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

[Benno Lossin]

On Mon Mar 17, 2025 at 12:25 PM CET, Tamir Duberstein wrote:
> On Mon, Mar 17, 2025 at 6:48 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>>
>> On Mon, Mar 17, 2025 at 09:58:35AM +0000, Benno Lossin wrote:
>> > 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".
>
> Why? Is it possible to set it any other way?

Yeah when `truncate` and `resize` land. It conveys that this is a
low-level operation.

>> > >      ///
>> > >      /// # 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.
>>
>> I don't think we do.
>
> Rearranged as
>
>         debug_assert!(additional <= self.capacity() - self.len());

LGTM

> It should be impossible for this to underflow because capacity must be
>>= len. Interestingly this isn't a documented invariant, but it is
> relied upon by `spare_capacity_mut`.

Oh yeah that should be an invariant. Feel free to open an issue or send
a patch.

>> > > 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.
>>
>> The buffer has length zero as it was just created with:
>>
>> let mut buf = KVec::with_capacity(size, GFP_KERNEL)?;

Ahh, I didn't look at the context. Makes sense.

> Indeed. Added to the commit message.

Thanks.

---
Cheers,
Benno