linux-kernel - [PATCH v3 2/2] rust: disallow use of `CStr::as_ptr` and `CStr::from

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Message-ID: <20260203130745.868762-2-gary@kernel.org>
Date: Tue,  3 Feb 2026 13:06:27 +0000
From: Gary Guo <gary@...nel.org>
To: Miguel Ojeda <ojeda@...nel.org>,
	Boqun Feng <boqun@...nel.org>,
	Gary Guo <gary@...yguo.net>,
	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>,
	Panagiotis Foliadis <pfoliadis@...teo.net>,
	Shankari Anand <shankari.ak0208@...il.com>,
	FUJITA Tomonori <fujita.tomonori@...il.com>,
	Tamir Duberstein <tamird@...nel.org>
Cc: kernel test robot <lkp@...el.com>,
	rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v3 2/2] rust: disallow use of `CStr::as_ptr` and `CStr::from_ptr`

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.

Similarly, the dual `from_ptr` is also disallowed.

Signed-off-by: Gary Guo <gary@...yguo.net>
---
Changes since v2:
- Also disallow `CStr::from_ptr`
---
 .clippy.toml       | 10 ++++++++++
 rust/kernel/str.rs |  3 +++
 2 files changed, 13 insertions(+)

diff --git a/.clippy.toml b/.clippy.toml
index 137f41d203de..a51de9a46380 100644
--- a/.clippy.toml
+++ b/.clippy.toml
@@ -9,3 +9,13 @@ 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"
+
+[[disallowed-methods]]
+path = "core::ffi::CStr::from_ptr"
+replacement = "kernel::prelude::CStrExt::from_char_ptr"
+reason = "kernel's `char` is always unsigned, use `from_char_ptr` instead"
diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
index fa87779d2253..97bf9427af59 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()
 }
@@ -319,6 +320,7 @@ unsafe fn to_bytes_mut(s: &mut CStr) -> &mut [u8] {
 
 impl CStrExt for CStr {
     #[inline]
+    #[expect(clippy::disallowed_methods, reason = "internal implementation")]
     unsafe fn from_char_ptr<'a>(ptr: *const c_char) -> &'a Self {
         // SAFETY: The safety preconditions are the same as for `CStr::from_ptr`.
         unsafe { CStr::from_ptr(ptr.cast()) }
@@ -334,6 +336,7 @@ unsafe fn from_bytes_with_nul_unchecked_mut(bytes: &mut [u8]) -> &mut Self {
     }
 
     #[inline]
+    #[expect(clippy::disallowed_methods, reason = "internal implementation")]
     fn as_char_ptr(&self) -> *const c_char {
         self.as_ptr().cast()
     }
-- 
2.51.2