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Date: Thu, 19 Jun 2025 13:01:16 +0200
Message-Id: <DAQGIG20TJV6.3O4G0F4O72327@kernel.org>
Cc: <ojeda@kernel.org>, <alex.gaynor@gmail.com>, <aliceryhl@google.com>,
 <daniel.almeida@collabora.com>, <boqun.feng@gmail.com>, <gary@garyguo.net>,
 <bjorn3_gh@protonmail.com>, <a.hindborg@kernel.org>, <tmgross@umich.edu>,
 <alyssa@rosenzweig.io>, <lyude@redhat.com>,
 <rust-for-linux@vger.kernel.org>, <dri-devel@lists.freedesktop.org>
Subject: Re: [PATCH] rust: drm: Drop the use of Opaque for ioctl arguments
From: "Benno Lossin" <lossin@kernel.org>
To: "Danilo Krummrich" <dakr@kernel.org>, "Beata Michalska"
 <beata.michalska@arm.com>
X-Mailer: aerc 0.20.1
References: <20250619102102.750668-1-beata.michalska@arm.com>
 <aFPsjIFIC1J2qtdc@pollux>
In-Reply-To: <aFPsjIFIC1J2qtdc@pollux>

On Thu Jun 19, 2025 at 12:55 PM CEST, Danilo Krummrich wrote:
> On Thu, Jun 19, 2025 at 12:21:02PM +0200, Beata Michalska wrote:
>> diff --git a/rust/kernel/drm/ioctl.rs b/rust/kernel/drm/ioctl.rs
>> index 445639404fb7..12b296131672 100644
>> --- a/rust/kernel/drm/ioctl.rs
>> +++ b/rust/kernel/drm/ioctl.rs
>> @@ -139,7 +139,7 @@ pub mod internal {
>>                              // asserted above matches the size of this =
type, and all bit patterns of
>>                              // UAPI structs must be valid.
>>                              let data =3D unsafe {
>> -                                &*(raw_data as *const $crate::types::Op=
aque<$crate::uapi::$struct>)
>> +                                &mut *(raw_data as *mut $crate::uapi::$=
struct)
>
> I think we have to document the guarantees we rely on to create this muta=
ble
> reference.

If the C side is using pointers to read/write the value concurrently,
this is wrong, it needs to be wrapped in Opaque.

---
Cheers,
Benno