[v1] rust: alloc: add Vec::resize and Vec::truncate

[PATCH 1/3] rust: alloc: add Vec::truncate method

Posted by Andrew Ballance 9 months, 1 week ago

implements the equivalent to the std's Vec::truncate
on the kernel's Vec type.

Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
---
 rust/kernel/alloc/kvec.rs | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index ae9d072741ce..75e9feebb81f 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -452,6 +452,42 @@ pub fn reserve(&mut self, additional: usize, flags: Flags) -> Result<(), AllocEr
 
         Ok(())
     }
+
+    /// Shortens the vector, setting the length to `len` and drops the removed values.
+    /// If `len` is greater than or equal to the current length, this does nothing.
+    ///
+    /// This has no effect on the capacity and will not allocate.
+    /// # Examples
+    /// ```
+    /// let mut v = kernel::kvec![1, 2, 3]?;
+    /// v.truncate(1);
+    /// assert_eq!(v.len(), 1);
+    /// assert_eq!(&v, &[1]);
+    ///
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn truncate(&mut self, len: usize) {
+        if len >= self.len() {
+            return;
+        }
+
+        // [new_len, len) is guaranteed to be valid because [0, len) is guaranteed to be valid
+        let drop_range = len..self.len();
+
+        // SAFETY:
+        // we can safely ignore the bounds check because we already did our own check
+        let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };
+
+        // SAFETY:
+        // it is safe to shrink the length because the new length is
+        // guaranteed to be less than the old length
+        unsafe { self.set_len(len) };
+
+        // SAFETY:
+        // - the dropped values are valid `T`s
+        // - we are allowed to invalidate [new_len, old_len) because we just changed the len
+        unsafe { ptr::drop_in_place(ptr) };
+    }
 }
 
 impl<T: Clone, A: Allocator> Vec<T, A> {
-- 
2.48.1

Re: [PATCH 1/3] rust: alloc: add Vec::truncate method

Posted by Danilo Krummrich 9 months, 1 week ago

On Fri, Mar 14, 2025 at 09:42:33PM -0500, Andrew Ballance wrote:
> implements the equivalent to the std's Vec::truncate
> on the kernel's Vec type.
> 
> Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
> ---
>  rust/kernel/alloc/kvec.rs | 36 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 36 insertions(+)
> 
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index ae9d072741ce..75e9feebb81f 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -452,6 +452,42 @@ pub fn reserve(&mut self, additional: usize, flags: Flags) -> Result<(), AllocEr
>  
>          Ok(())
>      }
> +
> +    /// Shortens the vector, setting the length to `len` and drops the removed values.
> +    /// If `len` is greater than or equal to the current length, this does nothing.
> +    ///
> +    /// This has no effect on the capacity and will not allocate.

Nit: Please also add an empty line here.

> +    /// # Examples
> +    /// ```
> +    /// let mut v = kernel::kvec![1, 2, 3]?;
> +    /// v.truncate(1);
> +    /// assert_eq!(v.len(), 1);
> +    /// assert_eq!(&v, &[1]);
> +    ///
> +    /// # Ok::<(), Error>(())
> +    /// ```
> +    pub fn truncate(&mut self, len: usize) {
> +        if len >= self.len() {
> +            return;
> +        }
> +
> +        // [new_len, len) is guaranteed to be valid because [0, len) is guaranteed to be valid

We typically use markdown for comments.

> +        let drop_range = len..self.len();
> +
> +        // SAFETY:
> +        // we can safely ignore the bounds check because we already did our own check
> +        let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };
> +
> +        // SAFETY:
> +        // it is safe to shrink the length because the new length is
> +        // guaranteed to be less than the old length
> +        unsafe { self.set_len(len) };

I just sent out a fix [1] for the safety requirements of set_len() in [1], which
I think would be good to consider.

[1] https://lore.kernel.org/rust-for-linux/20250315154436.65065-1-dakr@kernel.org/

