[Haskell-cafe] chroot a single thread
jeremy at n-heptane.com
Sat Dec 27 21:01:52 EST 2008
I am working on an application where I would like to chroot a thread,
but I am not seeing a way to do it.
I already have code which can run an IO action inside a chroot. The
type signature is:
fchroot :: FilePath -> IO a -> IO a
The first argument is the new root, the second argument is the action
to run with that root.
You can see the implementation here if you are curious:
The problem with that function is that chroot affects the root of the
whole process. In a single-threaded program this is (possibly) ok,
because the original root is restored after the IO action completes
(though with unsafeInterleaveIO, perhaps bad stuff will happen). In a
multithreaded program it can be disasterous.
As far as I know, the only solution would be to implement a function
like fchroot which looks more like:
pchroot :: FilePath -> IO () -> IO ExitCode
Here the IO action would be forked off and run in a whole new process
so that changing its root does not affect other 'threads'. Of course,
you also can't use any of the nifty Haskell intra-thread communication
stuff. Basically you can pass some values in and get back an exit
However, I am not even sure how to implement a useful version of
pchroot. In theory, I can just use forkProcess, but, the Giant Warning
for forkProcess indicates that this will not be very useful in
forkProcess comes with a giant warning: since any other running
threads are not copied into the child process, it's easy to go
wrong: e.g. by accessing some shared resource that was held by
another thread in the parent. Another example is the I/O manager
thread: since the I/O manager isn't running in the child,
attempting to do any Handle-based I/O will deadlock.
Anyone have an ideas ?
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