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.

- Jamie
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