Re: fixST lost bottoms

9 Jul 2018


      I do find it scary, but I personally am in no position to express opinion
on what should happen (I'm not on any committee and I'm not even a regular
Haskell user).

On Mon, 9 Jul 2018 at 05:08, David Feuer  wrote:
...
That's a very scary, albeit contrived, example. I wonder if there are more
realistic functions that are susceptible to this attack from "safe"
Haskell. In light of your exploit, I'm leaning toward saying we probably
*should* fix this, but I'd like to hear your opinion.
On Sun, Jul 8, 2018, 6:51 PM Arseniy Alekseyev <
arseniy.alekseyev@gmail.com> wrote:
...
This might be just me expecting too much guarantees from ST monad, but it
seems to me that this is a potential violation of memory-safety.
Consider the program below.
The use of [unsafeWrite] in [silly_function] is guarded by the code
above (we know we wrote [sz] so we should read back [sz]). It ought to
be safe if you can rely on sequential execution of ST monad with no
preemption.
However, it does end up preempted due to the use of [mfix] elsewhere in
the program, which leads to a segmentation fault.
import Data.Array.ST
import Data.Array.Base
import Control.Monad.ST.Strict
import Control.Monad.Fix
silly_function :: STArray s Int Int -> Int -> ST s ()
silly_function arr a = do
  (0, sz) <- getBounds arr
  writeArray arr 0 sz
  let !res = a
  sz <- readArray arr 0
  unsafeWrite arr sz res
foo :: ST s Int
foo = do
  arr <- newArray (0, 10) 0
  mfix $ \res -> do
    n <- readArray arr 0
    writeArray arr 0 1000000000
    if n > 0
    then
      return 666
    else do
      silly_function arr res
      readArray arr 10
main = print $ runST foo
On Sun, 8 Jul 2018 at 22:26, Levent Erkok  wrote:
...
Hi David,
Wonderful example. I'm afraid no-eager-blackholing also breaks the "no
spooky action at a distance" rule. Since `x` is not used recursively, we
should be able to pull it out of the `mfix` call, transforming the original
to:
foo :: ST s Int
foo = do
  ref <- newSTRef True
  x <- readSTRef ref
  mfix $ \res -> do
    if x
      then do
        writeSTRef ref False
        return $! res + 5 -- force the final result
      else return 10
I believe this variant will produce <<loop>> with or without
eager-blackholing, as it should. By this argument alone, I'd say the
no-eager-blackholing breaks mfix axioms for strict-state.
This example is also interesting from a pure termination point of view:
Moving things "out-of" mfix usually improves termination. In this case, the
opposite is happening.
Strictly speaking, this is in violation of the mfix-axioms. But I doubt
it's worth losing sleep over. I suggest we add this as an example in the
value-recursion section on how eager-blackholing can change things.
Cheers,
-Levent.
On Sun, Jul 8, 2018 at 9:40 AM, David Feuer 
wrote:
...
I filed a ticket[*] for this, but I think maybe the libraries list
should weigh in on whether it is something that should be fixed. In
general, fixST f is supposed to bottom out if f forces its argument.
However, the lazy way GHC blackholes thunks under evaluation sometimes
leads to the computation being run again. In certain contrived situations,
this can allow the computation to succeed!
The example I give in the ticket:
import Control.Monad.ST.Strict
import Control.Monad.Fix
import Data.STRef
foo :: ST s Int
foo = do
  ref <- newSTRef True
  mfix $ \res -> do
    x <- readSTRef ref
    if x
      then do
        writeSTRef ref False
        return $! res + 5 -- force the final result
      else return 10
main = print $ runST foo
Here, the computation writes to an STRef before forcing the final
result. Forcing the final result causes the computation to run again, this
time taking the other branch. The program prints 15. When compiled with -O
-feager-blackholing, however, the expected <<loop>> exception occurs.
As far as I know, this weirdness never changes the value produced by a
non-bottoming computation, and never changes a non-bottoming computation
into a bottoming one. The fix (defining fixST the way fixIO is currently
defined) would have a slight performance impact. Is it worth it?
[*] https://ghc.haskell.org/trac/ghc/ticket/15349
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