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    SubjectRe: [RFC] Raceless module interface

    On Fri, 13 Sep 2002, Daniel Phillips wrote:

    > > The exit itself can fail as well, so it has to be done by the module code
    > > anyway (until it suceeds).
    > That's debatable. Arguably, a failed ->module_cleanup() should be
    > retried on every rmmod -a, but expecting module.c to just keep
    > retrying mindlessly on its own sounds too much like a busy wait.

    That's not what I meant, if module_init fails the module goes directly to
    the cleanup state and the module code calls module_exit. Depending on this
    return value it continues to the exit state. Further exit attempts (if
    necessary) are done on user request.

    > > What DoS opportunities are there?
    > Suppose the module exit relies on synchronize_kernel. The attacker
    > can force repeated synchronize_kernels, knowing that module.c will
    > mindlessly do a synchronize_kernel every time a module init fails,
    > whether needed or not. Each synchronize_kernel takes an unbounded
    > amount of time to complete, across which module.c holds a lock.

    This can't happen:

    if (hook) {
    hook = NULL;

    > > Module init failure is the exception
    > > case and usally needs further attention, so we could actually disable
    > > further attempts to load this module, unless the user tells us
    > > specifically so.
    > Sure, you can fix it by lathering on more complexity. What you have
    > to do is explain why we should do that, when there is a simpler and
    > faster approach that doesn't introduce the problem in the first
    > place.

    It doesn't add any complexity (at least not to the kernel). A simple
    approach might be that a failed kernel module cannot be loaded with
    modprobe anymore, this sort of policy can be done in userspace.

    > I take it that the points you didn't reply to are points that you
    > agree with? (The main point being, that we both advocate a simple,
    > two-method interface for module load/unload.)

    Basically yes, it's just that your initial RFC was more confusing than

    bye, Roman

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