linux-kernel - Re: [PATCHv2 1/1] Documentation: describe how to add a system call

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Date:	Fri, 31 Jul 2015 15:42:53 +0100
From:	David Drysdale <drysdale@...gle.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Michael Kerrisk <mtk.manpages@...il.com>,
	Linux API <linux-api@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	Shuah Khan <shuahkh@....samsung.com>,
	Jonathan Corbet <corbet@....net>,
	Eric B Munson <emunson@...mai.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Al Viro <viro@...iv.linux.org.uk>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Peter Zijlstra <peterz@...radead.org>,
	Vivek Goyal <vgoyal@...hat.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	David Herrmann <dh.herrmann@...il.com>,
	"Theodore Ts'o" <tytso@....edu>, Kees Cook <keescook@...omium.org>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Milosz Tanski <milosz@...in.com>, Fam Zheng <famz@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-doc@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv2 1/1] Documentation: describe how to add a system call

On Fri, Jul 31, 2015 at 2:06 PM, Josh Triplett <josh@...htriplett.org> wrote:
> On Fri, Jul 31, 2015 at 10:48:32AM +0100, David Drysdale wrote:
>> On Thu, Jul 30, 2015 at 7:50 PM, Josh Triplett <josh@...htriplett.org> wrote:
>> > On Thu, Jul 30, 2015 at 08:52:11AM +0100, David Drysdale wrote:
>> >> +needs to be governed by the appropriate Linux capability bit (checked with a
>> >> +call to capable()), as described in the capabilities(7) man page.
>> >> +
>> >> + - If there is an existing capability that governs related functionality, then
>> >> +   use that.  However, avoid combining lots of only vaguely related functions
>> >> +   together under the same bit, as this goes against capabilities' purpose of
>> >> +   splitting the power of root.  In particular, avoid adding new uses of the
>> >> +   already overly-general CAP_SYS_ADMIN capability.
>> >> + - If there is no related capability, then consider adding a new capability
>> >> +   bit -- but bear in mind that the numbering space is limited, and each new
>> >> +   bit needs to be understood and administered by sysadmins.
>> >
>> > Many previous discussions on this topic seem to have concluded that it's
>> > almost impossible to add a new capability without breaking existing
>> > programs.  I would suggest not even mentioning this possibility.
>>
>> I'm not particularly knowledgable about capabilities (at least, not the
>> POSIX.1e/Linux kind), so I'll confess that I got this suggestion from
>> Michael Kerrisk.
>>
>> Michael, do you agree that we should just drop the possibility of adding
>> new capability bits?
>>
>> Also, Josh, do you have any references to the earlier discussions on the
>> topic so I can update the Sources section?
>
> No direct links handy at the moment without some searching, but one
> iteration of it came up when Matthew Garrett suggested adding
> CAP_COMPROMISE_KERNEL, and the ensuing discussion of capability
> semantics suggested that the way the capability space was defined and
> controlled by userspace meant that adding any new capability would
> effectively break userspace ABI.  The userspace ABI for capabilities is
> not clear; some applications drop all possible capabilities and could
> get surprised by a new capability being dropped, while other
> applications drop only capabilities they know about and could get
> surprised by a new capability *not* being dropped.

BTW, I left this paragraph unchanged in the v3 version I just sent
out -- I'll update it for v4 when I get back from vacation, depending
on what discussion occurs in the meantime...
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