linux-kernel - Re: [PATCH v11 2/3] rust: add dma coherent allocator abstraction.

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CAH5fLgic6kdOQyw+=S49p4rVnA7x6maKPVakd1uqA5fkGxZiXA@mail.gmail.com>
Date: Mon, 27 Jan 2025 10:45:06 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Petr Tesařík <petr@...arici.cz>
Cc: Abdiel Janulgue <abdiel.janulgue@...il.com>, rust-for-linux@...r.kernel.org, 
	daniel.almeida@...labora.com, dakr@...nel.org, robin.murphy@....com, 
	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 <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Valentin Obst <kernel@...entinobst.de>, 
	open list <linux-kernel@...r.kernel.org>, Christoph Hellwig <hch@....de>, 
	Marek Szyprowski <m.szyprowski@...sung.com>, airlied@...hat.com, 
	"open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>
Subject: Re: [PATCH v11 2/3] rust: add dma coherent allocator abstraction.

On Mon, Jan 27, 2025 at 7:16 AM Petr Tesařík <petr@...arici.cz> wrote:
>
> On Fri, 24 Jan 2025 08:27:36 +0100
> Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> > On Thu, Jan 23, 2025 at 11:43 AM Abdiel Janulgue
> > <abdiel.janulgue@...il.com> wrote:
> > >
> > > Add a simple dma coherent allocator rust abstraction. Based on
> > > Andreas Hindborg's dma abstractions from the rnvme driver, which
> > > was also based on earlier work by Wedson Almeida Filho.
> > >
> > > Signed-off-by: Abdiel Janulgue <abdiel.janulgue@...il.com>
> >
> > Overall I think it looks pretty good, but a follow-up from our
> > discussion the other day.
> >
> > > +    /// Returns the device, base address, dma handle, attributes and the size of the
> > > +    /// allocated region.
> > > +    ///
> > > +    /// The caller takes ownership of the returned resources, i.e., will have the responsibility
> > > +    /// in calling `bindings::dma_free_attrs`. The allocated region is valid as long as
> > > +    /// the returned device exists.
> > > +    pub fn into_parts(
> > > +        self,
> > > +    ) -> (
> > > +        ARef<Device>,
> > > +        *mut T,
> > > +        bindings::dma_addr_t,
> > > +        crate::ffi::c_ulong,
> > > +        usize,
> > > +    ) {
> > > +        let size = self.count * core::mem::size_of::<T>();
> > > +        let ret = (
> > > +            // SAFETY: `&self.dev` is valid for reads.
> > > +            unsafe { core::ptr::read(&self.dev) },
> >
> > This safety comment should explain why this will not lead to
> > decrementing the refcount too many times. Which is because this
> > doesn't actually duplicate the ARef due to the use of mem::forget.
> >
> > That said, I think wrapping self in ManuallyDrop is slightly easier to
> > read than using mem::forget. I refer you to this pattern from Tokio
> > that solves a similar problem:
> > https://github.com/tokio-rs/tokio/blob/ee19b0ed7371b069112b9c9ef9280b81f3438d26/tokio/src/sync/rwlock/owned_write_guard.rs#L38-L51
> >
> > > +            self.cpu_addr,
> > > +            self.dma_handle,
> > > +            self.dma_attrs.as_raw(),
> >
> > Why not return the Attrs type instead of an integer?
> >
> > > +            size,
> > > +        );
> > > +        core::mem::forget(self);
> > > +        ret
> > > +    }
> >
> > > +    /// Reads data from the region starting from `offset` as a slice.
> > > +    /// `offset` and `count` are in units of `T`, not the number of bytes.
> > > +    ///
> > > +    /// Due to the safety requirements of slice, the data returned should be regarded by the
> > > +    /// caller as a snapshot of the region when this function is called, as the region could
> > > +    /// be modified by the device at anytime. For ringbuffer type of r/w access or use-cases
> > > +    /// where the pointer to the live data is needed, `start_ptr()` or `start_ptr_mut()`
> > > +    /// could be used instead.
> > > +    ///
> > > +    /// # Safety
> > > +    ///
> > > +    /// Callers must ensure that no hardware operations that involve the buffer are currently
> > > +    /// taking place while the returned slice is live.
> > > +    pub unsafe fn as_slice(&self, offset: usize, count: usize) -> Result<&[T]> {
> >
> > You were asked to rename this function because it returns a slice, but
> > I wonder if it's better to take an `&mut [T]` argument and to have
> > this function copy data into that argument. That way, we could make
> > the function itself safe. Perhaps the actual copy needs to be
> > volatile?
> >
> > Well ... I understand that we did this previously and that we want to
> > avoid it because it causes too much reading if T is a struct and we
> > just want to read one of its fields. How about an API like this?
> >
> > dma_read!(my_alloc[7].foo)
> >
> > which expands to something that reads the value of the foo field of
> > the 7th element, and
> >
> > dma_write!(my_alloc[7].foo = 13);
> >
> > That expands to something that writes 13 to field foo of the 7th element.
> >
> > Thoughts? I'm proposing this to avoid going in circles between the
> > same solutions.
>
> I don't have a strong opinion, but maybe you could elaborate a bit on
> what exactly you dislike about as_slice() / as_slice_mut().

The as_slice() method is unsafe as currently defined. The purpose of
this proposal is so we can have a method that is not unsafe.

Alice