linux-kernel - Re: [PATCH] rust: disallow use of `CStr::as

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Message-ID: <CAJ-ks9=J564pjPbMBaq5MOHy2W5MhyghR-0B1B4XwWQVVnEOSQ@mail.gmail.com>
Date: Wed, 21 Jan 2026 12:55:25 -0500
From: Tamir Duberstein <tamird@...nel.org>
To: Gary Guo <gary@...yguo.net>
Cc: Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: disallow use of `CStr::as_ptr`

On Wed, Jan 21, 2026 at 11:58 AM Gary Guo <gary@...nel.org> wrote:
>
> From: Gary Guo <gary@...yguo.net>
>
> As kernel always use unsigned char and not the platform ABI's default, an
> user should always use `as_char_ptr` provided via `CStrExt` instead.
> Therefore configure `disallow-methods` feature of clippy to catch incorrect
> usage.
>
> Signed-off-by: Gary Guo <gary@...yguo.net>
> ---
>  .clippy.toml       | 5 +++++
>  rust/kernel/str.rs | 2 ++
>  2 files changed, 7 insertions(+)
>
> diff --git a/.clippy.toml b/.clippy.toml
> index 137f41d203de..fd934bc04242 100644
> --- a/.clippy.toml
> +++ b/.clippy.toml
> @@ -9,3 +9,8 @@ disallowed-macros = [
>      # it here, see: https://github.com/rust-lang/rust-clippy/issues/11303.
>      { path = "kernel::dbg", reason = "the `dbg!` macro is intended as a debugging tool", allow-invalid = true },
>  ]
> +
> +[[disallowed-methods]]
> +path = "core::ffi::CStr::as_ptr"
> +replacement = "kernel::prelude::CStrExt::as_char_ptr"
> +reason = "Kernel's `char` is always unsigned. Use `as_char_ptr` instead."

In the disallowed-macros section above we seem to use sentence
fragments (no capitalization, no trailing period).

> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
> index fa87779d2253..08b8e2ebc8ad 100644
> --- a/rust/kernel/str.rs
> +++ b/rust/kernel/str.rs
> @@ -189,6 +189,7 @@ macro_rules! b_str {
>  //
>  // - error[E0379]: functions in trait impls cannot be declared const
>  #[inline]
> +#[expect(clippy::disallowed_methods, reason = "internal implementation")]
>  pub const fn as_char_ptr_in_const_context(c_str: &CStr) -> *const c_char {
>      c_str.as_ptr().cast()
>  }
> @@ -334,6 +335,7 @@ unsafe fn from_bytes_with_nul_unchecked_mut(bytes: &mut [u8]) -> &mut Self {
>      }
>
>      #[inline]
> +    #[expect(clippy::disallowed_methods, reason = "internal implementation")]

Can this be inside the function, on the `as_ptr` call itself?

>      fn as_char_ptr(&self) -> *const c_char {
>          self.as_ptr().cast()
>      }
>
> --
> 2.51.2
>

Acked-by: Tamir Duberstein <tamird@...nel.org>