Re: [PATCH man-pages v2] capabilities.7, prctl.2: Document ambient capabilities

Michael Kerrisk (man-pages)
2015-05-19T19:13:59Z

Hi Andy,

On 05/19/2015 07:21 PM, Andy Lutomirski wrote:
> On Tue, May 19, 2015 at 12:56 AM, Michael Kerrisk (man-pages)
> <mtk.manpages@gmail.com> wrote:
>> Hi Andy,
>>
>> Thanks for this patch. There are some broken pieces though. Also,
>> I have some minor questions about the API design. See below.
>>
>> On 05/15/2015 08:43 AM, Andy Lutomirski wrote:
>>> Signed-off-by: Andy Lutomirski <luto@kernel.org>
>>> ---
>>>
>>> There was no v1. I'm calling this v2 to keep it in sync with the kernel
>>> patch versioning.
>>>
>>> man2/prctl.2 | 10 ++++++++++
>>> man7/capabilities.7 | 32 ++++++++++++++++++++++++++------
>>> 2 files changed, 36 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/man2/prctl.2 b/man2/prctl.2
>>> index b352f6283624..5861e3aefe9a 100644
>>> --- a/man2/prctl.2
>>> +++ b/man2/prctl.2
>>> @@ -949,6 +949,16 @@ had been called.
>>> For further information on Intel MPX, see the kernel source file
>>> .IR Documentation/x86/intel_mpx.txt .
>>> .\"
>>> +.TP
>>> +.BR PR_CAP_AMBIENT " (since Linux 4.2)"
>>> +Reads or changes the ambient capability set. If arg2 is PR_CAP_AMBIENT_RAISE,
>>> +then the capability specified in arg3 is added to the ambient set. This will
>>> +fail, returning EPERM, if the capability is not already both permitted and
>>> +inheritable or if the SECBIT_NO_CAP_AMBIENT_RAISE securebit is set. If arg2
>>> +is PR_CAP_AMBIENT_LOWER, then the capability specified in arg3 is removed
>>> +from the ambient set. If arg2 is PR_CAP_AMBIENT_GET, then
>>> +.BR prctl (2)
>>> +will return 1 if the capability in arg3 is in the ambient set and 0 if not.
>>
>> Some API design questions:
>>
>> 1. We already have prctl() operations that work on some capability sets:
>> PR_CAPBSET_READ and PR_CAPBSET_DROP. These don't use arg3; the operation
>> is directly encoded in the first argument of prctl(). Just to keep some
>> consistency, why not do things the same way for these new operations?
>
> I'm torn. On the one hand, consistency is nice. On the other hand,
> prctl is a mess

Agreed.

> and trying to organize new additions seems like a good
> idea.

Sure, but what is your organizing principle here? (I don't feel strongly
about it, but it's not clear to me what trumps the (mild) degree of
consistency that I suggest.)

>> Also, you could opt for some consistency in the naming, so using "READ"
>> rather than "GET", for example. On the other hand, both "READ" and "GET"
>> are suboptimal names: this is really a test operation. So, maybe a
>> clean break to a good name, PR_CAP_AMBIENT_IS_SET, is best?
>
> I like IS_SET.

Okay.

>> Thus:
>>
>> prctl(PR_CAP_AMBIENT_READ, cap, 0, 0, 0); // or PR_CAP_AMBIENT_IS_SET?
>> prctl(PR_CAP_AMBIENT_RAISE, cap, 0, 0, 0);
>> prctl(PR_CAP_AMBIENT_LOWER, cap, 0, 0, 0);
>>
>> 2. In terms of the API design, would it be useful to have a prctl() operation
>> that clears the entire ambient set?
>>
>> prctl(PR_CAP_AMBIENT_ZERO, 0, 0, 0, 0); // or PR_CAP_AMBIENT_EMPTY?
>
> Seems like a good idea. How about _CLEAR?

Also good. …