There are only so many places to fit code in a binary, and for any sophisticated virus, the parasite is going to be at least a few thousand bytes and will require enlarging the size of the host executable. In ELF executables, there aren't a whole lot of code caves (such as in the PE format), so you are not likely to be able to shove more than just a meager amount of shellcode into existing code slots (such as areas that have 0s or NOPS for function padding).
This infection method was conceived by Silvio Cesare in the late '90s and has since shown up in various Linux viruses, such as Brundle Fly and the POCs produced by Silvio himself. This method is inventive, but it limits the infection payload to one page size. On 32-bit Linux systems, this is 4096 bytes, but on 64-bit systems, the executables use large pages that measure 0x200000 bytes, which allows for about a 2-MB infection. The way that this infection works is by...