return-to-csu: A New Method to Bypass 64-bit Linux ASLR BlackHat talk is a must-read (https://www.blackhat.com/docs/asia-18/asia-18-Marco-return-to-csu-a-new-method-to-bypass-the-64-bit-Linux-ASLR.pdf).
There are two useful gadgets in all the binaries that are not being reported by ROP tools such as ropper2.