Re: [PATCH] [RFC] rust: add PidNamespace wrapper

[Christian Brauner <brauner@kernel.org>]

On Fri, Sep 27, 2024 at 02:04:13PM GMT, Alice Ryhl wrote:
> On Thu, Sep 26, 2024 at 6:36 PM Christian Brauner <brauner@kernel.org> wrote:
> >
> > Ok, so here's my feeble attempt at getting something going for wrapping
> > struct pid_namespace as struct pid_namespace indirectly came up in the
> > file abstraction thread.
> 
> This looks great!

Thanks!

> 
> > The lifetime of a pid namespace is intimately tied to the lifetime of
> > task. The pid namespace of a task doesn't ever change. A
> > unshare(CLONE_NEWPID) or setns(fd_pidns/pidfd, CLONE_NEWPID) will not
> > change the task's pid namespace only the pid namespace of children
> > spawned by the task. This invariant is important to keep in mind.
> >
> > After a task is reaped it will be detached from its associated struct
> > pids via __unhash_process(). This will also set task->thread_pid to
> > NULL.
> >
> > In order to retrieve the pid namespace of a task task_active_pid_ns()
> > can be used. The helper works on both current and non-current taks but
> > the requirements are slightly different in both cases and it depends on
> > where the helper is called.
> >
> > The rules for this are simple but difficult for me to translate into
> > Rust. If task_active_pid_ns() is called on current then no RCU locking
> > is needed as current is obviously alive. On the other hand calling
> > task_active_pid_ns() after release_task() would work but it would mean
> > task_active_pid_ns() will return NULL.
> >
> > Calling task_active_pid_ns() on a non-current task, while valid, must be
> > under RCU or other protection mechanism as the task might be
> > release_task() and thus in __unhash_process().
> 
> Just to confirm, calling task_active_pid_ns() on a non-current task
> requires the rcu lock even if you own a refcont on the task?

Interesting question. Afaik, yes. task_active_pid_ns() goes via
task->thread_pid which is a shorthand for task->pid_links[PIDTYPE_PID].

This will be NULLed when the task exits and is dead (so usually when
someone has waited on it - ignoring ptrace for sanity reasons and
autoreaping the latter amounts to the same thing just in-kernel):

T1                      T2                                                   T3
exit(0);
                        wait(T1)
                        -> wait_task_zombie()
                           -> release_task()
                              -> __exit_signals()
                                 -> __unash_process()
                                    // sets task->thread_pid == NULL         task_active_pid_ns(T1)
                                    // task->pid_links[PIDTYPE_PID] == NULL

So having a reference to struct task_struct doesn't prevent
task->thread_pid becoming NULL.

And you touch upon a very interesting point. The lifetime of struct
pid_namespace is actually tied to struct pid much tighter than it is to
struct task_struct. So when a task is released (transitions from zombie
to dead in the common case) the following happens:

release_task()
-> __exit_signals()
   -> thread_pid = get_pid(task->thread_pid)
      -> __unhash_process()
         -> detach_pid(PIDTYPE_PID)
            -> __change_pid()
               {
                       task->thread_pid = NULL;
                       task->pid_links[PIDTYPE_PID] = NULL;
                       free_pid(thread_pid)
               }
         put_pid(thread_pid)

And the free_pid() in __change_pid() does a delayed_put_pid() via
call_rcu().

So afaiu, taking the rcu_read_lock() synchronizes against that
delayed_put_pid() in __change_pid() so the call_rcu() will wait until
everyone who does

rcu_read_lock()
task_active_pid_ns(task)
rcu_read_unlock()

and sees task->thread_pid non-NULL, is done. This way no additional
reference count on struct task_struct or struct pid is needed before
plucking the pid namespace from there. Does that make sense or have I
gotten it all wrong?