Hooking

/*
 This idea is based on chrom-lib approach, Distributed under GNU LGPL License.
 Source chrom-lib: https://github.com/linuxexp/chrom-lib
 Copyright (C) 2011  Raja Jamwal
*/
#include <windows.h>  
#define SIZE 6

 typedef int (WINAPI *pMessageBoxW)(HWND, LPCWSTR, LPCWSTR, UINT);  // Messagebox prototype
 int WINAPI MyMessageBoxW(HWND, LPCWSTR, LPCWSTR, UINT);            // Our detour

 void BeginRedirect(LPVOID);                                        
 pMessageBoxW pOrigMBAddress = NULL;                                // address of original
 BYTE oldBytes[SIZE] = {0};                                         // backup
 BYTE JMP[SIZE] = {0};                                              // 6 byte JMP instruction
 DWORD oldProtect, myProtect = PAGE_EXECUTE_READWRITE;

 INT APIENTRY DllMain(HMODULE hDLL, DWORD Reason, LPVOID Reserved)  
 {  
   switch (Reason)  
   {  
   case DLL_PROCESS_ATTACH:                                        // if attached
     pOrigMBAddress = (pMessageBoxW)                      
       GetProcAddress(GetModuleHandleA("user32.dll"),              // get address of original 
               "MessageBoxW");  
     if (pOrigMBAddress != NULL)  
       BeginRedirect(MyMessageBoxW);                               // start detouring
     break;

   case DLL_PROCESS_DETACH:  
     VirtualProtect((LPVOID)pOrigMBAddress, SIZE, myProtect, &oldProtect);   // assign read write protection
     memcpy(pOrigMBAddress, oldBytes, SIZE);                                 // restore backup
     VirtualProtect((LPVOID)pOrigMBAddress, SIZE, oldProtect, &myProtect);   // reset protection

   case DLL_THREAD_ATTACH:  
   case DLL_THREAD_DETACH:  
     break;  
   }  
   return TRUE;  
 }

 void BeginRedirect(LPVOID newFunction)  
 {  
   BYTE tempJMP[SIZE] = {0xE9, 0x90, 0x90, 0x90, 0x90, 0xC3};              // 0xE9 = JMP 0x90 = NOP 0xC3 = RET
   memcpy(JMP, tempJMP, SIZE);                                             // store jmp instruction to JMP
   DWORD JMPSize = ((DWORD)newFunction - (DWORD)pOrigMBAddress - 5);       // calculate jump distance
   VirtualProtect((LPVOID)pOrigMBAddress, SIZE,                            // assign read write protection
           PAGE_EXECUTE_READWRITE, &oldProtect);  
   memcpy(oldBytes, pOrigMBAddress, SIZE);                                 // make backup
   memcpy(&JMP[1], &JMPSize, 4);                                           // fill the nop's with the jump distance (JMP,distance(4bytes),RET)
   memcpy(pOrigMBAddress, JMP, SIZE);                                      // set jump instruction at the beginning of the original function
   VirtualProtect((LPVOID)pOrigMBAddress, SIZE, oldProtect, &myProtect);   // reset protection
 }

 int WINAPI MyMessageBoxW(HWND hWnd, LPCWSTR lpText, LPCWSTR lpCaption, UINT uiType)  
 {  
   VirtualProtect((LPVOID)pOrigMBAddress, SIZE, myProtect, &oldProtect);   // assign read write protection
   memcpy(pOrigMBAddress, oldBytes, SIZE);                                 // restore backup
   int retValue = MessageBoxW(hWnd, lpText, lpCaption, uiType);            // get return value of original function
   memcpy(pOrigMBAddress, JMP, SIZE);                                      // set the jump instruction again
   VirtualProtect((LPVOID)pOrigMBAddress, SIZE, oldProtect, &myProtect);   // reset protection
   return retValue;                                                        // return original return value
 }

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/skbuff.h>

#include <linux/ip.h>
#include <linux/tcp.h>
#include <linux/in.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>

/* Port we want to drop packets on */
static const uint16_t port = 25;

