Re: [PATCH 1/2] iouring: one capable call per iouring instance

[Ming Lei <ming.lei@redhat.com>]

On Mon, Dec 04, 2023 at 11:57:55AM -0700, Keith Busch wrote:
> On Mon, Dec 04, 2023 at 01:40:58PM -0500, Jeff Moyer wrote:
> > I added a CC: linux-security-module@vger
> > Keith Busch <kbusch@meta.com> writes:
> > > From: Keith Busch <kbusch@kernel.org>
> > >
> > > The uring_cmd operation is often used for privileged actions, so drivers
> > > subscribing to this interface check capable() for each command. The
> > > capable() function is not fast path friendly for many kernel configs,
> > > and this can really harm performance. Stash the capable sys admin
> > > attribute in the io_uring context and set a new issue_flag for the
> > > uring_cmd interface.
> > 
> > I have a few questions.  What privileged actions are performance
> > sensitive? I would hope that anything requiring privileges would not
> > be in a fast path (but clearly that's not the case).
> 
> Protocol specifics that don't have a generic equivalent. For example,
> NVMe FDP is reachable only through the uring_cmd and ioctl interfaces,
> but you use it like normal reads and writes so has to be as fast as the
> generic interfaces.

But normal read/write pt command doesn't require ADMIN any more since 
commit 855b7717f44b ("nvme: fine-granular CAP_SYS_ADMIN for nvme io commands"),
why do you have to pay the cost of checking capable(CAP_SYS_ADMIN)?


Thanks, 
Ming