linux-kernel - Re: [PATCH v14 02/11] rust: add dma coherent allocator abstraction.

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Message-ID: <D8DRCM2FOEBN.3IVEVKW9A65AL@proton.me>
Date: Tue, 11 Mar 2025 21:34:19 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Boqun Feng <boqun.feng@...il.com>, Abdiel Janulgue <abdiel.janulgue@...il.com>
Cc: rust-for-linux@...r.kernel.org, daniel.almeida@...labora.com, dakr@...nel.org, robin.murphy@....com, aliceryhl@...gle.com, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 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 v14 02/11] rust: add dma coherent allocator abstraction.

On Tue Mar 11, 2025 at 7:12 PM CET, Boqun Feng wrote:
> On Tue, Mar 11, 2025 at 07:47:58PM +0200, Abdiel Janulgue wrote:
> [...]
>> +    /// Reads the value of `field` and ensures that its type is [`FromBytes`].
>> +    ///
>> +    /// # Safety
>> +    ///
>> +    /// This must be called from the [`dma_read`] macro which ensures that the `field` pointer is
>> +    /// validated beforehand.
>> +    ///
>> +    /// Public but hidden since it should only be used from [`dma_read`] macro.
>> +    #[doc(hidden)]
>> +    pub unsafe fn field_read<F: FromBytes>(&self, field: *const F) -> F {
>> +        // SAFETY: By the safety requirements field is valid.
>> +        unsafe { field.read_volatile() }
>
> I agree with Andreas that we should document the exception of usage on
> {read,write}_volatile() here. How about:
>
> When dealing with a potential race from a hardware or code outside
> kernel (e.g. user-space program), we need that read and write on a valid
> memory are not UBs. Currently {read,write}_volatile() are used for this,

I would use the singular `UB` here and below.

> and the rationale behind is that they should generate the same code as
> READ_ONCE() and WRITE_ONCE() which kernel already relies on to avoid UBs

s/kernel/the kernel/

> on data races. Note that the usage of {read,write}_volatile() is limited
> to this particular case, they cannot be used to emit the UBs caused by

s/emit/prevent/

> racing between two kernel functions nor do they provide atomicity.
>
> Thoughts? One problem is that I don't know where to put this document
> :-( Any suggestion?

I am a bit out of the loop on this one, but why not put into the safety
comment? I.e. explicitly state that this is *not* sound as per the usual
rules and it is a special exception?