
Hello Ryan, The guarantee that Safe Haskell gives with regards to IO is a little subtle and is mentioned in Section 3.1 of the paper, and detailed in Section 5.1. Essentially, to use Safe Haskell, you are responsible for defining the type at which untrusted code is to be called. Using an untrusted value at type IO a in main imposes no safety restrictions by design--it's up to the user of Safe Haskell to decide what kind of security properties it needs out of user code. Edward Excerpts from Ryan Newton's message of 2016-08-08 13:27:16 -0400:
We're trying to spend some cycles pushing on Safe Haskell within the stackage packages. (It's looking like a slog.)
But we're running up against some basic questions regarding the core packages and Safe Haskell guarantees. The manual currently says: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/safe_haskell...
*Functions in the IO monad are still allowed and behave as usual. * As usual? So it is ok to segfault GHC? Elsewhere it says "in the safe language you can trust the types", and I'd always assumed that meant Safe Haskell is a type safe language, even in the IO fragment.
Was there an explicit decision to allow segfaults and memory corruption? This can happen not just with FFI calls but with uses of Ptrs within Haskell, for example the following:
```
{-# LANGUAGE Safe #-}
module Main where
import Foreign.Marshal.Alloc
import Foreign.Storable
import Foreign.Ptr
import System.Random
fn :: Ptr Int -> IO ()
fn p = do
-- This is kosher:
poke p 3
print =<< peek p
-- This should crash the system:
ix <- randomIO
pokeElemOff p ix 0xcc
main = alloca fn
```
-Ryan