linux-kernel - Re: [PATCH] rust: kunit: use C-string literals to clean warning

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Message-ID: <20240928193632.063d62a6.gary@garyguo.net>
Date: Sat, 28 Sep 2024 19:36:32 +0100
From: Gary Guo <gary@...yguo.net>
To: Miguel Ojeda <ojeda@...nel.org>
Cc: Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, Benno Lossin
 <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, Alice
 Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
 rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
 patches@...ts.linux.dev
Subject: Re: [PATCH] rust: kunit: use C-string literals to clean warning

On Fri, 27 Sep 2024 18:44:14 +0200
Miguel Ojeda <ojeda@...nel.org> wrote:

> Starting with upstream Rust commit a5e3a3f9b6bd ("move
> `manual_c_str_literals` to complexity"), to be released in Rust 1.83.0
> [1], Clippy now warns on `manual_c_str_literals` by default, e.g.:
> 
>     error: manually constructing a nul-terminated string
>       --> rust/kernel/kunit.rs:21:13  
>        |
>     21 |             b"\x013%pA\0".as_ptr() as _,
>        |             ^^^^^^^^^^^^^ help: use a `c""` literal: `c"\x013%pA"`
>        |
>        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_c_str_literals
>        = note: `-D clippy::manual-c-str-literals` implied by `-D warnings`
>        = help: to override `-D warnings` add `#[allow(clippy::manual_c_str_literals)]`
> 
> Apply the suggestion to clean up the warnings.
> 
> Link: https://github.com/rust-lang/rust-clippy/pull/13263 [1]
> Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
> ---
> The cast could also be cleaned up, but that is already done in Gary's
> FFI series, so I didn't include another patch.

Note that the cast actually need to be re-introduced instead of
cleaned-up -- `c"".as_ptr()` returns `core::ffi::c_char` which might be
signed, while after my series `*const u8` is expected by `printf`.

An alternative is to use `c_str!(...).as_char_ptr()` which will
guarantee the type is what we expected.

Best,
Gary

> 
> C-string literals are available since Rust 1.77.0, which is higher than
> our minimum version, but LTS kernels do not have Rust 1.77.0. But
> perhaps for these is not a big deal, and we could also enable
> `feature(c_str_literals)` instead.
> 
>  rust/kernel/kunit.rs | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/rust/kernel/kunit.rs b/rust/kernel/kunit.rs
> index 0ba77276ae7e..824da0e9738a 100644
> --- a/rust/kernel/kunit.rs
> +++ b/rust/kernel/kunit.rs
> @@ -18,7 +18,7 @@ pub fn err(args: fmt::Arguments<'_>) {
>      #[cfg(CONFIG_PRINTK)]
>      unsafe {
>          bindings::_printk(
> -            b"\x013%pA\0".as_ptr() as _,
> +            c"\x013%pA".as_ptr() as _,
>              &args as *const _ as *const c_void,
>          );
>      }
> @@ -34,7 +34,7 @@ pub fn info(args: fmt::Arguments<'_>) {
>      #[cfg(CONFIG_PRINTK)]
>      unsafe {
>          bindings::_printk(
> -            b"\x016%pA\0".as_ptr() as _,
> +            c"\x016%pA".as_ptr() as _,
>              &args as *const _ as *const c_void,
>          );
>      }
> 
> base-commit: 570172569238c66a482ec3eb5d766cc9cf255f69
> --
> 2.46.2