incorrect storage layout for contracts containing large arrays

Impact

contracts containing large arrays might underallocate the number of slots they need. prior to v0.3.8, the calculation to determine how many slots a storage variable needed used math.ceil(type_.size_in_bytes / 32):

https://github.com/vyperlang/vyper/blob/6020b8bbf66b062d299d87bc7e4eddc4c9d1c157/vyper/semantics/validation/data_positions.py#L197

the intermediate floating point step can produce a rounding error if there are enough bits set in the IEEE-754 mantissa. roughly speaking, if type_.size_in_bytes is large (> 2**46), and slightly less than a power of 2, the calculation can overestimate how many slots are needed. if type_.size_in_bytes is slightly more than a power of 2, the calculation can underestimate how many slots are needed.

the following two example contracts can result in overwriting of the variable vulnerable:

large_array: address[2**64 + 1]  # type_.size_in_bytes == 32 * (2**64 + 1); math.ceil(type_.size_in_bytes / 32) < 2**64 + 1
vulnerable: uint256

# writing to self.large_array[2**64] will overwrite self.vulnerable

large_dynarray: DynArray[address, 2**64]  # Dynarray has a length word in front, its size in bytes is 32 * (2**64 + 1)
vulnerable: uint256

# writing to self.large_dynarray[2**64 - 1] will overwrite self.vulnerable

note that in the latter case, the risk of vulnerable being overwritten is relatively small, since it would cost roughly $1.45 million trillion USD at today's gas prices (gas price 20gwei, ETH ~= $1800) in order to extend the DynArray to its full container size.

Patches

patched by v0.3.8, specifically in commit https://github.com/vyperlang/vyper/commit/0bb7203b584e771b23536ba065a6efda457161bb.