linux-kernel - Re: [PATCH v5 6/6] rust: use strict provenance APIs

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Message-ID: <Z9lnIJCcVSza6UVo@google.com>
Date: Tue, 18 Mar 2025 12:29:20 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Tamir Duberstein <tamird@...il.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nicolas Schier <nicolas@...sle.eu>, 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>, 
	Danilo Krummrich <dakr@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Brendan Higgins <brendan.higgins@...ux.dev>, 
	David Gow <davidgow@...gle.com>, Rae Moar <rmoar@...gle.com>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Luis Chamberlain <mcgrof@...nel.org>, 
	Russ Weight <russ.weight@...ux.dev>, Rob Herring <robh@...nel.org>, 
	Saravana Kannan <saravanak@...gle.com>, linux-kbuild@...r.kernel.org, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com, 
	linux-pci@...r.kernel.org, linux-block@...r.kernel.org, 
	devicetree@...r.kernel.org
Subject: Re: [PATCH v5 6/6] rust: use strict provenance APIs

On Mon, Mar 17, 2025 at 10:23:56AM -0400, Tamir Duberstein wrote:
> Throughout the tree, use the strict provenance APIs stabilized in Rust
> 1.84.0[1]. Retain backwards-compatibility by introducing forwarding
> functions at the `kernel` crate root along with polyfills for rustc <
> 1.84.0.
> 
> Use `#[allow(clippy::incompatible_msrv)]` to avoid warnings on rustc <
> 1.84.0 as our MSRV is 1.78.0.
> 
> In the `kernel` crate, enable the strict provenance lints on rustc >=
> 1.84.0; do this in `lib.rs` rather than `Makefile` to avoid introducing
> compiler flags that are dependent on the rustc version in use.
> 
> Link: https://blog.rust-lang.org/2025/01/09/Rust-1.84.0.html#strict-provenance-apis [1]
> Suggested-by: Benno Lossin <benno.lossin@...ton.me>
> Link: https://lore.kernel.org/all/D8EIXDMRXMJP.36TFCGWZBRS3Y@proton.me/
> Signed-off-by: Tamir Duberstein <tamird@...il.com>

I'm not convinced that the pros of this change outweigh the cons. I
think this is going to be too confusing for the C developers who look at
this code.

> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index 719b0a48ff55..96393bcf6bd7 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -226,7 +226,9 @@ pub fn read_raw(&mut self, out: &mut [MaybeUninit<u8>]) -> Result {
>          }
>          // SAFETY: `out_ptr` points into a mutable slice of length `len`, so we may write
>          // that many bytes to it.
> -        let res = unsafe { bindings::copy_from_user(out_ptr, self.ptr as *const c_void, len) };
> +        let res = unsafe {
> +            bindings::copy_from_user(out_ptr, crate::with_exposed_provenance(self.ptr), len)
> +        };
>          if res != 0 {
>              return Err(EFAULT);
>          }
> @@ -264,7 +266,7 @@ pub fn read<T: FromBytes>(&mut self) -> Result<T> {
>          let res = unsafe {
>              bindings::_copy_from_user(
>                  out.as_mut_ptr().cast::<c_void>(),
> -                self.ptr as *const c_void,
> +                crate::with_exposed_provenance(self.ptr),
>                  len,
>              )
>          };

That's especially true for cases like this. These are userspace pointers
that are never dereferenced. It's not useful to care about provenance
here.

Alice