The assembly code of the Linux kernel is complicated and poorly maintained, according to some Linux developers. So a lot of code is rewritten in C. Linux kernel 4.1 comes with many x86 ASM code changes and one of the developers; Andy Lutomirski explains what kind of issues he encountered. “The exit-to-usermode code is copied in several places and is written in a nasty combination of asm and C. It’s not at all clear what it’s supposed to do, and the way it’s structured makes it very hard to work with.” Andy Lutomirski explains.