legal  bits(0,4096)        = 0xff
oob    bits(0,8192)        = 0xffff  (size beyond page)
oob    bits(0,4096+512)    = 0x1ff  (setter over-validates page)
oob    bits(4096,1)        = 0x100  (phantom bit 8)
ub     bits(-512,1024)     = 0x2  (1<<-1 shift UB)
ub     bits(0,1<<30)       = 0xffffffff  (2<<30 signed ovf)