linux-kernel - Re: [PATCH] [RFC] rust: add PidNamespace wrapper

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Message-ID: <CAH5fLghUj3-8eZMOVhhk0c9x29B7uMj=9dHWsRJYC1ghxqUdxg@mail.gmail.com>
Date: Fri, 27 Sep 2024 14:04:13 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Christian Brauner <brauner@...nel.org>
Cc: Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>, 
	"Serge E. Hallyn" <serge@...lyn.com>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	Bjoern Roy Baron <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, 
	Andreas Hindborg <a.hindborg@...sung.com>, Peter Zijlstra <peterz@...radead.org>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Arve Hjonnevag <arve@...roid.com>, Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, 
	Joel Fernandes <joel@...lfernandes.org>, Carlos Llamas <cmllamas@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Dan Williams <dan.j.williams@...el.com>, 
	Matthew Wilcox <willy@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Daniel Xu <dxu@...uu.xyz>, 
	Martin Rodriguez Reboredo <yakoyoku@...il.com>, Trevor Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org, 
	linux-security-module@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH] [RFC] rust: add PidNamespace wrapper

On Thu, Sep 26, 2024 at 6:36 PM Christian Brauner <brauner@...nel.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