Create a proxy DLL with GCC/MinGW - c++

Question

With the Visual C++ compiler, it is possible to create a DLL file, that can imitate another DLL file and redirect all function calls to the original DLL. Here is an article with a tool that can generate Visual C++ code automatically.
The generated function-stubs work (tested) and look like this:
extern "C" __declspec(naked) void __stdcall __E__0__()
{
__asm
{
jmp p[0]; // p[0] = GetProcAddress(hL,"AcceptEx");
}
}
Now I want to do the same thing with MinGW/GCC instead of MSVC.
__declspec(naked) isn't supported by GCC on i386, so we need another way.
As suggested here, I could override functions by writing assembly code in the global scope. Here's my code that should do the trick:
__asm__
(
"jmp *%0"
: /* empty output list */
: "r" (pointer_to_original_function) /* p[0] in the example above */
);
My snippet uses GCC's extended ASM. But unfortunatelly this is only allowed inside of functions, not in the global scope!
So... how do I do that? My next approach would be to try it without extended ASM, but how do I get the pointer address in assembly then?
Here i'm trying to get it from a global variable, but it segfaults at repace_this_stub():
#include <stdio.h>
void try_to_jump_to_me()
{
printf("you made the jump\n");
}
void* target_pointer = try_to_jump_to_me;
__asm__ (
"replace_this_stub:"
"jmp target_pointer"
);
void replace_this_stub();
int main(int argc, char** argv)
{
printf("starting in main. \n");
replace_this_stub();
printf("back in main?\n");
}

Answer 1

If the pointer is in a global variable, you can just use its name. Be sure to apply any name mangling. Also put your code in the applicable code section and give it a name. Sample code:
#include <stdio.h>
void* p = printf;
asm(
".section .text\n\t"
"proxy: jmp *p\n\t"
".previous\n\t");
extern void proxy();
int main()
{
proxy("Hello world!\n");
return 0;
}
If you want to use an array, just add the appropriate displacement. Extended sample:
#include <stdio.h>
#include <string.h>
void* p[] = { printf, strcpy };
#define str(x) #x
#define PROXY(name, index) asm( \
".section .text\n\t" \
str(proxy_##name) ": jmp *p + " str(index) " * 4\n\t" \
".previous\n\t"); \
extern void proxy_##name()
PROXY(printf, 0);
PROXY(strcpy, 1);
int main()
{
char buf[128];
proxy_strcpy(buf, "Hello world!\n");
proxy_printf(buf);
return 0;
}