On Wed, Feb 10, 2021 at 08:40:30PM +0100, Bodo Stroesser wrote: > If uio_unregister_device() is called while userspace daemon > still holds the uio device open or mmap'ed, uio will not call > uio_info->release() on later close / munmap. > > At least one user of uio (tcmu) should not free resources (pages > allocated by tcmu which are mmap'ed to userspace) while uio > device still is open, because that could cause userspace daemon > to be killed by SIGSEGV or SIGBUS. Therefore tcmu frees the > pages only after it called uio_unregister_device _and_ the device > was closed. > So, uio not calling uio_info->release causes trouble. > tcmu currently leaks memory in that case. > > Just waiting for userspace daemon to exit before calling > uio_unregister_device I think is not the right solution, because > daemon would not become aware of kernel code wanting to destroy > the uio device. > After uio_unregister_device was called, reading or writing the > uio device returns -EIO, which normally results in daemon exit. > > This patch adds new callback pointer 'late_release' to struct > uio_info. If uio user sets this callback, it will be called by > uio if userspace closes / munmaps the device after > uio_unregister_device was executed. > > That way we can use uio_unregister_device() to notify userspace > that we are going to destroy the device, but still get a call > to late_release when uio device is finally closed. > > Signed-off-by: Bodo Stroesser <bostroesser@xxxxxxxxx> > --- > Documentation/driver-api/uio-howto.rst | 10 ++++++++++ > drivers/uio/uio.c | 4 ++++ > include/linux/uio_driver.h | 4 ++++ > 3 files changed, 18 insertions(+) > > diff --git a/Documentation/driver-api/uio-howto.rst b/Documentation/driver-api/uio-howto.rst > index 907ffa3b38f5..a2d57a7d623a 100644 > --- a/Documentation/driver-api/uio-howto.rst > +++ b/Documentation/driver-api/uio-howto.rst > @@ -265,6 +265,16 @@ the members are required, others are optional. > function. The parameter ``irq_on`` will be 0 to disable interrupts > and 1 to enable them. > > +- ``int (*late_release)(struct uio_info *info, struct inode *inode)``: > + Optional. If you define your own :c:func:`open()`, you will > + in certain cases also want a custom :c:func:`late_release()` > + function. If uio device is unregistered - by calling > + :c:func:`uio_unregister_device()` - while it is open or mmap'ed by > + userspace, the custom :c:func:`release()` function will not be > + called when userspace later closes the device. An optionally > + specified :c:func:`late_release()` function will be called in that > + situation. > + > Usually, your device will have one or more memory regions that can be > mapped to user space. For each region, you have to set up a > ``struct uio_mem`` in the ``mem[]`` array. Here's a description of the > diff --git a/drivers/uio/uio.c b/drivers/uio/uio.c > index ea96e319c8a0..0b2636f8d373 100644 > --- a/drivers/uio/uio.c > +++ b/drivers/uio/uio.c > @@ -532,6 +532,8 @@ static int uio_release(struct inode *inode, struct file *filep) > mutex_lock(&idev->info_lock); > if (idev->info && idev->info->release) > ret = idev->info->release(idev->info, inode); > + else if (idev->late_info && idev->late_info->late_release) > + ret = idev->late_info->late_release(idev->late_info, inode); > mutex_unlock(&idev->info_lock); Why can't release() be called here? Why doesn't your driver define a release() if it cares about this information? Why do we need 2 different callbacks that fire at exactly the same time? This feels really wrong. greg k-h
