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

[Benno Lossin]

On Mon Mar 17, 2025 at 3:23 PM CET, 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@xxxxxxxxx>
> Link: https://lore.kernel.org/all/D8EIXDMRXMJP.36TFCGWZBRS3Y@xxxxxxxxx/
> Signed-off-by: Tamir Duberstein <tamird@xxxxxxxxx>

One comment below, with that fixed:

Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx>

> ---
>  init/Kconfig           |   3 ++
>  rust/kernel/alloc.rs   |   2 +-
>  rust/kernel/devres.rs  |   4 +-
>  rust/kernel/io.rs      |  14 +++----
>  rust/kernel/lib.rs     | 108 +++++++++++++++++++++++++++++++++++++++++++++++++
>  rust/kernel/of.rs      |   2 +-
>  rust/kernel/pci.rs     |   4 +-
>  rust/kernel/str.rs     |  16 +++-----
>  rust/kernel/uaccess.rs |  12 ++++--
>  9 files changed, 138 insertions(+), 27 deletions(-)


> +#[cfg(not(CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE))]
> +mod strict_provenance {
> +    /// Gets the "address" portion of the pointer.
> +    ///
> +    /// See https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.addr.
> +    #[inline]
> +    pub fn addr<T>(ptr: *const T) -> usize {
> +        // This is core's implementation from
> +        // https://github.com/rust-lang/rust/commit/4291332175d12e79e6061cdc3f5dccac2e28b969 through
> +        // https://github.com/rust-lang/rust/blob/1.84.0/library/core/src/ptr/const_ptr.rs#L172
> +        // which is the first version that satisfies `CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE`.
> +        #[allow(clippy::undocumented_unsafe_blocks)]
> +        unsafe {
> +            #[allow(clippy::transmutes_expressible_as_ptr_casts)]
> +            core::mem::transmute(ptr.cast::<()>())
> +        }

I think we should just use `ptr as usize` here instead. It's going away
at some point and it will only affect optimizations (I don't even know
if they exist at the moment) of old versions.

---
Cheers,
Benno

> +    }
> +
> +    /// Exposes the "provenance" part of the pointer for future use in
> +    /// [`with_exposed_provenance`] and returns the "address" portion.
> +    ///
> +    /// See https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.expose_provenance.
> +    #[inline]
> +    pub fn expose_provenance<T>(ptr: *const T) -> usize {
> +        ptr.cast::<()>() as usize
> +    }
> +
> +    /// Converts an address back to a pointer, picking up some previously 'exposed'
> +    /// provenance.
> +    ///
> +    /// See https://doc.rust-lang.org/stable/core/ptr/fn.with_exposed_provenance.html.
> +    #[inline]
> +    pub fn with_exposed_provenance<T>(addr: usize) -> *const T {
> +        addr as *const T
> +    }
> +
> +    /// Converts an address back to a mutable pointer, picking up some previously 'exposed'
> +    /// provenance.
> +    ///
> +    /// See https://doc.rust-lang.org/stable/core/ptr/fn.with_exposed_provenance_mut.html
> +    #[inline]
> +    pub fn with_exposed_provenance_mut<T>(addr: usize) -> *mut T {
> +        addr as *mut T
> +    }
> +
> +    /// Creates a pointer with the given address and no [provenance][crate::ptr#provenance].
> +    ///
> +    /// See https://doc.rust-lang.org/stable/core/ptr/fn.without_provenance_mut.html.
> +    #[inline]
> +    pub fn without_provenance_mut<T>(addr: usize) -> *mut T {
> +        addr as *mut T
> +    }
> +}
> +
> +pub use strict_provenance::*;
> +
>  // Ensure conditional compilation based on the kernel configuration works;
>  // otherwise we may silently break things like initcall handling.
>  #[cfg(not(CONFIG_RUST))]