linux-kernel - Re: [PATCH 13/13] rust: sync: introduce `LockedBy`

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Message-ID: <8a5d6cf1-17eb-6cb9-fb45-0a4d454d385e@protonmail.com>
Date:   Thu, 30 Mar 2023 21:10:18 +0000
From:   Benno Lossin <y86-dev@...tonmail.com>
To:     Wedson Almeida Filho <wedsonaf@...il.com>
Cc:     rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org,
        Wedson Almeida Filho <walmeida@...rosoft.com>
Subject: Re: [PATCH 13/13] rust: sync: introduce `LockedBy`

On 30.03.23 23:04, Wedson Almeida Filho wrote:
> On Thu, 30 Mar 2023 at 08:45, Benno Lossin <y86-dev@...tonmail.com> wrote:
>>
>> On 30.03.23 13:28, Benno Lossin wrote:
>>       struct Outer {
>>           mtx1: Mutex<()>,
>>           mtx2: Mutex<()>,
>>           inners: Vec<Inner>,
>>       }
>>
>>       struct Inner {
>>           count: LockedBy<usize, ()>,
>>       }
>>
>>       fn new_inner(outer: &Outer) -> Inner {
>>           Inner { count: LockedBy::new(&outer.mtx1, 0) }
>>       }
>>
>>       fn evil(outer: &Outer) {
>>           let inner = outer.inners.get(0).unwrap();
>>           let mut guard1 = outer.mtx1.lock();
>>           let mut guard2 = outer.mtx2.lock();
>>          // The pointee of `guard1` and `guard2` have the same address.
>>           let ref1 = inner.count.access_mut(&mut *guard1);
>>           let ref2 = inner.count.access_mut(&mut *guard2);
>>           mem::swap(ref1, ref2);
>>       }
>
> This doesn't reproduce the issue because `mtx2` itself is not a ZST
> (it contains a `struct mutex` before the data it protects).
>
> Something like the following should reproduce it though:
>
>      struct Outer {
>           mtx1: Mutex<()>,
>           zst: (),
>       }
>
>       fn evil(outer: &Outer) {
>           let lb = LockedBy::new(&outer.mtx1, 0u8);
>           let value = lb.access(&outer.zst);
>           // Accessing "value" without holding `mtx1`.
>           pr_info!("{}", *value);
>       }

You are correct, but in your example you also cannot be sure that it
works, since the layout of the `Mutex` and `Outer` is `repr(Rust)`.
And so you cannot be sure that `zst` has the same address as `value`
inside of the `Mutex` (since the `struct mutex` could be in between).
But regardless, lets just deny ZSTs in `LockedBy` since the fix is
easy and it would be weird to put a ZST in a lock in the first place.
(Not that you have argued against it)

--
Cheers,
Benno