i am a beginner in the PE( portable executable format,winxp). I have a doubt.suppose we are declaring a global variable in a dll and to access that variable we need to have an address
mov eax,
but this address depends , in which position of virtual address space the dll get loaded..
so how they are managing this situtaion?
mov eax,
but this address depends , in which position of virtual address space the dll get loaded..
so how they are managing this situtaion?
you would use GetProcAddress to get the address (assuming its exported) from the dll... where the windows pe loader would use the reloc's to obtain the right address for you..
other (unsafe) approaches would be to use LoadLibrary to load the dll (if it is loaded already use GetModuleHandle) to get the base address, and then add on the displacement to calculate the real address at runtime
other (unsafe) approaches would be to use LoadLibrary to load the dll (if it is loaded already use GetModuleHandle) to get the base address, and then add on the displacement to calculate the real address at runtime
Assuming you created the DLL, why not just add a function to return the variable's address?
nasm -f win32 baka.asm
nasm -f win32 dlltest.asm
golink /dll /export:getVar /entry:DllMain baka.obj
golink /entry:Start dlltest.obj kernel32.dll user32.dll baka.dll
bits 32
section .data
someVar DD 1234
section .text
global getVar
global DllMain
getVar:
mov eax, someVar
ret
DllMain:
mov eax, 1
ret 12bits 32
extern getVar
extern wsprintfA
extern MessageBoxA
extern ExitProcess
global Start
section .data
strCaption: DB "DLL Example.", 13, 10, 0
strFormat: DB "%d", 13, 10, 0
strBuffer: TIMES 512 DB 0
section .text
Start:
call getVar
push dword
push dword strFormat
push dword strBuffer
call wsprintfA
add esp, 12
push dword 0 ; MK_OK
push dword strCaption
push dword strBuffer
push dword 0 ; HWND(NULL)
call MessageBoxA
push dword 0
call ExitProcessnasm -f win32 baka.asm
nasm -f win32 dlltest.asm
golink /dll /export:getVar /entry:DllMain baka.obj
golink /entry:Start dlltest.obj kernel32.dll user32.dll baka.dll