[PATCH v2] xf86drm: Add drmIsMaster()

[Daniel Vetter]

On Thu, Jan 24, 2019 at 10:45:04AM +0100, Daniel Vetter wrote:
> On Thu, Jan 24, 2019 at 09:56:51AM +1300, Christopher James Halse Rogers wrote:
> > On 24 January 2019 6:18:32 am NZDT, Emil Velikov <emil.l.velikov at gmail.com> wrote:
> > >On Wed, 23 Jan 2019 at 04:39, Christopher James Halse Rogers
> > ><christopher.halse.rogers at canonical.com> wrote:
> > >>
> > >> We can't use drmSetMaster to query whether or not a drm fd is master
> > >> because it requires CAP_SYS_ADMIN, even if the fd *is* a master fd.
> > >>
> > >> Pick DRM_IOCTL_MODE_ATTACHMODE as a long-deprecated ioctl that is
> > >> DRM_MASTER but not DRM_ROOT_ONLY as the probe by which we can detect
> > >> whether or not the fd is master.
> > >>
> > >> This is useful for code that might get master by open()ing the drm
> > >device
> > >> while no other master exists, but can't call drmSetMaster itself
> > >because
> > >> it's not running as root or is in a container, where container-root
> > >isn't
> > >> real-root.
> > >>
> > >> v2: Use the AUTH_MAGIC request rather than MODE_ATTACHMODE, as it's
> > >more
> > >>     clearly related to master status.
> > >>
> > >> Signed-off-by: Christopher James Halse Rogers
> > ><christopher.halse.rogers at canonical.com>
> > >> ---
> > >>  xf86drm.c | 15 +++++++++++++++
> > >>  xf86drm.h |  2 ++
> > >>  2 files changed, 17 insertions(+)
> > >>
> > >> diff --git a/xf86drm.c b/xf86drm.c
> > >> index 10df682b..adee5bd9 100644
> > >> --- a/xf86drm.c
> > >> +++ b/xf86drm.c
> > >> @@ -2741,6 +2741,21 @@ drm_public int drmDropMaster(int fd)
> > >>          return drmIoctl(fd, DRM_IOCTL_DROP_MASTER, NULL);
> > >>  }
> > >>
> > >> +drm_public bool drmIsMaster(int fd)
> > >> +{
> > >> +        /* Detect master by attempting something that requires
> > >master.
> > >> +         *
> > >> +         * Authenticating magic tokens requires master and 0 is
> > >> +         * guaranteed to be an invalid magic number. Attempting this
> > >on
> > >> +         * a master fd will fail therefore fail with EINVAL because
> > >0 is
> > >> +         * invalid.
> > >> +         *
> > >> +         * A non-master fd will fail with EACCESS, as the kernel
> > >checks for
> > >> +         * master before attempting to do anything else.
> > >> +         */
> > >> +        return drmAuthMagic(fd, 0) == EINVAL;
> > >What magic value is valid, is a DRM implementation detail, which we
> > >don't need to depend upon.
> > >
> > >Instead we can check for EACCES, since we care if we have permissions
> > >- aka are we master.
> > >The function returns a negative errno, so I'd make this a:
> > >
> > >        return drmAuthMagic(fd, 0) != -EACCES;
> > >
> > >If you and Daniel agree, I'll squash this locally and push.
> > 
> > That's a much better idea, thanks!
> 
> I don't like checks for "something else happened", I much prefer checking
> for something specific. Hence == -EINVAL over != EACCESS. But I guess in
> this case here it doesn't matter.
> 
> We just need to make sure that the igt tests both ways, to make sure we'll
> never ever break this. I'd still prefer the current version, imo easier to
> write an igt to make sure it keeps working.

Since this landed now ... is the igt anywhere to make sure this keeps
working?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch