Re: [PATCH 1/2] uio: Add late_release callback to uio_info

On 10.02.21 20:47, Greg Kroah-Hartman wrote:

On Wed, Feb 10, 2021 at 08:40:30PM +0100, Bodo Stroesser wrote:

quoted

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@gmail.com>
---
  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

tcmu has a release callback. But uio can't call it after
uio_unregister_device was executed, because in uio_unregister_device
uio sets the uio_device::info to NULL.

So, uio would never call both callbacks for the same release action,
but would call release before uio_unregister_device is executed, and
late_release after that.

Of course it would be good for tcmu if uio would call uio_info:release 
even after uio_unregister_device, but changing this AFAICS could cause
trouble in other drivers using uio.

Bodo
after uio_unregister_device.