Re: clone3: allow creation of time namespace with offset

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

On 4/1/20 1:46 PM, Christian Brauner wrote:
> On Wed, Apr 01, 2020 at 01:40:25PM +0200, Michael Kerrisk (man-pages) wrote:
>> On 3/25/20 12:26 PM, Christian Brauner wrote:
>>> On Wed, Mar 25, 2020 at 08:58:36AM +0100, Adrian Reber wrote:
>>>> On Tue, Mar 24, 2020 at 06:56:49PM +0100, Christian Brauner wrote:
>>>>> On Tue, Mar 24, 2020 at 05:25:46PM +0100, Adrian Reber wrote:
>>>>>> On Tue, Mar 24, 2020 at 05:09:45PM +0100, Christian Brauner wrote:
>>>>>>> On Fri, Mar 20, 2020 at 11:33:55AM -0700, Andrei Vagin wrote:
>>>>>>>> On Thu, Mar 19, 2020 at 11:29:55AM +0100, Christian Brauner wrote:
>>>>>>>>> On Thu, Mar 19, 2020 at 09:16:43AM +0100, Arnd Bergmann wrote:
>>>>>>>>>> On Thu, Mar 19, 2020 at 9:11 AM Adrian Reber <areber@redhat.com> wrote:
>>>>>>>>>>
>>>>>>>>>>> With Arnd's idea of only using nanoseconds, timens_offset would then
>>>>>>>>>>> contain something like this:
>>>>>>>>>>>
>>>>>>>>>>> struct timens_offset {
>>>>>>>>>>>         __aligned_s64 monotonic_offset_ns;
>>>>>>>>>>>         __aligned_s64 boottime_offset_ns;
>>>>>>>>>>> };
>>>>>>>>>>>
>>>>>>>>>>> I kind of prefer adding boottime and monotonic directly to struct clone_args
>>>>>>>>>>>
>>>>>>>>>>>         __aligned_u64 tls;
>>>>>>>>>>>         __aligned_u64 set_tid;
>>>>>>>>>>>         __aligned_u64 set_tid_size;
>>>>>>>>>>> +       __aligned_s64 monotonic_offset_ns;
>>>>>>>>>>> +       __aligned_s64 boottime_offset_ns;
>>>>>>>>>>>  };
>>>>>>>>>>
>>>>>>>>>> I would also prefer the second approach using two 64-bit integers
>>>>>>>>>> instead of a pointer, as it keeps the interface simpler to implement
>>>>>>>>>> and simpler to interpret by other tools.
>>>>>>>>>
>>>>>>>>> Why I don't like has two reasons. There's the scenario where we have
>>>>>>>>> added new extensions after the new boottime member and then we introduce
>>>>>>>>> another offset. Then you'd be looking at:
>>>>>>>>>
>>>>>>>>> __aligned_u64 tls;
>>>>>>>>> __aligned_u64 set_tid;
>>>>>>>>> __aligned_u64 set_tid_size;
>>>>>>>>> + __aligned_s64 monotonic_offset_ns;
>>>>>>>>> + __aligned_s64 boottime_offset_ns;
>>>>>>>>> __aligned_s64 something_1
>>>>>>>>> __aligned_s64 anything_2
>>>>>>>>> + __aligned_s64 sometime_offset_ns
>>>>>>>>>
>>>>>>>>> which bothers me just by looking at it. That's in addition to adding two
>>>>>>>>> new members to the struct when most people will never set CLONE_NEWTIME.
>>>>>>>>> We'll also likely have more features in the future that will want to
>>>>>>>>> pass down more info than we want to directly expose in struct
>>>>>>>>> clone_args, e.g. for a long time I have been thinking about adding a
>>>>>>>>> struct for CLONE_NEWUSER that allows you to specify the id mappings you
>>>>>>>>> want the new user namespace to get. We surely don't want to force all
>>>>>>>>> new info into the uppermost struct. So I'm not convinced we should here.
>>>>>>>>
>>>>>>>> I think here we can start thinking about a netlink-like interface.
>>>>>>>
>>>>>>> I think netlink is just not a great model for an API and I would not
>>>>>>> want us to go down that route.
>>>>>>>
>>>>>>> I kept thinking about this for a bit and I think that we will end up
>>>>>>> growing more namespace-related functionality. So one thing that came to
>>>>>>> my mind is the following layout:
>>>>>>>
>>>>>>> struct {
>>>>>>> 	struct {
>>>>>>> 		__s64 monotonic;
>>>>>>> 		__s64 boot;
>>>>>>> 	} time;
>>>>>>> } namespaces;
>>>>>>>
>>>>>>> struct _clone_args {
>>>>>>> 	__aligned_u64 flags;
>>>>>>> 	__aligned_u64 pidfd;
>>>>>>> 	__aligned_u64 child_tid;
