[PATCH] [RFC] rust: add PidNamespace wrapper

[Alice Ryhl]

On Thu, Sep 26, 2024 at 6:36 PM Christian Brauner <brauner at 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!

> 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?

Alice