[Devel] Re: [PATCH 2/2] exec: move core_pattern pipe helper into the crashing namespace

[Will Drewry]

On Fri, Sep 17, 2010 at 9:34 PM, Will Drewry <wad at chromium.org> wrote:
> On Fri, Sep 17, 2010 at 8:29 PM, Oleg Nesterov <oleg at redhat.com> wrote:
>> On 09/17, Will Drewry wrote:
>>>
>>> Instead, this change implements the more complex option two.  It
>>> migrates the ____call_usermodehelper() thread into the same namespaces
>>> as the dumping process.  It does not assign a pid in that namespace so
>>> the collector will appear to be pid 0.
>>
>> Hmm... You mean, it won't visible in that namespace? Afacis, it should
>> have the correct pid in the init ns, no?
>
> Exactly - it will be unmapped in its new pid namespace, returning 0
> only in that case, as you point out below!
>
>>
>> I am a bit worried task_active_pid_ns() != nsproxy->pid_ns, but perhaps
>> this is OK... Say, sys_getpid() returns 0, strange.
>>
>>> +             /* Run the core_collector in the crashing namespaces */
>>> +             if (copy_namespaces_unattached(0, current,
>>> +                     &pipe_params.nsproxy, &pipe_params.fs)) {
>>> +                     printk(KERN_WARNING "%s failed to copy namespaces\n",
>>> +                            __func__);
>>> +                     argv_free(helper_argv);
>>> +                     goto fail_dropcount;
>>> +             }
>>
>> This looks overcomplicated to me, or I missed something.
>>
>> I do not understand why should we do this beforehand, and why we need
>> copy_namespaces_unattached().
>>
>> Can't you just pass current to umh_pipe_setup() (or another helper) as
>> the argument? Then this helper can copy ->fs and ->nsproxy itself.
>
> I wasn't sure if it was reasonable to pass the current task_struct
> over, but I certainly can.
>
>> In fact, I do not understand why create_new_namespaces() is used. It
>> is called with flags == 0 anyway, can't we just do
>>
>>        ns = coredumping_task->nsproxy;
>>        get_nsproxy(ns);
>>        switch_task_namespaces(current, ns);
>>
>> ?
>
> So that was my first thought (which I tried).  I did exactly what you
> suggested to the khelper thread, and the lack of the fs struct bit me.
>  Since the older patch from Eric Biederman (setns()) had taken the
> route of deflecting the work through create_new_namespaces(), I did
> too.  I figured it would ensure that any namespacing behavior that
> needed to be done would be done.
>
> In practice, this seems to amount to just adding a refcount to all the
> namespaces and creating a new nsproxy which isn't really needed.  Most
> likely, doing what you've suggested above plus the copy_fs_struct and
> the swap out will do the trick.  I'll try it out and see.  That's make
> it much clearer I think.

That seems to work -- I'll post an update shortly with your comments
and Neil's integrated - hopefully accurately.  Any other thoughts on
style/cleanup/etc will certainly be appreciated.

cheers!
will
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