bpf: teach verifier to recognize zero initialized stack

programs with function calls are often passing various
pointers via stack. When all calls are inlined llvm
flattens stack accesses and optimizes away extra branches.
When functions are not inlined it becomes the job of
the verifier to recognize zero initialized stack to avoid
exploring paths that program will not take.
The following program would fail otherwise:

ptr = &buffer_on_stack;
*ptr = 0;
...
func_call(.., ptr, ...) {
  if (..)
    *ptr = bpf_map_lookup();
}
...
if (*ptr != 0) {
  // Access (*ptr)->field is valid.
  // Without stack_zero tracking such (*ptr)->field access
  // will be rejected
}

since stack slots are no longer uniform invalid | spill | misc
add liveness marking to all slots, but do it in 8 byte chunks.
So if nothing was read or written in [fp-16, fp-9] range
it will be marked as LIVE_NONE.
If any byte in that range was read, it will be marked LIVE_READ
and stacksafe() check will perform byte-by-byte verification.
If all bytes in the range were written the slot will be
marked as LIVE_WRITTEN.
This significantly speeds up state equality comparison
and reduces total number of states processed.

                    before   after
bpf_lb-DLB_L3.o       2051    2003
bpf_lb-DLB_L4.o       3287    3164
bpf_lb-DUNKNOWN.o     1080    1080
bpf_lxc-DDROP_ALL.o   24980   12361
bpf_lxc-DUNKNOWN.o    34308   16605
bpf_netdev.o          15404   10962
bpf_overlay.o         7191    6679

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>