I've discovered a problem in the array libraries that allows one to read arbitrary memory locations without the use of any unsafeFoo functions. Both GHC and Hugs are affected.
import Data.Array.IArray import Data.Array.Unboxed
Here is a poorly behaved instance of Ix: inRange and index ignore the bounds supplied.
newtype EvilIx = E Int deriving (Eq, Ord) instance Ix EvilIx where inRange _ _ = True index _ (E i) = i range (E x, E y) = map E $ range (x, y)
One can read arbitrary memory locations, eventually indexing far enough to cause a segmentation fault.
main = print [a ! E (2^i) | i <- [0..]] where a :: UArray EvilIx Int a = array (E 0, E 0) []
This problem is not specific to UArrays:
main' = print [a ! E (2^i) | i <- [0..]] where a :: Array EvilIx String a = array (E 0, E 0) []
The issue is that the array operators trust the Ix instance to manage boundaries correctly. The solution is to double check the value returned by index with the actual length of the array. I volunteer to write the fix, if I can extract some hints from more knowledgeable folk. There's sizeOfByteArray#, is there an analog for an Array#? I need to know how to find the length of Hugs array primitives too. Cheers, Spencer Janssen