Re: [RFC v2 1/2] USB: core: add a way to revoke access to open USB devices

[Alan Stern <stern@rowland.harvard.edu>]

On Fri, Aug 05, 2022 at 02:38:13PM +0200, Bastien Nocera wrote:
> On Thu, 2022-08-04 at 15:25 -0400, Alan Stern wrote:
> > Have you considered what should happen if two processes share the
> > same 
> > file descriptor (and hence the same usb_dev_state structure) and you
> > want 
> > to revoke access for one of the processes but not the other?
> 
> No, because this isn't something that happens in practice, as it's
> simpler for each programme to open their own file descriptor and claim
> the interfaces they need on their own.

But it is possible for a program to open a USB device and then fork 
several children.  The children would all share the same file descriptor.

I have no idea how often this happens in practice.  But kernel design is 
supposed to be based on correctness, not on handling only things that 
crop up "in practice".

> > I have the feeling that this part of the function (matching the
> > busnum 
> > and devnum values) doesn't belong here but rather with the iteration 
> > routines in your 2/2 patch.  Filtering of devices generally is done
> > as 
> > part of the iteration.  As an added bonus, doing it that way means
> > you 
> > don't need to pass around pointers to usb_revoke_match structures.
> 
> I felt it better to have the filtering done in one place, to avoid
> passing just a uid to check to that function.

There's nothing wrong with passing just a uid.  Especially since the same 
device might be open multiple times, for varying uids.

> Should I rename the function something like usb_revoke_for_uid() ?

Up to you.

Alan Stern