Re: For review: pid_namespaces(7) man page

Eric W. Biederman
2013-03-04T17:52:30Z

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

> On Fri, Mar 1, 2013 at 4:35 PM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>
>>> Hi Rob,
>>>
>>> On Fri, Mar 1, 2013 at 5:01 AM, Rob Landley <rob@landley.net> wrote:
>>>> On 02/28/2013 05:24:07 AM, Michael Kerrisk (man-pages) wrote:
>>> [...]
>>>
>>>>> DESCRIPTION
>>>>> For an overview of namespaces, see namespaces(7).
>>>>>
>>>>> PID namespaces isolate the process ID number space, meaning
>>>>> that processes in different PID namespaces can have the same
>>>>> PID.
>>>>
>>>>
>>>> Um, perhaps "different processes"? Slightly repetitive, but trying to avoid
>>>> the potential misreading that "a processes can have the same PID in
>>>> different namespaces". (A single process can't be a member of more than one
>>>> namespace. This is not about selective visibility.)
>>>
>>> I'm not sure this clarifies things...
>>>
>>>>> PID namespaces allow containers to migrate to a new host
>>>>> while the processes inside the container maintain the same
>>>>> PIDs.
>>>>
>>>>
>>>> I thought suspend/resume a container was the simple case. Migration to a new
>>>> host is built on top of that. (On resume in a new container on the same
>>>> system, if other stuff is going on in the system so the available PIDs have
>>>> shifted.)
>>>
>>> I'll add some words here on suspend/resume.
>>>
>>>>> Likewise, a process in an ancestor namespace can—subject to the
>>>>> usual permission checks described in kill(2)—send signals to
>>>>> the "init" process of a child PID namespace only if the "init"
>>>>> process has established a handler for that signal. (Within the
>>>>> handler, the siginfo_t si_pid field described in sigaction(2)
>>>>> will be zero.) SIGKILL or SIGSTOP are treated exceptionally:
>>>>> these signals are forcibly delivered when sent from an ancestor
>>>>> PID namespace. Neither of these signals can be caught by the
>>>>> "init" process, and so will result in the usual actions associ‐
>>>>> ated with those signals (respectively, terminating and stopping
>>>>> the process).
>>>>
>>>>
>>>> If SIGKILL to init is propogated to all the children of init, is SIGSTOP
>>>> also propogated to all the children? (I.E. will SIGSTOP to container's init
>>>> suspend the whole container, and will SIGCONT resume the whole container? If
>>>> the latter, will it only resume processes that weren't previously stopped?
>>>> :)
>>>
>>> Covered by Eric.
>>>
>>>>> To put things another way: a process's PID namespace membership
>>>>> is determined when the process is created and cannot be changed
>>>>> thereafter. Among other things, this means that the parental
>>>>> relationship between processes mirrors the parental between PID
>>>>
>>>>
>>>> mirrors the relationship
>>>
>>> Thanks.
>>>
>>>>> namespaces: the parent of a process is either in the same
>>>>> namespace or resides in the immediate parent PID namespace.
>>>>>
>>>>> Every thread in a process must be in the same PID namespace.
>>>>> For this reason, the two following call sequences will fail:
>>>>>
>>>>> unshare(CLONE_NEWPID);
>>>>> clone(..., CLONE_VM, ...); /* Fails */
>>>>>
>>>>> setns(fd, CLONE_NEWPID);
>>>>> clone(..., CLONE_VM, ...); /* Fails */
>>>>
>>>>
>>>> They fail with -EUNDOCUMENTED
>>>
>>> Added EINVAL, as per Eric's reply. (Eric does that error also apply
>>> for the two new cases you added?).
>>>
>>>>> Because the above unshare(2) and setns(2) calls only change the
>>>>> PID namespace for created children, the clone(2) calls neces‐
>>>>> sarily put the new thread in a different PID namespace from the
>>>>> calling thread.
>>>>
>>>>
>>>> Um, no they don't. They fail. That's the point.
>>>
>>> (Good catch.)
>>>
>>>> They _would_ put the new
>>>> thread in a different PID namespace, which breaks the definition of threads.
>>>>
>>>> How about:
>>>>
>>>> The above unshare(2) and setns(2) calls change the PID namespace of
>>>> children created by subsequent clone(2) calls, which is incompatible
>>>> with CLONE_VM.
>>>
>>> I decided on:
>>>
>>> The point here is that unshare(2) and setns(2) change the PID
>>> namespace for created children but not for the calling process,
>>> while clone(2) CLONE_VM specifies the creation of a new thread
>>> in the same process.
>>
>> Can we make that "for all new tasks created" instead of "created
>> children"
>>
>> Othewise someone might expect CLONE_THREAD would work as you
>> CLONE_THREAD creates a thread and not a child...
>
> The term "task" is kernel-space talk that rarely appears in man pages,
> so I am reluctant to use it.

With respect to clone and in this case I am not certain we can properly
describe what happens without talking about tasks. But it is worth
a try.

> How about this:
>
> The point here is that unshare(2) and setns(2) change the PID
> namespace for processes subsequently created by the caller, but
> not for the calling process, while clone(2) CLONE_VM specifies
> the creation of a new thread in the same process.

Hmm. How about this.

The point here is that unshare(2) and setns(2) change the PID
namespace that will be used by in all subsequent calls to clone
and fork by the caller, but not for the calling process, and
that all threads in a process must share the same PID
namespace. Which makes a subsequent clone(2) CLONE_VM
specify the creation of a new thread in the a different PID
namespace but in the same process which is impossible.

Eric