Re: [PATCH v4] pidns: introduce syscall translate_pid

[Nagarathnam Muthusamy]

On 10/16/2017 09:24 AM, Oleg Nesterov wrote:
> On 10/13, Konstantin Khlebnikov wrote:
> > On 13.10.2017 19:05, Oleg Nesterov wrote:
> > > I won't insist, but this suggests we should add a new helper,
> > > get_ns_by_fd_type(fd, type), and convert get_net_ns_by_fd() to use it
> > > as well.
> > That was in v3.
> >
> > I'll prefer to this later, separately. And replace fget with fdget which
> > allows to do this without atomic operations if task is single-threaded.
> OK, agreed,
>
> > > Stupid question. Can't we make a simpler API which doesn't need /proc/ ?
> > > I mean,
> > >
> > > 	sys_translate_pid(pid_t pid, pid_t source_pid, pid_t target_pid)
> > > 	{
> > > 		struct pid_namespace *source_ns, *target_ns;
> > >
> > > 		source_ns = task_active_pid_ns(find_task_by_vpid(source_pid));
> > > 		target_ns = task_active_pid_ns(find_task_by_vpid(target_pid));
> > >
> > > 		...
> > > 	}
> > > > Yes, this is more limited... Do you have a use-case when this is not enough?
> > That was in v1 but considered too racy.
> Hmm, I don't understand...
>
> Yes sure, this is racy but open("/proc/$pid/ns/pid") is racy too?
>
> OK, once you do fd=open("/proc/$pid/ns/pid") you can use this fd even after
> its owner exits, while find_task_by_vpid() will fail or find another task if
> this pid was already reused.
>
> But once again, do you have a use-case when this is important?

I believe that in V1 Eric pointed out that pid in general is not a clean 
way to represent
namespace. (https://lkml.org/lkml/2015/9/22/1087) Few old interfaces 
used pid only because at that time there was no better way to represent 
namespaces.

> > But we could merge both ways:
> >
> > source >= 0 - pidns fs
> > source < 0  - task_pid = -source
> But for what? I must have missed something...
>
> Oleg.