linux-kernel - Re: [PATCH v10 7/8] rust: file: add `Kuid` wrapper

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Message-ID: <20240926-bewundere-beseitigen-59808f199f82@brauner>
Date: Thu, 26 Sep 2024 18:33:28 +0200
From: Christian Brauner <brauner@...nel.org>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Gary Guo <gary@...yguo.net>, 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>, 
	Björn 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 Hjønnevåg <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 v10 7/8] rust: file: add `Kuid` wrapper

On Mon, Sep 23, 2024 at 11:13:56AM GMT, Alice Ryhl wrote:
> On Mon, Sep 16, 2024 at 12:02 AM Gary Guo <gary@...yguo.net> wrote:
> >
> > On Sun, 15 Sep 2024 14:31:33 +0000
> > Alice Ryhl <aliceryhl@...gle.com> wrote:
> > > +    /// Returns the given task's pid in the current pid namespace.
> > > +    pub fn pid_in_current_ns(&self) -> Pid {
> > > +        // SAFETY: We know that `self.0.get()` is valid by the type invariant, and passing a null
> > > +        // pointer as the namespace is correct for using the current namespace.
> > > +        unsafe { bindings::task_tgid_nr_ns(self.0.get(), ptr::null_mut()) }
> >
> > Do we want to rely on the behaviour of `task_tgid_nr_ns` with null
> > pointer as namespace, or use `task_tgid_vnr`?
> 
> Hmm. Looks like C Binder actually does:
> trd->sender_pid = task_tgid_nr_ns(sender, task_active_pid_ns(current));
> 
> Not sure why I'm using a null pointer here.

Passing a NULL pointer for task_tgid_nr_ns() is fine. Under the hood
it's just __task_pid_nr_ns(task, PIDTYPE_TGID, NULL) which causes
task_active_pid_ns(current) to be called internally. So it's equivalent.

In any case, I did add Rust wrappers for struct pid_namespace just to
see how far I would get as task_active_pid_ns() is rather subtle even if
it isn't obvious at first glance. Sending that in a second.