Re: devfs initial permissions

Richard Gooch (rgooch@atnf.CSIRO.AU)
Mon, 12 Jan 1998 08:08:33 +1100

Andreas Jellinghaus writes:
> >People are devising all kinds of schemes for sensible devfs initial
> >defalut values. In particular, one person had a table of uids and gids
> >compiled into the kernel. I think this is a Bad Thing.
>
> >Using gid 0 is not a horrible idea, but I think that using any uid or gid
> >*except* 0 is a Bad Thing.
>
> i agree.
>
> >I'd prefer having a default mode of 000, to make it clear that you really
> >have to make a decision about the permissions, and the kernel isn't going
> >to do it for you.
>
> i disagree. /dev/null should be always write and readable and /dev/zero
> always readable.
>
> the device driver should suggest a permission using this table :
> first 3 bits (suid, sgid, t) always 0
> all execute bits always 0
> owner read 1 if driver supports read
> owner write 1 if driver supports write
> global read 0 default
> 1 read access will not give additional resources,
> driver was designed to offer public read
> (e.g. null zero random urandom)
> global write 0 default
> 1 write access will not give additional resource
> driver was designed to offer public write
> (e.g. null random urandowm)
> user read 0 default
> 1 if reading access will give resources, but not
> compromisse system security
> (audio, mouse and serial devices, but not disk
> or /dev/kmem)
> user write 0 default
> 1 if write access will give resources, but not
> compromisse system security
> (audio, mouse and serial devices, but not disk
> or /dev/kmem)
>
> summary : device drivers should give default permission.
> give a permission for "group", if it makes sence to have a special group
> on your system (e.g. "audio"), some privileged users in that group (e.g.
> me) so they can access additional resources.
>
> this does not make sence for things like hard disks or /dev/kmem :
> access to devices could be used to gain root status (in case of the root
> hard disk or the kmem this should be easy).

So my system where the device driver provides a default mode,uid,gid
does everything you want, easily. There's no need to invent new
permission variable types. It's up to the driver writer to choose
permissions.

Regards,

Richard....