linux-kernel - Re: [PATCH 5/5] rust: device: Add a stub abstraction for devices

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Message-ID: <bd93ed8c-b020-5883-4baa-f9743e395b97@asahilina.net>
Date:   Sat, 25 Feb 2023 00:51:38 +0900
From:   Asahi Lina <lina@...hilina.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        Hector Martin <marcan@...can.st>,
        Sven Peter <sven@...npeter.dev>, Arnd Bergmann <arnd@...db.de>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Neal Gompa <neal@...pa.dev>, rust-for-linux@...r.kernel.org,
        linux-kernel@...r.kernel.org, asahi@...ts.linux.dev
Subject: Re: [PATCH 5/5] rust: device: Add a stub abstraction for devices

On 25/02/2023 00.34, Greg Kroah-Hartman wrote:
> On Sat, Feb 25, 2023 at 12:10:47AM +0900, Asahi Lina wrote:
>>>> +impl Device {
>>>> +    /// Creates a new device instance.
>>>> +    ///
>>>> +    /// # Safety
>>>> +    ///
>>>> +    /// Callers must ensure that `ptr` is valid, non-null, and has a non-zero reference count.
>>>> +    pub unsafe fn new(ptr: *mut bindings::device) -> Self {
>>>> +        // SAFETY: By the safety requirements, ptr is valid and its refcounted will be incremented.
>>>> +        unsafe { bindings::get_device(ptr) };
>>>
>>> You don't check the return value of get_device()?  What if it failed
>>> (hint, it can)?
>>
>> Really? I looked at the implementation and I don't see how it can fail,
>> as long as the argument is non-NULL and valid... (which is a
>> precondition of this unsafe function). Did I miss something?
> 
> This function has changed a bit over time, but yes, it could fail if
> someone else just dropped the last reference right before you tried to
> grab a new one (look at the implementation of kobject_get()).
> 
> Yes, if you "know" you have a non-zero reference count first, it should
> never fail, but how do you know this?  We have the ability to check the
> return value of the function, shouldn't that happen?

Well, we know this because it is part of the invariant. If we don't
uphold invariants, things fall apart... that's why we create safe
abstractions that don't let you break those invariants after all!

In this particular case though, I don't see what we can usefully do
here. `device_get()` is going to be part of Clone impls and things like
that which are non-fallible. At most we can assert!() and rust-panic
(which is a BUG as far as I know) if it fails... would that be preferable?

~~ Lina