On Thu, Jun 27, 2019 at 01:00:03AM +0000, Song Liu wrote:
>
>
> > On Jun 26, 2019, at 5:08 PM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Wed, Jun 26, 2019 at 03:17:47PM +0000, Song Liu wrote:
> >>>> +static struct miscdevice bpf_dev = {
> >>>> + .minor = MISC_DYNAMIC_MINOR,
> >>>> + .name = "bpf",
> >>>> + .fops = &bpf_chardev_ops,
> >>>> + .mode = 0440,
> >>>> + .nodename = "bpf",
> >>>
> >>> Here's what kvm does:
> >>>
> >>> static struct miscdevice kvm_dev = {
> >>> KVM_MINOR,
> >>> "kvm",
> >>> &kvm_chardev_ops,
> >>> };
> >
> > Ick, I thought we converted all of these to named initializers a long
> > time ago :)
> >
> >>> Is there an actual reason that mode is not 0 by default in bpf case? Why
> >>> we need to define nodename?
> >>
> >> Based on my understanding, mode of 0440 is what we want. If we leave it
> >> as 0, it will use default value of 0600. I guess we can just set it to
> >> 0440, as user space can change it later anyway.
> >
> > Don't rely on userspace changing it, set it to what you want the
> > permissions to be in the kernel here, otherwise you have to create a new
> > udev rule and get it merged into all of the distros. Just do it right
> > the first time and there is no need for it.
> >
> > What is wrong with 0600 for this? Why 0440?
>
> We would like root to own the device, and let users in a certain group
> to be able to open it. So 0440 is what we need.
But you are doing a "write" ioctl here, right? So don't you really need
0660 at the least? And if you "know" the group id, I think you can
specify it too so udev doesn't have to do a ton of work, but that only
works for groups that all distros number the same.
And why again is this an ioctl instead of a syscall? What is so magic
about the file descriptor here?
thanks
greg k-h
