Functions will have the stack-protector canary logic added in any
of the following conditions:

- local variable's address used as part of the right hand side of an
assignment or function argument
- local variable is an array (or union containing an array),
regardless of array type or length
- uses register local variables

This feature requires gcc version 4.9 or above, or a distribution
gcc with the feature backported ("-fstack-protector-strong").

On an x86 "defconfig" build, this feature adds canary checks to
about 20% of all kernel functions, which increases the kernel code
size by about 2%.