Re: [PATCH 1/2] rust: alloc: replace `Vec::set_len` with `inc_len`

Benno Lossin
2025-03-17T09:58:43Z

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(())
> }
> }