>>>>>>> 	__aligned_u64 parent_tid;
>>>>>>> 	__aligned_u64 exit_signal;
>>>>>>> 	__aligned_u64 stack;
>>>>>>> 	__aligned_u64 stack_size;
>>>>>>> 	__aligned_u64 tls;
>>>>>>> 	__aligned_u64 set_tid;
>>>>>>> 	__aligned_u64 set_tid_size;
>>>>>>> 	__aligned_u64 namespaces;
>>>>>>> 	__aligned_u64 namespaces_size;
>>>>>>> };
>>>>>>>
>>>>>>> Then when we end up adding id mapping support for CLONE_NEWUSER we can
>>>>>>> extend this with:
>>>>>>>
>>>>>>> struct {
>>>>>>> 	struct {
>>>>>>> 		__aligned_u64 monotonic;
>>>>>>> 		__aligned_u64 boot;
>>>>>
>>>>> s/__aligned_u64/__s64/g
>>>>>
>>>>> Sorry, leftover from my first draft.
>>>>>
>>>>>>> 	} time;
>>>>>>>
>>>>>>> 	struct {
>>>>>>> 		/* id mapping members */
>>>>>>> 	} user;
>>>>>>> } namespaces;
>>>>>>>
>>>>>>> Thoughts? Other ideas?
>>>>>>
>>>>>> Works for me.
>>>>>>
>>>>>> If we add the user namespace id mappings and then at some point a third
>>>>>> element for the time namespace appears it would also start to be mixed.
>>>>>> Just as you mentioned that a few mails ago.
>>>>>
>>>>> I think you misunderstand me or I'm misunderstanding you. That new time
>>>>> namespace member would go into struct time {} so
>>>>>
>>>>> struct {
>>>>> 	struct {
>>>>> 		__s64 monotonic;
>>>>> 		__s64 boot;
>>>>> 		__s64 someothertime;
>>>>> 	} time;
>>>>>
>>>>> 	struct {
>>>>> 		/* id mapping members */
>>>>> 	} user;
>>>>> } namespaces;
>>
>> So far, this seems like the least worst approach to me :-).
>>
>> I think it's reasonable to assume that there will be another
>> time NS offset to add one day. I don't think anyone expected
>> CLOCK_BOOTIME (added in 2011) at the time that CLOCK_MONOTONIC
>> appeared (as part of the POSIX timers API in Linux 2.6.0 2003);
>> similarly, we probably can't conceive now what clock might be
>> added in the future that should also be governed by time
>> namespaces.
>>
>>
>> But...
>>
>>>> My question was about how does the kernel know how 'struct namespaces'
>>>> is structured. How can an older kernel (which only is aware of two
>>>> clocks) deal with a, like in this example, third clock. Will the size
>>>> '__aligned_u64 namespaces_size' be used for versioning?
>>>
>>> Yes, that would be the idea.
>>
>> The idea implied here (if I understand correctly) of "binary chop
>> on the structure size to see what your kernel supports" is pretty
>> clunky, IMO. It's worth at least considering alternatives. 
>> For example, seccomp has a number of interesting interfaces to
>> discover what the running kernel supports [1]. Maybe it is worth
>> considering something similar for clone3()?
> 
> Yes, that's definitely something I've been thinking about and I've also
> discussed this publicly on the libc-alpha mailing list since Florian
> (Weimer) wants this for glibc. _But_ I didn't want to simply go my own
> way and so Aleksa and I stuck our heads together and tried to come up
> with a generic way that e.g. would work for both openat2() and for
> clone3(), in fact we tried to come up with a generic way that could
> potentially be used by any syscall that makes use of the new
> copy_struct_from_user() helper that Aleksa and I pushed upstream. We
> haven't yet gotten around to actually try and implementing our ideas.
> I'll try to get around to this during this cycle (/me makes note to "sit
> down" with Aleksa). We'll definitely take a look at seccomp and cgroups
> too.

Great! Thanks for thinking about this. Just by the way, on 
discussions such as the one you had with Florian, it would
be great if you CCed linux-api@ (and also for the patch series
that implements this idea).

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/