Re: [PATCH] block: Check ADMIN before NICE for IOPRIO_CLASS_RT

[Serge E. Hallyn]

On Tue, Nov 16, 2021 at 09:30:12AM +0000, David Laight wrote:
> From: Alistair Delva
> > Sent: 15 November 2021 19:09
> ...
> > > > -                       if (!capable(CAP_SYS_NICE) && !capable(CAP_SYS_ADMIN))
> > > > +                       if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_NICE))
> > > >                                 return -EPERM;
> 
> Isn't the real problem that you actually want to test:
> 		if (!capable(CAP_SYS_NICE | CAP_SYS_ADMIN))
> 			return -EPERM;
> so that you only get the fail 'splat' when neither is set.
> 
> This will be true whenever more than one capability enables something.
> 
> Possibly this needs something like:
> int capabale_or(unsigned int, ...);
> #define capabale_or(...) capabable_or(__VA_LIST__, ~0u)
> 
> 	David

Right, that's what i was suggesting yesterday.  We do this in other
places, where we split off a more fine-grained version of a gross
capability.  If we care enough about the audit messages, then we
probably do need a new primitive.

-serge