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

[Alice Ryhl <alice@ryhl.io>]

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@gmail.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