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

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Message-Id: <DFUJ9ECRB2GJ.ZK9ZCKMIFQYJ@garyguo.net>
Date: Wed, 21 Jan 2026 20:01:02 +0000
From: "Gary Guo" <gary@...yguo.net>
To: "Tamir Duberstein" <tamird@...nel.org>, "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 7:45 PM GMT, Tamir Duberstein wrote:
> On Wed, Jan 21, 2026 at 2:38 PM Gary Guo <gary@...yguo.net> wrote:
>>
>> On Wed Jan 21, 2026 at 5:55 PM GMT, Tamir Duberstein wrote:
>> > 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).
>>
>> Ok.
>>
>> >
>> >> 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?
>>
>> You cannot apply attributes on expressions (yet). If this is to be moved into
>> the function, there needs an additional block, which I think is just
>> unnecessary.
>
> Thought so. Should we add a TODO that links to the relevant upstream
> issue, or track it in
> https://github.com/Rust-for-Linux/linux/issues/2?

I don't think we really this need this.

If Miguel thinks this is useful then he can add it to the tracking issue.

Best,
Gary

>>
>> >
>> >>      fn as_char_ptr(&self) -> *const c_char {
>> >>          self.as_ptr().cast()
>> >>      }
>> >>
>> >> --
>> >> 2.51.2
>> >>
>> >
>> > Acked-by: Tamir Duberstein <tamird@...nel.org>
>>
>>