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
