This achieves two things:
- Programs can now intentionally perform arbitrary syscalls by calling
syscall(). This allows us to work on things like syscall fuzzing.
- It restricts the ability of userspace to make syscalls to a single
4KB page of code. In order to call the kernel directly, an attacker
must now locate this page and call through it.
This allows us to implement mkstemp() with open() directly, instead of
first lstat()'ing, and then open()'ing the filename.
Also implement tmpfile() in terms of mkstemp() instead of mktemp().
