linux-kernel - Re: [PATCH 1/2] rust: alloc: replace `Vec::set_len` with `inc

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Message-ID: <D8IGB61NVPBT.1I3IZYPVPIEMU@proton.me>
Date: Mon, 17 Mar 2025 09:58:35 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Tamir Duberstein <tamird@...il.com>, Danilo Krummrich <dakr@...nel.org>, Andrew Ballance <andrewjballance@...il.com>, Alice Ryhl <aliceryhl@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] rust: alloc: replace `Vec::set_len` with `inc_len`

On Sun Mar 16, 2025 at 11:32 PM CET, Tamir Duberstein wrote:
> Rename `set_len` to `inc_len` and simplify its safety contract.
> ---
>  rust/kernel/alloc/kvec.rs | 19 +++++++++----------
>  rust/kernel/str.rs        |  2 +-
>  rust/kernel/uaccess.rs    |  2 +-
>  3 files changed, 11 insertions(+), 12 deletions(-)
>
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index ae9d072741ce..d43a1d609434 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -183,17 +183,16 @@ pub fn len(&self) -> usize {
>          self.len
>      }
>  
> -    /// Forcefully sets `self.len` to `new_len`.
> +    /// Increments `self.len` by `additional`.

I would keep the "Forcefully".

>      ///
>      /// # Safety
>      ///
> -    /// - `new_len` must be less than or equal to [`Self::capacity`].
> -    /// - If `new_len` is greater than `self.len`, all elements within the interval
> -    ///   [`self.len`,`new_len`) must be initialized.
> +    /// - `self.len + additional` must be less than or equal to [`Self::capacity`].
> +    /// - All elements within the interval [`self.len`,`self.len + additional`) must be initialized.
>      #[inline]
> -    pub unsafe fn set_len(&mut self, new_len: usize) {
> -        debug_assert!(new_len <= self.capacity());
> -        self.len = new_len;
> +    pub unsafe fn inc_len(&mut self, additional: usize) {
> +        debug_assert!(self.len() + additional <= self.capacity());

What if this overflows? Do we always have overflow debugging on when
debug assertions are enabled? If yes, then this is fine.

> +        self.len += additional;
>      }
>  
>      /// Returns a slice of the entire vector.

> diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
> index 28e2201604d6..005713839e9e 100644
> --- a/rust/kernel/str.rs
> +++ b/rust/kernel/str.rs
> @@ -840,7 +840,7 @@ pub fn try_from_fmt(args: fmt::Arguments<'_>) -> Result<Self, Error> {
>  
>          // SAFETY: The number of bytes that can be written to `f` is bounded by `size`, which is
>          // `buf`'s capacity. The contents of the buffer have been initialised by writes to `f`.
> -        unsafe { buf.set_len(f.bytes_written()) };
> +        unsafe { buf.inc_len(f.bytes_written()) };

This change seems wrong unless the code was wrong to begin with.

Otherwise the change looks good.

---
Cheers,
Benno

>  
>          // Check that there are no `NUL` bytes before the end.
>          // SAFETY: The buffer is valid for read because `f.bytes_written()` is bounded by `size`
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index 719b0a48ff55..0aa5455a18be 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -291,7 +291,7 @@ pub fn read_all<A: Allocator>(mut self, buf: &mut Vec<u8, A>, flags: Flags) -> R
>  
>          // SAFETY: Since the call to `read_raw` was successful, so the next `len` bytes of the
>          // vector have been initialized.
> -        unsafe { buf.set_len(buf.len() + len) };
> +        unsafe { buf.inc_len(len) };
>          Ok(())
>      }
>  }