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While any of the GPIO cdev syscalls is in progress, the kernel can call gpiochip_remove() (for instance, when a USB GPIO expander is disconnected) which will set gdev->chip to NULL after which any subsequent access will cause a crash. To avoid that: use an RW-semaphore in which the syscalls take it for reading (so that we don't needlessly prohibit the user-space from calling syscalls simultaneously) while gpiochip_remove() takes it for writing so that it can only happen once all syscalls return. Fixes:d7c51b47ac("gpio: userspace ABI for reading/writing GPIO lines") Fixes:3c0d9c635a("gpiolib: cdev: support GPIO_V2_GET_LINE_IOCTL and GPIO_V2_LINE_GET_VALUES_IOCTL") Fixes:aad955842d("gpiolib: cdev: support GPIO_V2_GET_LINEINFO_IOCTL and GPIO_V2_GET_LINEINFO_WATCH_IOCTL") Fixes:a54756cb24("gpiolib: cdev: support GPIO_V2_LINE_SET_CONFIG_IOCTL") Fixes:7b8e00d981("gpiolib: cdev: support GPIO_V2_LINE_SET_VALUES_IOCTL") Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org> [Nick: fixed a build failure with CDEV_V1 disabled] Co-authored-by: Nick Hainke <vincent@systemli.org> Reviewed-by: Kent Gibson <warthog618@gmail.com> Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com> Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.