
Intercepting and Manipulating Function and System Calls in Linux
================================================================

Option 1: Preloading (`LD_PRELOAD`) {#preloading}
-------------------------------------------------

From the [ENVIRONMENT section in the Linux manual page ld.so(8)](https://www.man7.org/linux/man-pages/man8/ld.so.8.html#ENVIRONMENT):

> **`LD_PRELOAD`**
>
> A list of additional, user-specified, ELF shared objects to be loaded before all others.
> This feature can be used to selectively override functions in other shared objects.
> [...]

* No need to re-link
* Works for *all* functions
* Works only on dynamically linked executables
* Intercepts all calls (including calls inside libraries etc.)

Example (`preload.c`):
```c
#include <stdlib.h>
#include <dlfcn.h>
#include <errno.h>

void *malloc(size_t size) {
    // before call to malloc
    void *(*_malloc)(size_t);
    if ((_malloc = dlsym(RTLD_NEXT, "malloc")) == NULL) {
        errno = ENOSYS;
        return NULL;
    }
    void *ret = _malloc(size);
    // after call to malloc
    return ret;
}
```

```shell
# ./main is already compiled and ready
gcc -shared -fPIC -o preload.so preload.c
LD_PRELOAD="$(pwd)/preload.so" ./main
```


Option 2: Wrapper functions (`gcc -Wl,--wrap=`, `ld --wrap=`) {#wrapper-functions}
----------------------------------------------------------------------------------

From the [OPTIONS section in the Linux manual page ld(1)](https://www.man7.org/linux/man-pages/man1/ld.1.html#OPTIONS):

> **`--wrap=symbol`**
>
> Use a wrapper function for *symbol*.
> Any undefined reference to *symbol* will be resolved to `__wrap_<symbol>`.
> Any undefined reference to `__real_<symbol>` will be resolved to *symbol*.
>
> This can be used to provide a wrapper for a system function.
> The wrapper function should be called `__wrap_<symbol>`.
> If it wishes to call the system function, it should call `__real_<symbol>`.
> [...]

From the [OPTIONS section in the Linux manual page gcc(1)](https://www.man7.org/linux/man-pages/man1/gcc.1.html#OPTIONS):

> **`-Wl,option`**
>
> Pass *option* as an option to the linker.
> If *option* contains commas, it is split into multiple options at the commas.
> You can use this syntax to pass an argument to the option.
> For example, `-Wl,-Map,output.map` passes `-Map output.map` to the linker.
> When using the GNU linker, you can also get the same effect with `-Wl,-Map=output.map`.

* Need to re-link(/-compile)
* Relatively simple code:
  * Function name: `__wrap_<symbol>`
  * Call to real function inside wrapper: `__real_<symbol>`
* Works for *all* functions
* Works only on dynamically linked executables
* Intercepts only calls inside the given source file

Example (`wrap.c`):
```c
extern void *__real_malloc(size_t size);

void *__wrap_malloc(size_t size) {
    // before call to malloc
    void *ret = __real_malloc(size);
    // after call to malloc
    return ret;
}
```

```shell
gcc -o main_wrapped main.c wrap.c -Wl,--wrap=malloc
./main_wrapped
```


Option 3: Kernel module {#kernel-module}
----------------------------------------

* Only works with Linux system calls
* Also works with statically linked executables
* Only possible with some "hacks" inside the kernel module (to access the Syscall Table)
* [litux.nl?](https://litux.nl/mirror/networksecuritytools/0596007949/networkst-CHP-7-SECT-2.html)


Option 4: Emulating {#emulating}
--------------------------------

* Valgrind, GDB


Option 5: Modifying the kernel {#kernel}
----------------------------------------

* Add a special Syscall to intercept/modify other Syscalls
