linux-kernel - Re: RFC: capabilities(7): notes for kernel developers

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Message-ID: <1901394.l2S6b8SCmT@aspire.rjw.lan>
Date:   Mon, 19 Dec 2016 21:20:24 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     John Stultz <john.stultz@...aro.org>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jann Horn <jann@...jh.net>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-man <linux-man@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: RFC: capabilities(7): notes for kernel developers

On Friday, December 16, 2016 01:16:15 PM John Stultz wrote:
> On Fri, Dec 16, 2016 at 1:05 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
> > Quoting John Stultz (john.stultz@...aro.org):
> >> On Fri, Dec 16, 2016 at 12:10 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
> >> > Quoting Michael Kerrisk (man-pages) (mtk.manpages@...il.com):
> >> >> On 12/16/2016 01:44 AM, Casey Schaufler wrote:
> >> >> > On 12/15/2016 4:31 PM, John Stultz wrote:
> >> >> >> On Thu, Dec 15, 2016 at 12:40 PM, Casey Schaufler
> >> >> >> <casey@...aufler-ca.com> wrote:
> >> >> >>> On 12/15/2016 11:41 AM, Michael Kerrisk (man-pages) wrote:
> >> >> >>>> On 12/15/2016 05:29 PM, Casey Schaufler wrote:
> >> >> >>>>> CAP_WAKE_ALARM could readily be CAP_TIME.
> >> >> >>>> Actually, I don't quite understand what you mean with that sentence.
> >> >> >>>> Could you elaborate?
> >> >> >>> Should have said CAP_SYS_TIME
> >> >> >>>
> >> >> >>> Setting an alarm could be considered a time management function,
> >> >> >>> depending on what it actually does.
> >> >> >> Just a nit here. CAP_WAKE_ALARM is more about the privilege of waking
> >> >> >> a system from suspend, while CAP_SYS_TIME covers the ability to set
> >> >> >> the time. One wouldn't necessarily want to give applications which
> >> >> >> could wake a system up the capability to also set the time.
> >> >> >
> >> >> > Doesn't really matter, except that an ignorant developer
> >> >> > might make the mistake I did and assume that WAKE_ALARM
> >> >> > was somehow related to time management. If you want to use
> >> >> > it as an example don't let my dunderheadedness get in your
> >> >> > way.
> >> >>
> >> >> Actually, I decided it wasn't such a good example anyway.
> >> >> That capability could potentially be generic. (But it probably
> >> >> should better have been named something like 'CAP_WAKE_SYSTEM'.)
> >> >
> >> > How about:
> >> >
> >> > Subject: [PATCH 1/1] capabilities: alias CAP_WAKE_SYSTEM to CAP_WAKE_ALARM
> >> >
> >> > As suggested by Michael Kerrisk his is a less confusing name, and
> >> > this won't break any old userspace.
> >> >
> >> > Signed-off-by: Serge Hallyn <serge@...lyn.com>
> >> > Cc: Michael Kerrisk <mtk.manpages@...il.com>
> >> > ---
> >> >  include/uapi/linux/capability.h | 2 ++
> >> >  1 file changed, 2 insertions(+)
> >> >
> >> > diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
> >> > index fd4f87d..ba972ff 100644
> >> > --- a/include/uapi/linux/capability.h
> >> > +++ b/include/uapi/linux/capability.h
> >> > @@ -357,6 +357,8 @@ struct vfs_ns_cap_data {
> >> >
> >> >  #define CAP_WAKE_ALARM            35
> >> >
> >> > +#define CAP_WAKE_SYSTEM      CAP_WAKE_ALARM
> >> > +
> >>
> >> I was thinking of the same thing. Although I might rename the
> >> numerical define to WAKE_SYSTEM and put WAKE_ALARM as the alias (along
> >> with a comment as to WAKE_ALARM being deprecated), so its more clear
> >> which is the one that ought to be used by new code.
> >>
> >> However, in the spirit of this thread, we might even consider
> >> broadening the cap silo a bit further, to something like
> >> CAP_WAKE_SUSPEND, such that it might also be able to cover broader PM
> >> actions?
> >
> > Or just CAP_UNSUSPEND?
> 
> I guess I was trying to capture it could be use for actions like both
> waking and suspending the system.

Well, CAP_SYS_PM comes to mind then.

Thanks,
Rafael