Assembly Language for Beginners

6.4. LINUX

vprintf (fmt, va);
exit(0);
};

static void find_original_functions ()
{
if (inited)
return;

libc_handle = dlopen ("libc.so.6", RTLD_LAZY);

if (libc_handle==NULL)

die ("can't open libc.so.6\n");

open_ptr = dlsym (libc_handle, "open");

if (open_ptr==NULL)

die ("can't find open()\n");

close_ptr = dlsym (libc_handle, "close");

if (close_ptr==NULL)

die ("can't find close()\n");

read_ptr = dlsym (libc_handle, "read");

if (read_ptr==NULL)

die ("can't find read()\n");

inited = true;
}

static int opened_fd=0;

int open(const char *pathname, int flags)
{
find_original_functions();

int fd=(*open_ptr)(pathname, flags);
if (strcmp(pathname, "/proc/uptime")==0)
opened_fd=fd; // that's our file! record its file descriptor
else
opened_fd=0;
return fd;
};

int close(int fd)
{
find_original_functions();

if (fd==opened_fd)
opened_fd=0; // the file is not opened anymore
return (*close_ptr)(fd);
};

ssize_t read(int fd, void *buf, size_t count)
{
find_original_functions();

if (opened_fd!=0 && fd==opened_fd)
{
// that's our file!
return snprintf (buf, count, "%d %d", 0x7fffffff, 0x7fffffff)+1;
};
// not our file, go to real read() function
return (*read_ptr)(fd, buf, count);
};

(Source code at GitHub)

Let’s compile it as common dynamic library:

gcc -fpic -shared -Wall -o fool_uptime.so fool_uptime.c -ldl