Shim DLL Generation

genshim.cpp

// Code originally found on http://www.hulver.com/scoop/story/2006/2/18/125521/185
// Created by Rogerborg

#include <stdio.h>
#include <windows.h>
#include <list>
using namespace std;

// Change this to the name of the DLL that you want to clone
#define CLONE_DLL "WS2_32"

// Define this to insert basic fprintf debug into the cloned DLL
#define INSERT_DEBUG_OUTPUT 1

typedef struct
{
    int ordinal;
    char name[64];
} Symbol;

static list<Symbol> gs_symbols;

main()
{
    char originalDLL[256];
    (void)GetSystemDirectory(originalDLL, sizeof(originalDLL) - 1);
    strcat(originalDLL, "\\" CLONE_DLL ".dll");

    char exportFile[256];
    (void)GetTempPath(sizeof(exportFile), exportFile);
    strcat(exportFile, "exports.txt");

    char dumpbin[512];
    sprintf(dumpbin, "dumpbin /exports %s > %s", originalDLL, exportFile);
    (void)system(dumpbin);

    FILE * exports = fopen(exportFile, "r");
    FILE * cpp = fopen(CLONE_DLL ".cpp", "w");
    FILE * def = fopen(CLONE_DLL ".def", "w");

    while(!feof(exports))
    {
        char line[256];
        Symbol newSymbol;

        if(fgets(line, sizeof(line) - 1, exports))
            if(2 == sscanf(line, "%d %*x %*x %s",
                &newSymbol.ordinal,
                newSymbol.name))
                gs_symbols.push_back(newSymbol);
    }

    fprintf(def, "LIBRARY " CLONE_DLL "\n\nEXPORTS\n");

    for(list<Symbol>::iterator symbol = gs_symbols.begin();
        symbol != gs_symbols.end();
        ++symbol)
        fprintf(def, "\t%s = _I_%s @%d\n",
                symbol->name,
                symbol->name,
                symbol->ordinal);

    fprintf(cpp, "#include <stdio.h>\n");
    fprintf(cpp, "#include <windows.h>\n");
    fprintf(cpp, "static HINSTANCE gs_hDLL = 0;\n\n");

#if INSERT_DEBUG_OUTPUT
    fprintf(cpp, "#if defined (_DEBUG)\n");
    fprintf(cpp, "static FILE * debug;\n");
    fprintf(cpp, "#define DEBUG(x) fprintf x\n");
    fprintf(cpp, "#else // DEBUG\n");
    fprintf(cpp, "#define DEBUG(x)\n");
    fprintf(cpp, "#endif // _DEBUG\n\n");
#endif // INSERT_DEBUG_OUTPUT

    for(symbol = gs_symbols.begin();
        symbol != gs_symbols.end();
        ++symbol)
    {
        fprintf(cpp, "int (__stdcall * _O_%s)();\n", symbol->name);
        fprintf(cpp, "extern \"C\" void __declspec(naked) __stdcall _I_%s()\n", symbol->name);       
        fprintf(cpp, "{\n");
#if INSERT_DEBUG_OUTPUT
        fprintf(cpp, "\tDEBUG((debug, \"%s\\n\"));\n", symbol->name);
#endif // INSERT_DEBUG_OUTPUT
        fprintf(cpp, "\t__asm { jmp _O_%s }\n", symbol->name);
        fprintf(cpp, "}\n\n");
    }

    fprintf(cpp, "BOOL WINAPI DllMain(HINSTANCE hI, DWORD reason, LPVOID notUsed)\n");
    fprintf(cpp, "{\n");
    fprintf(cpp, "    if (reason == DLL_PROCESS_ATTACH)\n");
    fprintf(cpp, "    {\n");

#if INSERT_DEBUG_OUTPUT   
    fprintf(cpp, "#if defined(_DEBUG)\n");
    fprintf(cpp, "        if(!debug)\n");
    fprintf(cpp, "            debug = fopen(\"debug.txt\", \"w\");\n");
    fprintf(cpp, "#endif // DEBUG\n");
    fprintf(cpp, "        DEBUG((debug, \"DllMain\\n\"));\n\n");
#endif // INSERT_DEBUG_OUTPUT

    fprintf(cpp, "        char realDLL[MAX_PATH];\n");
    fprintf(cpp, "        (void)GetSystemDirectory(realDLL, sizeof(realDLL) - 1);\n");
    fprintf(cpp, "        strcat(realDLL, \"\\\\" CLONE_DLL ".dll\");\n");
    fprintf(cpp, "        gs_hDLL = LoadLibrary(realDLL);\n");
    fprintf(cpp, "        if (!gs_hDLL)\n");
    fprintf(cpp, "            return FALSE;\n\n");

    for(symbol = gs_symbols.begin();
        symbol != gs_symbols.end();
        ++symbol)
    {
        fprintf(cpp, "        _O_%s = GetProcAddress(gs_hDLL, \"%s\");\n",
                symbol->name, symbol->name);
    }

    fprintf(cpp, "\n        return TRUE;\n");
    fprintf(cpp, "    }\n");
    fprintf(cpp, "    else if (reason == DLL_PROCESS_DETACH)\n");
    fprintf(cpp, "    {\n");

#if INSERT_DEBUG_OUTPUT
    fprintf(cpp, "#if defined(_DEBUG)\n");
    fprintf(cpp, "        if(debug)\n");
    fprintf(cpp, "            fclose(debug);\n");
    fprintf(cpp, "#endif // DEBUG\n\n");
#endif // INSERT_DEBUG_OUTPUT

    fprintf(cpp, "        FreeLibrary(gs_hDLL);\n\n");
    fprintf(cpp, "    }\n");
    fprintf(cpp, "    return TRUE;\n");
    fprintf(cpp, "}\n\n");

    fclose(exports);
    fclose(cpp);
    fclose(def);

    return 0;
}