 2012, 06:25
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