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: <20250127071625.1882afa4@meshulam.tesarici.cz>
Date: Mon, 27 Jan 2025 07:16:25 +0100
From: Petr Tesařík <petr@...arici.cz>
To: Alice Ryhl <aliceryhl@...gle.com>
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 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().

Petr T