On Wed, Nov 28, 2012 at 11:38 AM, Leon Smith <
leon.p.smith@gmail.com> wrote:
> I have some code that reads (infrequently) small amounts of data from
> /dev/urandom, and because this is pretty infrequent, I simply open the
> handle and close it every time I need some random bytes.
>
> The problem is that I recently discovered that, thanks to buffering within
> GHC, I was actually reading 8096 bytes when I only need 16 bytes, and
> thus wasting entropy. Moreover calling hSetBuffering handle NoBuffering
> did not change this behavior.
>
> I'm not sure if this behavior is a bug or a feature, but in any case it's
> unacceptable for dealing with /dev/urandom. Probably the simplest way to
> fix this is to write a little C helper function that will read from
> /dev/urandom for me, so that I have precise control over the system calls
> involved. But I'm curious if GHC can manage this use case correctly;
> I've just started digging into the
GHC.IO code myself.
>
> Best,
> Leon
>
> {-# LANGUAGE BangPatterns, ViewPatterns #-}
>
> import Control.Applicative
> import Data.Bits
> import Data.Word(Word64)
> import qualified Data.ByteString as S
> import qualified Data.ByteString.Lazy as L
> import Data.ByteString.Internal (c2w)
> import qualified System.IO as IO
> import qualified Data.Binary.Get as Get
>
> showHex :: Word64 -> S.ByteString
> showHex n = s
> where
> (!s,_) = S.unfoldrN 16 f n
>
> f n = Just (char (n `shiftR` 60), n `shiftL` 4)
>
> char (fromIntegral -> i)
> | i < 10 = (c2w '0' - 0) + i
> | otherwise = (c2w 'a' - 10) + i
>
> twoRandomWord64s :: IO (Word64,Word64)
> twoRandomWord64s = IO.withBinaryFile "/dev/urandom" IO.ReadMode $ \handle ->
> do
> IO.hSetBuffering handle IO.NoBuffering
> Get.runGet ((,) <$> Get.getWord64host <*> Get.getWord64host) <$> L.hGet
> handle 16
>
> main = do
> (x,y) <- twoRandomWord64s
> S.hPutStrLn IO.stdout (S.append (showHex x) (showHex y))
>
>
> {- Relevant part of strace:
>
> open("/dev/urandom", O_RDONLY|O_NOCTTY|O_NONBLOCK) = 3
> fstat(3, {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 9), ...}) = 0
> ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7ffff367e528) = -1 EINVAL (Invalid
> argument)
> ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7ffff367e528) = -1 EINVAL (Invalid
> argument)
> read(3,
> "N\304\4\367/\26c\"\3218\237f\214yKg~i\310\r\262\"\224H\340y\n\376V?\265\344"...,
> 8096) = 8096
> close(3) = 0
>
> -}
>
>