Hello Greg,
Thanks for your feedback.
On Thu, Jul 31, 2025 at 06:32:35AM +0200, Greg KH wrote:
On Thu, Jul 31, 2025 at 03:19:19AM +0100, Salah Triki wrote:
The driver stores a reference to the `usb_device` structure (`udev`) in its private data (`data->udev`), which can persist beyond the immediate context of the `bfusb_probe()` function.
Without proper reference count management, this can lead to two issues:
- A `use-after-free` scenario if `udev` is accessed after its main reference count drops to zero (e.g., if the device is disconnected and the `data` structure is still active).
How can that happen as during the probe/remove cycle, the reference count is always properly incremetned.
- A `memory leak` if `udev`'s reference count is not properly decremented during driver disconnect, preventing the `usb_device` object from being freed.
There is no leak here at all, sorry.
I understand your concern about the existence of a memory leak or use-after-free scenario in the driver's current context.
My intention with this patch is to ensure the driver adheres to best practices for managing `usb_device` structure references, as outlined in the kernel's documentation. The `usb_get_dev()` function is explicitly designed for use when a driver stores a reference to a `usb_device` structure in its private data, which is the case here with `data->udev`.
As the documentation for `usb_get_dev()` states:
``Each live reference to a device should be refcounted. Drivers for USB interfaces should normally record such references in their probe() methods, when they bind to an interface, and release them by calling usb_put_dev(), in their disconnect() methods.``
By following this recommendation, adding `usb_get_dev(udev)` in `bfusb_probe()` and `usb_put_dev(data->udev)` in `bfusb_disconnect()` ensures the `udev` structure's lifetime is explicitly managed by the driver as long as it's being referenced. This proactively prevents potential issues that could arise in future scenarios, even if a specific problem hasn't been observed or reported yet.
To correctly manage the `udev` lifetime, explicitly increment its reference count with `usb_get_dev(udev)` when storing it in the driver's private data. Correspondingly, decrement the reference count with `usb_put_dev(data->udev)` in the `bfusb_disconnect()` callback.
This ensures `udev` remains valid while referenced by the driver's private data and is properly released when no longer needed.
How was this tested?
I'm not saying the change is wrong, just that I don't think it's actually a leak, or fix of anything real.
Or do you have a workload that shows this is needed? If so, what is the crash reported?
While I don't have a specific workload that reproduces a current crash or memory leak, this patch aims to enhance the driver's robustness by aligning its behavior with the established conventions for managing `usb_device` object references. It's a preventive measure to ensure the driver correctly handles the lifetime of the `usb_device` object it references, even in scenarios of unexpected disconnection or re-enumeration that might otherwise have unforeseen consequences.
Please let me know if you have any further questions.
Regards,
Salah Triki