linux-kernel - Re: [PATCH 6/7] rust: file: add `DeferredFdCloser`

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Date:   Sat, 02 Dec 2023 10:16:36 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Alice Ryhl <aliceryhl@...gle.com>
Cc:     a.hindborg@...sung.com, alex.gaynor@...il.com, arve@...roid.com,
        bjorn3_gh@...tonmail.com, boqun.feng@...il.com, brauner@...nel.org,
        cmllamas@...gle.com, dan.j.williams@...el.com, dxu@...uu.xyz,
        gary@...yguo.net, gregkh@...uxfoundation.org,
        joel@...lfernandes.org, keescook@...omium.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        maco@...roid.com, ojeda@...nel.org, peterz@...radead.org,
        rust-for-linux@...r.kernel.org, surenb@...gle.com,
        tglx@...utronix.de, tkjos@...roid.com, viro@...iv.linux.org.uk,
        wedsonaf@...il.com, willy@...radead.org
Subject: Re: [PATCH 6/7] rust: file: add `DeferredFdCloser`

On 12/1/23 12:35, Alice Ryhl wrote:
> Benno Lossin <benno.lossin@...ton.me> writes:
>>> +        // SAFETY: The `inner` pointer points at a valid and fully initialized task work that is
>>> +        // ready to be scheduled.
>>> +        unsafe { bindings::task_work_add(current, inner, TWA_RESUME) };
>>
>> I am a bit confused, when does `do_close_fd` actually run? Does
>> `TWA_RESUME` mean that `inner` is scheduled to run after the current
>> task has been completed?
> 
> When the current syscall returns to userspace.

What happens when I use `DeferredFdCloser` outside of a syscall? Will
it never run? Maybe add some documentation about that?

>>> +    // SAFETY: This function is an implementation detail of `close_fd`, so its safety comments
>>> +    // should be read in extension of that method.
>>> +    unsafe extern "C" fn do_close_fd(inner: *mut bindings::callback_head) {
>>> +        // SAFETY: In `close_fd` we use this method together with a pointer that originates from a
>>> +        // `Box<DeferredFdCloserInner>`, and we have just been given ownership of that allocation.
>>> +        let inner = unsafe { Box::from_raw(inner as *mut DeferredFdCloserInner) };
>>
>> In order for this call to be sound, `inner` must be an exclusive
>> pointer (including any possible references into the `callback_head`).
>> Is this the case?
> 
> Yes, when this is called, it's been removed from the linked list of task
> work. That's why we can kfree it.

Please add this to the SAFETY comment.

-- 
Cheers,
Benno