# cdl86

`cdl86` - Compact Detours Library 86

> **See it in action!** <br>
> [`gsctool`](https://github.com/lunarjournal/gsctool)

# Abstract
`cdl86` is a simple, single file, cross platform detours library written in `C` for Linux and Windows.

It allows for the interception of `x86` and `x86_64` `C/C++` functions in memory.

[`https://journal.lunar.sh/2022/linux-detours.html`](https://journal.lunar.sh/2022/linux-detours.html)

The library currently supports two types of function hooks:
* `JMP` patch - patches origin function with a `JMP` to detour.
* `INT3` patch - places software breakpoint (`SWBP`) at origin address. Handles control flow to detour.

> This project makes use of an internal `x86` instruction length disassembly engine.

# API
```
struct cdl_jmp_patch cdl_jmp_attach(void **target, void *detour);
struct cdl_swbp_patch cdl_swbp_attach(void **target, void *detour);
void cdl_jmp_detach(struct cdl_jmp_patch *jmp_patch);
void cdl_swbp_detach(struct cdl_swbp_patch *swbp_patch);
void cdl_jmp_dbg(struct cdl_jmp_patch *jmp_patch);
void cdl_swbp_dbg(struct cdl_swbp_patch *swbp_patch);
```
The API is documented in more detail in the corresponding header and source
files.

# Example Usage

Assuming we have a function `add()` that is defined as follows:
```
int add(
    __in int x,
    __in int y
)
{
    printf("Inside original function\n");
    return x + y;
}
```

Assuming we want to hook this function with `cdl86` the first step
is to define a function pointer typedef and declare our detour function:

```
typedef int add_t(
    __in int x,
    __in int y
);
add_t* addo = NULL;
```
```
// define detour function
int add_detour(
    __in int x,
    __in int y
)
{
    printf("Inside detour function\n");
    return addo(5,5);
}

```
Then in our `main()` function we create a `cdl_jmp_patch` struct and assign
a value to `addo` (our function pointer to the original function):
```
struct cdl_jmp_patch jmp_patch = {};
addo = (add_t*)add;
```

Finally call `cdl_jmp_attach` as follows:
```
jmp_patch = cdl_jmp_attach((void**)&addo, add_detour);
```

The original function `add` has now been hooked!

To dump debug info from the `cdl_jmp_patch` struct use `cdl_jmp_dbg`.

# Info

The compiler used for this project is [`tcc`](https://github.com/lunarjournal/tcc).

Files:
<br/>
* `cdl.c` - `C source file for CDL`.
* `cdl.h` - `CDL header file to include`.

Folders:
* `/tests` - `CDL test suite`.