linux-kernel - Re: [PATCH v4] pidns: introduce syscall translate

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Date:   Mon, 16 Oct 2017 18:24:36 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Serge Hallyn <serge.hallyn@...ntu.com>,
        Nagarathnam Muthusamy <nagarathnam.muthusamy@...cle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Eugene Syromiatnikov <esyr@...hat.com>
Subject: Re: [PATCH v4] pidns: introduce syscall translate_pid

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?

> But we could merge both ways:
>
> source >= 0 - pidns fs
> source < 0  - task_pid = -source

But for what? I must have missed something...

Oleg.