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

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Date:   Fri, 16 Dec 2016 13:16:15 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     "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>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: RFC: capabilities(7): notes for kernel developers

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.

thanks
-john