BSc-Thesis/doc/README.md

Intercepting Function/System Calls in Linux

Option 1: LD_PRELOAD

• No need to re-link
• Works for all functions
• Works only on dynamically linked executables
• Intercepts all calls (including stack allocations etc.)

Example (preload.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;
}

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

Option 2: gcc --wrap

• Need to re-link(/-comiple)
• 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):

#include <stdlib.h>

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;
}

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

Option 3: Linux kernel

• Only works with Linux system calls
• Also works with statically linked executables