A lot of software relies on the fact that mmap and shbuf memory is
zeroed out by the kernel, so we should consider it initialized from the
shadow bit perspective as well.
This patch introduces the concept of shadow bits. For every byte of
memory there is a corresponding shadow byte that contains metadata
about that memory.
Initially, the only metadata is whether the byte has been initialized
or not. That's represented by the least significant shadow bit.
Shadow bits travel together with regular values throughout the entire
CPU and MMU emulation. There are two main helper classes to facilitate
this: ValueWithShadow and ValueAndShadowReference.
ValueWithShadow<T> is basically a struct { T value; T shadow; } whereas
ValueAndShadowReference<T> is struct { T& value; T& shadow; }.
The latter is used as a wrapper around general-purpose registers, since
they can't use the plain ValueWithShadow memory as we need to be able
to address individual 8-bit and 16-bit subregisters (EAX, AX, AL, AH.)
Whenever a computation is made using uninitialized inputs, the result
is tainted and becomes uninitialized as well. This allows us to track
this state as it propagates throughout memory and registers.
This patch doesn't yet keep track of tainted flags, that will be an
important upcoming improvement to this.
I'm sure I've messed up some things here and there, but it seems to
basically work, so we have a place to start! :^)
