is it possible to call libc function with rop

Question

let say hacker want to come over the aslr or he want to call function that doesn't exists in user program ,can he insert to the return address address to the share library function or this functions being load only when the user program call them

Answer 1

This is the basics of Return Oriented Programming, I encourage you to look up methods to bypass stack canaries ASLR/PIE, and DEP. Various resources are available out there one of the more rudimentary techniques is known as return to libc.