linux-kernel - Re: rust: wrong SAFETY comments in group

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Message-ID: <aTobdw31eNj_ahrU@google.com>
Date: Thu, 11 Dec 2025 01:16:39 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Gary Guo <gary@...yguo.net>
Cc: Oleg Nesterov <oleg@...hat.com>, Christian Brauner <christian@...uner.io>, 
	Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, Panagiotis Foliadis <pfoliadis@...teo.net>, 
	Shankari Anand <shankari.ak0208@...il.com>, FUJITA Tomonori <fujita.tomonori@...il.com>, 
	Alexey Gladkov <legion@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: rust: wrong SAFETY comments in group_leader() and pid() + questions

On Mon, Dec 08, 2025 at 03:43:07PM +0000, Gary Guo wrote:
> On Fri, 5 Dec 2025 18:17:09 +0000
> Alice Ryhl <aliceryhl@...gle.com> wrote:
> 
> > On Fri, Dec 05, 2025 at 06:21:46PM +0100, Oleg Nesterov wrote:
> > > On 12/05, Alice Ryhl wrote:  
> > > >
> > > > pub fn group_leader(&self) -> &Task {
> > > >     // SAFETY: The lifetime of the returned task reference is tied to
> > > >     // the lifetime of `self`, and given that a task has a reference to
> > > >     // its group leader, we know it must be valid for the lifetime of
> > > >     // the returned task reference.
> > > >     unsafe { &*bindings::task_group_leader(self.as_ptr()).cast::<Task>() }
> > > > }  
> > > 
> > > Thanks again Alice, but the comment still looks misleading to me...
> > > OK, quite possibly this is because I don't understand what does the
> > > "lifetime of the returned task reference" actually mean in the rust code.
> > > Does it mean "lifetime of task_struct" of "lifetime of the process/thread" ?  
> > 
> > To start with, it's likely that this comment is not the right choice
> > for this function, given our discussion. Most likely group_leader()
> > needs to be moved to `impl CurrentTask {}` and the safety comment needs
> > to explain why being the current task ensures that the returned &Task
> > lives for long enough. I just took the safety comment from the code we
> > have today.
> 
> This indeed sounds like the right approach to take.
> 
> If `Task::pid` or `Task::group_leader` just gives the pid or group
> leader at the time of invocation and doesn't have any stability
> guarantee, then the user of the functions will likely be misusing these
> functions.
> 
> It's better to just have them on `CurrentTask` for now. When an user
> arises that need to retrieve them for another task, then we can come
> back and think about a proper solution taking the scenario into account.

I believe Binder just uses them for including the pid when it prints to
the console. But I'll look into this and figure out the best approach.

Alice