linux-kernel - Re: [PATCH v3 13/14] rust: drm: gem: Add export() callback

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Message-ID: <4163b536a80badc7f5bfc8ddcb453547d3327d0c.camel@redhat.com>
Date: Thu, 11 Sep 2025 18:32:26 -0400
From: Lyude Paul <lyude@...hat.com>
To: Daniel Almeida <daniel.almeida@...labora.com>
Cc: dri-devel@...ts.freedesktop.org, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Danilo Krummrich <dakr@...nel.org>, Maarten
 Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
 <mripard@...nel.org>, Thomas Zimmermann	 <tzimmermann@...e.de>, David
 Airlie <airlied@...il.com>, Simona Vetter	 <simona@...ll.ch>, 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>, Benno
 Lossin <lossin@...nel.org>, Andreas Hindborg	 <a.hindborg@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross	 <tmgross@...ch.edu>, Sumit
 Semwal <sumit.semwal@...aro.org>, Christian König	
 <christian.koenig@....com>, Asahi Lina <lina+kernel@...hilina.net>, "open
 list:DRM DRIVER FOR NVIDIA GPUS [RUST]"	 <nouveau@...ts.freedesktop.org>,
 linux-media@...r.kernel.org, 	linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH v3 13/14] rust: drm: gem: Add export() callback

On Fri, 2025-09-05 at 12:09 -0300, Daniel Almeida wrote:
> > +impl<T: IntoGEMObject> Drop for DmaBuf<T> {
> > +    #[inline]
> > +    fn drop(&mut self) {
> > +        // SAFETY:
> > +        // - `dma_buf::DmaBuf` is guaranteed to have an identical layout to `struct dma_buf`
> > +        //   by its type invariants.
> > +        // - We hold the last reference to this `DmaBuf`, making it safe to destroy.
> 
> How can we be sure of this?

DmaBuf objects created with drm_gem_dmabuf_export() are unique, e.g. if you
call the function twice you have two DmaBufs - not two references to the same
DmaBuf. Since we don't implement Clone on DmaBuf, we're also the only one who
could hold a reference to the respective dma_buf::DmaBuf.

Note that this is unlike SGTables with shmem, where you only have references
to a single shared SGTable for each gem object that's created dynamically.

> 
> > +        unsafe { bindings::drm_gem_dmabuf_release(self.0.cast().as_ptr()) }
> > +    }
> > +}
> > +
> > +impl<T: IntoGEMObject> DmaBuf<T> {
> > +    /// Leak the reference for this [`DmaBuf`] and return a raw pointer to it.
> > +    #[inline]
> > +    pub(crate) fn into_raw(self) -> *mut bindings::dma_buf {
> 
> Then this should perhaps be called leak()? At least if we’re following the std nomenclature.

Nope, into_raw() is correct actually! FWIW: I had to double check this against
the std, it goes like this:

https://doc.rust-lang.org/std/boxed/struct.Box.html#method.into_raw
into_raw() provides Box<T, A> → *mut T

https://doc.rust-lang.org/std/boxed/struct.Box.html#method.leak
leak<'a>() provides Box<T, A> -> &'a mut T

Since we're returning *mut here, it should be into_raw().

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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