A PRIV_* flag for /dev/mem?

[Konstantin Belousov]

On Fri, May 17, 2013 at 01:14:23PM -0600, Jamie Gritton wrote:
> I'm considering Alexander Leidinger's patch to make X11 work inside a 
> jail (http://leidinger.net/FreeBSD/current-patches/0_jail.diff).  It 
> allows a jail to optionally have access to /dev/io and DRI (provided the 
> requisite device files are visible in the devfs ruleset).
> 
> I'm planning on putting this under a single jail permission, which would 
> group those two together as device access that allows messing with 
> kernel memory.  It seems more complete to put /dev/mem under that same 
> umbrella, with the side benefit of letting me call it "allow.dev_mem".
> 
> Currently, access is controlled only by device file permission and a 
> securelevel check.  Jail access is allowed as long as the /dev/mem is in 
> the jail's ruleset (it isn't by default).  Adding a prison_priv_check() 
> call would allow some finer control over this.  Something like:
> 
> int
> memopen(struct cdev *dev __unused, int flags, int fmt __unused,
>      struct thread *td)
> {
>      int error;
> 
>      error = priv_check(td, PRIV_FOO);
>      if (error != 0 && (flags & FWRITE))
>          error = securelevel_gt(td->td_ucred, 0);
> 
>      return (error);
> }
> 
> The main question I'm coming up with here is, what PRIV_* flag should I 
> use.  Does PRIV_IO make sense?  PRIV_DRIVER?  Something new like 
> PRIV_KMEM?  Also, I'd appreciate if anyone familiar with this interface 
> can tell me if memopen() is the right/only place to make this change.

Why do we need the PRIV check there at all, esp. for DRM ?
Why the devfs rulesets are not enough ?
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