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

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Message-ID: <CALAqxLXzpntE=-9AY3iHiZRj4uMAi_tsXRM=exhSvBeajt=z9Q@mail.gmail.com>
Date:   Fri, 16 Dec 2016 12:20:57 -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 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?

thanks
-john