My trampoline won't bounce (detouring, C++, GCC) - c++

Question

It feels like I'm abusing Stackoverflow with all my questions, but it's a Q&A forum after all :) Anyhow, I have been using detours for a while now, but I have yet to implement one of my own (I've used wrappers earlier). Since I want to have complete control over my code (who doesn't?) I have decided to implement a fully functional detour'er on my own, so I can understand every single byte of my code.
The code (below) is as simple as possible, the problem though, is not. I have successfully implemented the detour (i.e a hook to my own function) but I haven't been able to implement the trampoline.
Whenever I call the trampoline, depending on the offset I use, I get either a "segmentation fault" or an "illegal instruction". Both cases ends the same though; 'core dumped'. I think it is because I've mixed up the 'relative address' (note: I'm pretty new to Linux so I have far from mastered GDB).
As commented in the code, depending on sizeof(jmpOp)(at line 66) I either get an illegal instruction or a segmentation fault. I'm sorry if it's something obvious, I'm staying up way too late...
// Header files
#include <stdio.h>
#include <sys/mman.h>
#include <unistd.h>
#include "global.h" // Contains typedefines for byte, ulong, ushort etc...
#include <cstring>
bool ProtectMemory(void * addr, int flags)
{
// Constant holding the page size value
const size_t pageSize = sysconf(_SC_PAGE_SIZE);
// Calculate relative page offset
size_t temp = (size_t) addr;
temp -= temp % pageSize;
// Update address
addr = (void*) temp;
// Update memory area protection
return !mprotect(addr, pageSize, flags);
}
const byte jmpOp[] = { 0xE9, 0x00, 0x00, 0x00, 0x00 };
int Test(void)
{
printf("This is testing\n");
return 5;
}
int MyTest(void)
{
printf("This is ******\n");
return 9;
}
typedef int (*TestType)(void);
int main(int argc, char * argv[])
{
// Fetch addresses
byte * test = (byte*) &Test;
byte * myTest = (byte*) &MyTest;
// Call original
Test();
// Update memory access for 'test' function
ProtectMemory((void*) test, PROT_EXEC | PROT_WRITE | PROT_READ);
// Allocate memory for the trampoline
byte * trampoline = new byte[sizeof(jmpOp) * 2];
// Do copy operations
memcpy(trampoline, test, sizeof(jmpOp));
memcpy(test, jmpOp, sizeof(jmpOp));
// Setup trampoline
trampoline += sizeof(jmpOp);
*trampoline = 0xE9;
// I think this address is incorrect, how should I calculate it? With the current
// status (commented 'sizeof(jmpOp)') the compiler complains about "Illegal Instruction".
// If I uncomment it, and use either + or -, a segmentation fault will occur...
*(uint*)(trampoline + 1) = ((uint) test - (uint) trampoline)/* + sizeof(jmpOp)*/;
trampoline -= sizeof(jmpOp);
// Make the trampoline executable (and read/write)
ProtectMemory((void*) trampoline, PROT_EXEC | PROT_WRITE | PROT_READ);
// Setup detour
*(uint*)(test + 1) = ((uint) myTest - (uint) test) - sizeof(jmpOp);
// Call 'detoured' func
Test();
// Call trampoline (crashes)
((TestType) trampoline)();
return 0;
}
In case of interest, this is the output during a normal run (with the exact code above):
This is testing
This is **
Illegal instruction (core dumped)
And this is the result if I use +/- sizeof(jmpOp) at line 66:
This is testing
This is ******
Segmentation fault (core dumped)
NOTE: I'm running Ubuntu 32 bit and compile with g++ global.cpp main.cpp -o main -Iinclude

Answer 1

You're not going to be able to indiscriminately copy the first 5 bytes of Test() into your trampoline, followed by a jump to the 6th instruction byte of Test(), because you don't know if the first 5 bytes comprise an integral number of x86 variable-length instructions. To do this, you're going to have to do at least a minimal amount of automated disassembling of the Test() function in order to find an instruction boundary that's 5 or more bytes past the beginning of the function, then copy an appropriate number of bytes to your trampoline, and THEN append your jump (which won't be at a fixed offset within your trampoline). Note that on a typical RISC processor (like PPC), you wouldn't have this problem, as all instructions are the same width.