Re: [PATCH v6 02/16] chardev: introduce cdev_get_by_path()

[Greg Kroah-Hartman]

On Thu, Jul 25, 2019 at 12:02:30PM -0700, Sagi Grimberg wrote:
> 
> > > > > Why do you have a "string" within the kernel and are not using the
> > > > > normal open() call from userspace on the character device node on the
> > > > > filesystem in your namespace/mount/whatever?
> > > > 
> > > > NVMe-OF is configured using configfs. The target is specified by the
> > > > user writing a path to a configfs attribute. This is the way it works
> > > > today but with blkdev_get_by_path()[1]. For the passthru code, we need
> > > > to get a nvme_ctrl instead of a block_device, but the principal is the same.
> > > 
> > > Why isn't a fd being passed in there instead of a random string?
> > 
> > I wouldn't know the answer to this but I assume because once we decided
> > to use configfs, there was no way for the user to pass the kernel an fd.
> 
> That's definitely not changing. But this is not different than how we
> use the block device or file configuration, this just happen to need the
> nvme controller chardev now to issue I/O.

So, as was kind of alluded to in another part of the thread, what are
you doing about permissions?  It seems that any user/group permissions
are out the window when you have the kernel itself do the opening of the
char device, right?  Why is that ok?  You can pass it _any_ character
device node and away it goes?  What if you give it a "wrong" one?  Char
devices are very different from block devices this way.

thanks,

greg k-h