Re: [PATCH] capabilities: add capability cgroup controller

[Djalal Harouni <tixxdz@gmail.com>]

On Fri, Jun 24, 2016 at 6:15 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Thu, Jun 23, 2016 at 6:14 PM, Topi Miettinen <toiwoton@gmail.com> wrote:
>> On 06/23/16 23:46, Andrew Morton wrote:
>>> On Thu, 23 Jun 2016 18:07:10 +0300 Topi Miettinen <toiwoton@gmail.com> wrote:
>>>
>>>> There are many basic ways to control processes, including capabilities,
>>>> cgroups and resource limits. However, there are far fewer ways to find
>>>> out useful values for the limits, except blind trial and error.
>>>>
>>>> Currently, there is no way to know which capabilities are actually used.
>>>> Even the source code is only implicit, in-depth knowledge of each
>>>> capability must be used when analyzing a program to judge which
>>>> capabilities the program will exercise.
>>>>
>>>> Add a new cgroup controller for monitoring of capabilities
>>>> in the cgroup.
>>>
>>> I'm having trouble understanding how valuable this feature is to our
>>> users, and that's a rather important thing!
>>>
>>> Perhaps it would help if you were to explain your motivation:
>>> particular use cases which benefited from this, for example.
>>>
>>
>> It's easy to control with for example systemd or many other tools, which
>> capabilities a service should have at the start. But how should a system
>> administrator, application developer or distro maintaner ever determine
>> a suitable value for this? Currently the only way seems to be to become
>> an expert on capabilities, make an educated guess how the set of
>> programs in question happen to work in this context and especially how
>> they could exercise the capabilites in all possible use cases. Even
>> then, the outcome is to just try something to see if that happens to
>> work. Reading the source code (if available) does not help very much,
>> because the use of capabilities is anything but explicit there.
>>
>> This is way too difficult, there must be some easier way. The
>> information which capabilities actually were used in a trial run gives a
>> much better starting point. The users can just use the list of used
>> capabilities with configuring the service or when developing or
>> maintaining the application. Of course, even that could still fail
>> eventually, but then you simply copy the new value of used capabilities
>> to the configuration, whereas currently you have to reconsider your
>> understanding of the capabilities and the programs in light of the
>> failure, which by itself might give no new useful information.
>>
>> One way to solve this for good would be to make the use of capabilities
>> explicit in the ABI. For example, there could be a system call
>> dac_override() which would be the only possible way ever to use the
>> capability CAP_DAC_OVERRIDE and so forth. Then reading source code,
>> tracing and many other approaches would be useful. But the OS with that
>> kind of ABI (not Linux) would not be Unix-like at all for any
>> (potentially) capability using programs, like find(1) or cat(1).
>
> The problem is that most of the capabilities are so powerful on their
> own that limiting services to just a few may be all but useless.

May be there is some gain _if_ the resources that a process interact
with _can_ also be made invisible with namespaces.



> --Andy
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tixxdz
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