Executing a ByteString

Why not!

{-# LANGUAGE ForeignFunctionInterface #-}
import Foreign ( FunPtr, withForeignPtr, castPtrToFunPtr )
import System  ( getArgs )
import qualified Data.ByteString as B ( pack, ByteString )
import qualified Data.ByteString.Internal as B ( toForeignPtr )

payload :: B.ByteString
payload = B.pack [
  0x55,                   -- push   %rbp
  0x48, 0x89, 0xe5,       -- mov    %rsp,%rbp
  0x89, 0x7d, 0xec,       -- mov    %edi,-0x14(%rbp)
  0xc7, 0x45, 0xfc,
    1, 0, 0, 0,           -- movl   $0x1,-0x4(%rbp)
  0xeb, 0x0e,             -- jmp    1e <fact+0x1e>
  0x8b, 0x45, 0xfc,       -- mov    -0x4(%rbp),%eax
  0x0f, 0xaf, 0x45, 0xec, -- imul   -0x14(%rbp),%eax
  0x89, 0x45, 0xfc,       -- mov    %eax,-0x4(%rbp)
  0x83, 0x6d, 0xec, 1,    -- subl   $0x1,-0x14(%rbp)
  0x83, 0x7d, 0xec, 1,    -- cmpl   $0x1,-0x14(%rbp)
  0x7f, 0xec,             -- jg     10 <fact+0x10>
  0x8b, 0x45, 0xfc,       -- mov    -0x4(%rbp),%eax
  0xc9,                   -- leaveq 
  0xc3 ]                  -- retq

type F = Int -> Int

foreign import ccall "dynamic"
  getF :: FunPtr F -> F

main :: IO ()
main = do
  [s] <- getArgs
  n   <- readIO s
  let (fptr, _, _) = B.toForeignPtr payload
  withForeignPtr fptr $ \ptr -> do
    let f = getF $ castPtrToFunPtr ptr
    print $ f n

Testing it:

$ runhaskell foo.hs 6
720

This code will only work on x86-64.