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    SubjectRe: [RFC] Input: Remove unsafe device module references
    On Tue, Nov 01, 2011 at 06:52:11PM +0100, David Herrmann wrote:
    > Hi Greg
    >
    > On Tue, Nov 1, 2011 at 6:01 PM, Greg KH <gregkh@suse.de> wrote:
    > > On Tue, Nov 01, 2011 at 04:41:40PM +0100, David Herrmann wrote:
    > >> Hi Dmitry and Greg
    > >>
    > >> It doesn't make sense to take a reference to our own module. When we call
    > >> module_put(THIS_MODULE) we cannot make sure that our module is still alive when
    > >> this function returns. Therefore, module_put() will return to invalid memory and
    > >> our input_dev_release() function is no longer available.
    > >>
    > >> It would be interesting if Greg could elaborate what else we could do to replace
    > >> this module-refcount as it is definitely needed here. However, "struct device"
    > >> doesn't provide an owner field so there is no way for us to let the device core
    > >> keep a reference to our module.
    > >
    > > For a bus module, yes, this is needed, so don't remove these calls, it's
    > > wrong to do so.
    > >
    > >> I have no clue what to do here but the current implementation is definitely
    > >> unsafe so this is marked as RFC. Currently, the device_attributes probably
    > >> already keep a reference to our module so applying this patch would probably not
    > >> break anything, however, this does not look like something we can trust on.
    > >
    > > Yes it is, why do you think it isn't?
    > >
    > >> My bug-thread kind of died (https://lkml.org/lkml/2011/10/29/75) so I now try to
    > >> show this with an example here.
    > >
    > > It died due to me traveling, sorry, I'll respond to them now.
    >
    > No problem. This is why I've resent this with an example.
    >
    > > I fail to see what the real problem you are trying to solve here is.  Is
    > > there something with the way the kernel works today that you are having
    > > problems with?  What is driving this?
    >
    > I am working on converting the hci stack to properly use sysfs APIs +
    > struct device. And my problem simply is the following:
    >
    > @@ -1417,8 +1417,6 @@ static void input_dev_release(struct device *device)
    > input_mt_destroy_slots(dev);
    > kfree(dev->absinfo);
    > kfree(dev);
    > -
    > - module_put(THIS_MODULE);
    > }
    >
    > If this module_put(THIS_MODULE) is needed as you said, then I can be
    > sure that this call does not release the last module-reference, can I?
    > Otherwise, this call may return to invalid memory.
    >
    > But, if I can be sure that this doesn't release the last reference,
    > why take this reference at all?
    >
    > The only reason I can think of is, that some other code calls
    > __get_module() after I called it, and it calls put_module() after I
    > call it. In all other cases, taking/releasing this reference is
    > needless as we can trust that our caller protects us.
    >
    > In other words, which code does this module_get/put() protect? It
    > cannot protect input_dev_release() because module_put(THIS_MODULE) is
    > called *inside* input_dev_release(). I need some way to protect the
    > input_dev_release() callback-code outside of this callback.
    > Or can I go sure that the caller of the input_dev_callback() takes a
    > reference to my module before calling this and releases it after? (But
    > then I wonder how does it know what module I am?)
    >
    > If this is the recommended way to protect the device_release
    > callbacks, I will just copy it into hci_dev, but currently I really
    > don't get why these are needed.
    > If you can tell me an example why the input-core breaks if this patch
    > is applied, I can probably better explain to you, why I think it still
    > breaks without this patch applied.
    >
    >
    >
    > For instance see my example:
    >
    > 1)
    > input-core-module is loaded
    > 2)
    > input-core-module creates a new input device and increases
    > module-refcnt inside input_allocate_device()
    > 4)
    > another subsystem grabs the "struct device" and increases its refcnt
    > (for any reason...)
    > 5)
    > input-core-module destroys the input-device but it still stays alive
    > until the other subsystem releases its refcnt of the "struct device".
    > 6)
    > input-core-module is unloaded
    > This doesn't succeed as the still living input-device has a module-refcnt
    > 7)
    > the other subsystem releases its refcnt of the input-device
    > 8)
    > The input-device is destroyed and its _release_ function is called
    > The release function destroys the input-device *and* frees the last
    > module-refcnt. Then *boom*, the release function cannot return as it
    > is no longer available as described above.

    The analysis is right except that this condition is very unlikely to
    trigger. By removing __module_get/module_put you are making this problem
    much much easier to hit.

    >
    > My solution: Some parent subsystem of us must take and release this
    > module-refcnt instead of us, so this bug doesn't occur.
    > Or: We simply wait for all these input_devices to be released before
    > exiting input_exit().

    How would you know that all input devices are released in the sense
    that all threads left (as in exited) all code in input.c completely?

    --
    Dmitry
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