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

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Message-ID: <CAJ-ks9nSNweaSCVAruo-zWxMAFKcRxcJRScV06NRNBoeEZggfA@mail.gmail.com>
Date: Mon, 17 Mar 2025 07:25:14 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Benno Lossin <benno.lossin@...ton.me>, Danilo Krummrich <dakr@...nel.org>, 
	Andrew Ballance <andrewjballance@...il.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>, 
	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 Mon, Mar 17, 2025 at 6:48 AM Alice Ryhl <aliceryhl@...gle.com> 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?

> >
> > >      ///
> > >      /// # 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());

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`.

> > > 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)?;

Indeed. Added to the commit message.