On 17/10/2021 16:40:14+0200, Greg KH wrote: > On Sun, Oct 17, 2021 at 04:02:42PM +0200, Alexandre Belloni wrote: > > On 17/10/2021 15:50:11+0200, Greg KH wrote: > > > Note, review of this now that it has been submitted in a pull request to > > > me, sorry I missed this previously... > > > > > > On Wed, Sep 29, 2021 at 12:15:59PM +0900, William Breathitt Gray wrote: > > > > +static int counter_chrdev_open(struct inode *inode, struct file *filp) > > > > +{ > > > > + struct counter_device *const counter = container_of(inode->i_cdev, > > > > + typeof(*counter), > > > > + chrdev); > > > > + > > > > + /* Ensure chrdev is not opened more than 1 at a time */ > > > > + if (!atomic_add_unless(&counter->chrdev_lock, 1, 1)) > > > > + return -EBUSY; > > > > > > I understand the feeling that you wish to stop userspace from doing > > > this, but really, it does not work. Eventhough you are doing this > > > correctly (you should see all the other attempts at doing this), you are > > > not preventing userspace from having multiple processes access this > > > device node at the same time, so please, don't even attempt to stop this > > > from happening. > > > > > > So you can drop the atomic "lock" you have here, it's not needed at all. > > > > > > > Could you elaborate a bit here because we've had a similar thing in the > > RTC subsystem: > > > > https://elixir.bootlin.com/linux/latest/source/drivers/rtc/dev.c#L28 > > Yeah, that too will not work :( Note, it does stop open from being > called from different processes, but think of the following sequence of > userspace calls: > open() > fork/exec() > both processes access the file descriptor > > or passing a fd across a socket? > > Or duplicating the file descriptor and sending it to a different task > (like across a socket or many other IPC ways)? > > Once userspace has a file descriptor, all bets are off as to where it > goes and what it does with it. There's no need to try to save userspace > from itself by preventing multiple opens when really, it doesn't stop > anyone who really wants to do this. > > If userspace does do multiple read/writes from different threads / > processes / whatever on the same file descriptor, it gets to keep the > pieces of the mess it causes. It's not the kernel's job to try to > "protect" userspace from itself here. > > Look at serial/tty connections as one example of this always being the > case. > > Does that help? > Thanks for the explanation, this is now clear to me. > > And it would mean I can remove rtc->flags completely. > > I think you can do that. > > thanks, > > greg k-h -- Alexandre Belloni, co-owner and COO, Bootlin Embedded Linux and Kernel engineering https://bootlin.com