From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lindbergh.monkeyblade.net (lindbergh.monkeyblade.net [23.128.96.19]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id EB9821170A for ; Mon, 25 Sep 2023 21:04:05 +0000 (UTC) Received: from mail-40131.protonmail.ch (mail-40131.protonmail.ch [185.70.40.131]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EFD5010F; Mon, 25 Sep 2023 14:04:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=proton.me; s=protonmail; t=1695675840; x=1695935040; bh=EUihDlu4xMCW4pJXMQT++Wi78DVs4G1Y5sE4xVXS7LI=; h=Date:To:From:Cc:Subject:Message-ID:In-Reply-To:References: Feedback-ID:From:To:Cc:Date:Subject:Reply-To:Feedback-ID: Message-ID:BIMI-Selector; b=CRf33zD4C8pjNv/GD/F85p0HUQw3v8OTbTEU2ZF+jvfUOqycYXwfLtBYCfUhjsx/t Ope6Pc881DenM1TOSpCxO3DsOe8vWXuBTx6itY0H2NK2G87xCAu2aTb7vTwnmbL35W 6z6CeoA5qwIBewfSTSgA6QjRVbSmcfLotvTUeNcZZBT3VqgqTvdPlIXv9tkIvSc3JX DHq6JRsrUTayn0/nto3uhKAD/4UX2ZHFAsysERo/lkqHIGY9v4JNy/c2wn4NXU/5YX fatZfNnSHRtr472RziosIsSQBnZZS2pVUHYO7zzAmK97VQgOHHOIlPUuumBK+tz8sX zttnQSU55HqZw== Date: Mon, 25 Sep 2023 21:03:52 +0000 To: Boqun Feng From: Benno Lossin Cc: Alice Ryhl , Wedson Almeida Filho , rust-for-linux@vger.kernel.org, Miguel Ojeda , Alex Gaynor , Gary Guo , =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= , Andreas Hindborg , linux-kernel@vger.kernel.org, Wedson Almeida Filho Subject: Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef` Message-ID: In-Reply-To: References: <20230923144938.219517-1-wedsonaf@gmail.com> <20230923144938.219517-3-wedsonaf@gmail.com> <969eab7f-ad40-0dfb-18b9-6002fc54e12b@proton.me> <14513589-cc31-8985-8ff6-a97d2882f593@proton.me> <9d6d6c94-5da6-a56d-4e85-fbf8da26a0b0@proton.me> Feedback-ID: 71780778:user:proton Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_BLOCKED, RCVD_IN_MSPIKE_H5,RCVD_IN_MSPIKE_WL,SPF_HELO_PASS,SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net On 25.09.23 20:51, Boqun Feng wrote: > On Mon, Sep 25, 2023 at 05:00:45PM +0000, Benno Lossin wrote: >> On 25.09.23 18:16, Boqun Feng wrote: >>> On Mon, Sep 25, 2023 at 03:07:44PM +0000, Benno Lossin wrote: >>>> ```rust >>>> struct MutatingDrop { >>>> value: i32, >>>> } >>>> >>>> impl Drop for MutatingDrop { >>>> fn drop(&mut self) { >>>> self.value =3D 0; >>>> } >>>> } >>>> >>>> let arc =3D Arc::new(MutatingDrop { value: 42 }); >>>> let wr =3D arc.as_with_ref(); // this creates a shared `&` reference t= o the MutatingDrop >>>> let arc2: Arc =3D wr.into(); // increments the reference= count to 2 >>> >>> More precisely, here we did a >>> >>> =09&WithRef<_> -> NonNull> >>> >>> conversion, and later on, we may use the `NonNull>` in >>> `drop` to get a `Box>`. >> >> Indeed. >> >=20 > Can we workaround this issue by (ab)using the `UnsafeCell` inside > `WithRef`? >=20 > impl From<&WithRef> for Arc { > fn from(b: &WithRef) -> Self { > // SAFETY: The existence of the references proves that > =09// `b.refcount.get()` is a valid pointer to `WithRef`. > =09let ptr =3D unsafe { NonNull::new_unchecked(b.refcount.get().cast::>()) }; >=20 > =09// SAFETY: see the SAFETY above `let ptr =3D ..` line. > ManuallyDrop::new(unsafe { Arc::from_inner(ptr) }) > .deref() > .clone() > } > } >=20 > This way, the raw pointer in the new Arc no longer derives from the > reference of `WithRef`. No, the code above only obtains a pointer that has provenance valid for a `bindings::refcount_t` (or type with the same layout, such as `Opaque`). But not the whole `WithRef`, so accessi= ng it by reading/writing will still be UB. --=20 Cheers, Benno