Re: [PATCH v2 2/2] rust: miscdevice: access the `struct miscdevice` from fops->open()

[Alice Ryhl]

On Mon, Dec 9, 2024 at 12:10 PM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Dec 09, 2024 at 11:50:57AM +0100, Alice Ryhl wrote:
> > On Mon, Dec 9, 2024 at 9:48 AM Greg Kroah-Hartman
> > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Mon, Dec 09, 2024 at 07:27:47AM +0000, Alice Ryhl wrote:
> > > > Providing access to the underlying `struct miscdevice` is useful for
> > > > various reasons. For example, this allows you access the miscdevice's
> > > > internal `struct device` for use with the `dev_*` printing macros.
> > > >
> > > > Note that since the underlying `struct miscdevice` could get freed at
> > > > any point after the fops->open() call, only the open call is given
> > > > access to it. To print from other calls, they should take a refcount on
> > > > the device to keep it alive.
> > >
> > > The lifespan of the miscdevice is at least from open until close, so
> > > it's safe for at least then (i.e. read/write/ioctl/etc.)
> >
> > How is that enforced? What happens if I call misc_deregister while
> > there are open fds?
>
> You shouldn't be able to do that as the code that would be calling
> misc_deregister() (i.e. in a module unload path) would not work because
> the module reference count is incremented at this point in time due to
> the file operation module reference.

Oh .. so misc_deregister must only be called when the module is being unloaded?

> Wait, we are plumbing in the module owner logic here, right?  That
> should be in the file operations structure.

Right ... it's missing but I will add it.

> Yeah, it's a horrid hack, and one day we will put "real" revoke logic in
> here to detach the misc device from the file operations if this were to
> happen.  It's a very very common anti-pattern that many subsystems have
> that is a bug that we all have been talking about for a very very long
> time.  Wolfram even has a plan for how to fix it all up (see his Japan
> LinuxCon talk from 2 years ago), but I don't think anyone is doing the
> work on it :(
>
> The media and drm layers have internal hacks/work-arounds to try to
> handle this issue, but luckily for us, the odds of a misc device being
> dynamically removed from the system is pretty low.
>
> Once / if ever, we get the revoke type logic implemented, then we can
> apply that to the misc device code and follow it through to the rust
> side if needed.

If dynamically deregistering is not safe, then we need to change the
Rust abstractions to prevent it.

Alice