linux-kernel - Re: [PATCH v8 3/8] rust: file: add Rust abstraction for `struct file`

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Message-ID: <51199e48-fd36-4669-a93a-97e5c10aea26@ryhl.io>
Date: Wed, 7 Aug 2024 23:59:47 +0200
From: Alice Ryhl <alice@...l.io>
To: Boqun Feng <boqun.feng@...il.com>, Alice Ryhl <aliceryhl@...gle.com>
Cc: Benno Lossin <benno.lossin@...ton.me>, Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Wedson Almeida Filho <wedsonaf@...il.com>, Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Andreas Hindborg <a.hindborg@...sung.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>,
 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,
 rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v8 3/8] rust: file: add Rust abstraction for `struct file`

On 8/7/24 4:46 PM, Boqun Feng wrote:
> On Wed, Aug 07, 2024 at 10:50:32AM +0200, Alice Ryhl wrote:
>> On Tue, Aug 6, 2024 at 9:30 PM Boqun Feng <boqun.feng@...il.com> wrote:
>>>
>>> On Tue, Aug 06, 2024 at 10:48:11AM +0200, Alice Ryhl wrote:
>>> [...]
>>>>>> +    /// Returns the flags associated with the file.
>>>>>> +    ///
>>>>>> +    /// The flags are a combination of the constants in [`flags`].
>>>>>> +    #[inline]
>>>>>> +    pub fn flags(&self) -> u32 {
>>>>>> +        // This `read_volatile` is intended to correspond to a READ_ONCE call.
>>>>>> +        //
>>>>>> +        // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
>>>>>> +        //
>>>>>> +        // FIXME(read_once): Replace with `read_once` when available on the Rust side.
>>>>>
>>>>> Do you know the status of this?
>>>>
>>>> It's still unavailable.
>>>>
>>>
>>> I think with our own Atomic API, we can just use atomic_read() here:
>>> yes, I know that to make this is not a UB, we need the C side to also do
>>> atomic write on this `f_flags`, however, my reading of C code seems to
>>> suggest that FS relies on writes to this field is atomic, therefore
>>> unless someone is willing to convert all writes to `f_flags` in C into
>>> a WRITE_ONCE(), nothing more we can do on Rust side. So using
>>> atomic_read() is the correct thing to begin with.
>>
>> Huh? The C side uses atomic reads for this?
>>
> 
> Well, READ_ONCE(->f_flags) is atomic, so I thought you want to use
> atomic here. However, after a quick look of `->f_flags` accesses, I find
> out they should be protected by `->f_lock` (a few cases rely on
> data race accesses, see p4_fd_open()), so I think what you should really
> do here is the similar: make sure Rust code only accesses `->f_flags`
> if `->f_lock` is held. Unless that's not the case for binder?


Binder just has an `if (filp->f_flags & O_NONBLOCK)` block somewhere in 
the ioctl, where filp is the `struct file *` passed to the ioctl. Binder 
doesn't take the lock.

Alice