/* This is the hook function itself */
static unsigned int hook_func(unsigned int hooknum,
                       struct sk_buff **pskb,
                       const struct net_device *in,
                       const struct net_device *out,
                       int (*okfn)(struct sk_buff *))
{
        struct iphdr *iph = ip_hdr(*pskb);
        struct tcphdr *tcph, tcpbuf;

        if (iph->protocol != IPPROTO_TCP)
                return NF_ACCEPT;

        tcph = skb_header_pointer(*pskb, ip_hdrlen(*pskb), sizeof(*tcph), &tcpbuf);
        if (tcph == NULL)
                return NF_ACCEPT;

        return (tcph->dest == port) ? NF_DROP : NF_ACCEPT;
}

/* Used to register our hook function */
static struct nf_hook_ops nfho = {
        .hook     = hook_func,
        .hooknum  = NF_IP_PRE_ROUTING,
        .pf       = NFPROTO_IPV4,
        .priority = NF_IP_PRI_FIRST,
};

static __init int my_init(void)
{
        return nf_register_hook(&nfho);
}

static __exit void my_exit(void)
{
    nf_unregister_hook(&nfho);
}

module_init(my_init);
module_exit(my_exit);

#include <windows.h>

typedef int(__stdcall *pMessageBoxA) (HWND hWnd, LPCSTR lpText, LPCSTR lpCaption, UINT uType); //This is the 'type' of the MessageBoxA call.
pMessageBoxA RealMessageBoxA; //This will store a pointer to the original function.

void DetourIATptr(const char* function, void* newfunction, HMODULE module);

int __stdcall NewMessageBoxA(HWND hWnd, LPCSTR lpText, LPCSTR lpCaption, UINT uType) { //Our fake function
    printf("The String Sent to MessageBoxA Was : %s\n", lpText);
    return RealMessageBoxA(hWnd, lpText, lpCaption, uType); //Call the real function
}

int main(int argc, CHAR *argv[]) {
   DetourIATptr("MessageBoxA",(void*)NewMessageBoxA,0); //Hook the function
   MessageBoxA(NULL, "Just A MessageBox", "Just A MessageBox", 0); //Call the function -- this will invoke our fake hook.
   return 0;
}

void **IATfind(const char *function, HMODULE module) { //Find the IAT (Import Address Table) entry specific to the given function.
	int ip = 0;
	if (module == 0)
		module = GetModuleHandle(0);
	PIMAGE_DOS_HEADER pImgDosHeaders = (PIMAGE_DOS_HEADER)module;
	PIMAGE_NT_HEADERS pImgNTHeaders = (PIMAGE_NT_HEADERS)((LPBYTE)pImgDosHeaders + pImgDosHeaders->e_lfanew);
	PIMAGE_IMPORT_DESCRIPTOR pImgImportDesc = (PIMAGE_IMPORT_DESCRIPTOR)((LPBYTE)pImgDosHeaders + pImgNTHeaders->OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT].VirtualAddress);

	if (pImgDosHeaders->e_magic != IMAGE_DOS_SIGNATURE)
		printf("libPE Error : e_magic is no valid DOS signature\n");

	for (IMAGE_IMPORT_DESCRIPTOR *iid = pImgImportDesc; iid->Name != NULL; iid++) {
		for (int funcIdx = 0; *(funcIdx + (LPVOID*)(iid->FirstThunk + (SIZE_T)module)) != NULL; funcIdx++) {
			char *modFuncName = (char*)(*(funcIdx + (SIZE_T*)(iid->OriginalFirstThunk + (SIZE_T)module)) + (SIZE_T)module + 2);
			const uintptr_t nModFuncName = (uintptr_t)modFuncName;
			bool isString = !(nModFuncName & (sizeof(nModFuncName) == 4 ? 0x80000000 : 0x8000000000000000));
			if (isString) {
				if (!_stricmp(function, modFuncName))
					return funcIdx + (LPVOID*)(iid->FirstThunk + (SIZE_T)module);
			}
		}
	}
	return 0;
}

void DetourIATptr(const char *function, void *newfunction, HMODULE module) {
	void **funcptr = IATfind(function, module);
	if (*funcptr == newfunction)
		 return;

	DWORD oldrights, newrights = PAGE_READWRITE;
	//Update the protection to READWRITE
	VirtualProtect(funcptr, sizeof(LPVOID), newrights, &oldrights);

	RealMessageBoxA = (pMessageBoxA)*funcptr; //Some compilers require the cast (like "MinGW"), not sure about MSVC though
	*funcptr = newfunction;

	//Restore the old memory protection flags.
	VirtualProtect(funcptr, sizeof(LPVOID), oldrights, &newrights);
}