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

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Date:   Fri,  1 Dec 2023 11:35:38 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     benno.lossin@...ton.me
Cc:     a.hindborg@...sung.com, alex.gaynor@...il.com,
        aliceryhl@...gle.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`

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.

>> +    // 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.

>> +        // SAFETY: Since `DeferredFdCloserInner` is `#[repr(C)]`, casting the pointers gives a
>> +        // pointer to the `twork` field.
>> +        let inner = Box::into_raw(self.inner) as *mut bindings::callback_head;
> 
> Here you can just use `.cast::<...>()`.

Will do.

Alice