How do I reimplement (or wrap) a syscall function on Linux?

Question

Suppose I want to completely take over the open() system call, maybe to wrap the actual syscall and perform some logging. One way to do this is to use LD_PRELOAD to load a (user-made) shared object library that takes over the open() entry point.

The user-made open() routine then obtains the pointer to the glibc function open() by dlsym()ing it, and calling it.

The solution proposed above is a dynamic solution, however. Suppose I want to link my own open() wrapper statically. How would I do it? I guess the mechanism is the same, but I also guess there will be a symbol clash between the user-defined open() and the libc open().

Please share any other techniques to achieve the same goal.

Answer 1

You can use the wrap feature provided by ld. From man ld:

--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.

So you just have to use the prefix __wrap_ for your wrapper function and __real_ when you want to call the real function. A simple example is:

malloc_wrapper.c:

#include <stdio.h>
void *__real_malloc (size_t);

/* This function wraps the real malloc */
void * __wrap_malloc (size_t size)
{
    void *lptr = __real_malloc(size);
    printf("Malloc: %lu bytes @%p\n", size, lptr);
    return lptr;
}

Test application testapp.c:

#include <stdio.h>
#include <stdlib.h>
int main()
{
    free(malloc(1024)); // malloc will resolve to __wrap_malloc
    return 0;
}

Then compile the application:

gcc -c malloc_wrapper.c
gcc -c testapp.c
gcc -Wl,-wrap,malloc testapp.o malloc_wrapper.o -o testapp

The output of the resulting application will be:

$ ./testapp
Malloc: 1024 bytes @0x20d8010