Re: [PATCH 1/3] rust: alloc: add Vec::truncate method

Posted by Benno Lossin 9 months, 1 week ago

On Sat Mar 15, 2025 at 3:42 AM CET, Andrew Ballance wrote:
> implements the equivalent to the std's Vec::truncate
> on the kernel's Vec type.
>
> Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
> ---
>  rust/kernel/alloc/kvec.rs | 36 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 36 insertions(+)
>
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index ae9d072741ce..75e9feebb81f 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -452,6 +452,42 @@ pub fn reserve(&mut self, additional: usize, flags: Flags) -> Result<(), AllocEr
>  
>          Ok(())
>      }
> +
> +    /// Shortens the vector, setting the length to `len` and drops the removed values.
> +    /// If `len` is greater than or equal to the current length, this does nothing.
> +    ///
> +    /// This has no effect on the capacity and will not allocate.
> +    /// # Examples
> +    /// ```
> +    /// let mut v = kernel::kvec![1, 2, 3]?;
> +    /// v.truncate(1);
> +    /// assert_eq!(v.len(), 1);
> +    /// assert_eq!(&v, &[1]);
> +    ///
> +    /// # Ok::<(), Error>(())
> +    /// ```
> +    pub fn truncate(&mut self, len: usize) {
> +        if len >= self.len() {
> +            return;
> +        }
> +
> +        // [new_len, len) is guaranteed to be valid because [0, len) is guaranteed to be valid
> +        let drop_range = len..self.len();
> +
> +        // SAFETY:
> +        // we can safely ignore the bounds check because we already did our own check
> +        let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };

What's this `get_unchecked_mut` method, I don't see it in `rust-next` or
`alloc-next`.

> +
> +        // SAFETY:
> +        // it is safe to shrink the length because the new length is
> +        // guaranteed to be less than the old length

Please take a look at the documentation of `set_len`, in the safety
section you'll find what you need to justify here.

> +        unsafe { self.set_len(len) };
> +
> +        // SAFETY:

A couple points missing:
- why is the pointer valid?

> +        // - the dropped values are valid `T`s
> +        // - we are allowed to invalidate [new_len, old_len) because we just changed the len

This should justify why the value will never be accessed again.

---
Cheers,
Benno

> +        unsafe { ptr::drop_in_place(ptr) };
> +    }
>  }
>  
>  impl<T: Clone, A: Allocator> Vec<T, A> {

Re: [PATCH 1/3] rust: alloc: add Vec::truncate method

Posted by Andrew Ballance 9 months, 1 week ago

On Sat, Mar 15, 2025 at 10:09:26AM +0000, Benno Lossin wrote:
> On Sat Mar 15, 2025 at 3:42 AM CET, Andrew Ballance wrote:
> > implements the equivalent to the std's Vec::truncate
> > on the kernel's Vec type.
> >
> > Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
> > ---
> >  rust/kernel/alloc/kvec.rs | 36 ++++++++++++++++++++++++++++++++++++
> >  1 file changed, 36 insertions(+)
> >
> > diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> > index ae9d072741ce..75e9feebb81f 100644
> > --- a/rust/kernel/alloc/kvec.rs
> > +++ b/rust/kernel/alloc/kvec.rs
> > @@ -452,6 +452,42 @@ pub fn reserve(&mut self, additional: usize, flags: Flags) -> Result<(), AllocEr
> >  
> >          Ok(())
> >      }
> > +
> > +    /// Shortens the vector, setting the length to `len` and drops the removed values.
> > +    /// If `len` is greater than or equal to the current length, this does nothing.
> > +    ///
> > +    /// This has no effect on the capacity and will not allocate.
> > +    /// # Examples
> > +    /// ```
> > +    /// let mut v = kernel::kvec![1, 2, 3]?;
> > +    /// v.truncate(1);
> > +    /// assert_eq!(v.len(), 1);
> > +    /// assert_eq!(&v, &[1]);
> > +    ///
> > +    /// # Ok::<(), Error>(())
> > +    /// ```
> > +    pub fn truncate(&mut self, len: usize) {
> > +        if len >= self.len() {
> > +            return;
> > +        }
> > +
> > +        // [new_len, len) is guaranteed to be valid because [0, len) is guaranteed to be valid
> > +        let drop_range = len..self.len();
> > +
> > +        // SAFETY:
> > +        // we can safely ignore the bounds check because we already did our own check
> > +        let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };
> 
> What's this `get_unchecked_mut` method, I don't see it in `rust-next` or
> `alloc-next`.

Vec derefs into a slice which implements get_uncheked_mut
https://rust.docs.kernel.org/next/kernel/alloc/kvec/struct.Vec.html#method.get_unchecked_mut

> > +
> > +        // SAFETY:
> > +        // it is safe to shrink the length because the new length is
> > +        // guaranteed to be less than the old length
> 
> Please take a look at the documentation of `set_len`, in the safety
> section you'll find what you need to justify here.
> 
> > +        unsafe { self.set_len(len) };
> > +
> > +        // SAFETY:
> 
> A couple points missing:
> - why is the pointer valid?
> 
> > +        // - the dropped values are valid `T`s
> > +        // - we are allowed to invalidate [new_len, old_len) because we just changed the len
> 
> This should justify why the value will never be accessed again.
> 

I will fixup the safety comments for the v2. Thanks.

> ---
> Cheers,
> Benno
> 
> > +        unsafe { ptr::drop_in_place(ptr) };
> > +    }
> >  }
> >  
> >  impl<T: Clone, A: Allocator> Vec<T, A> {
> 
>

Re: [PATCH 1/3] rust: alloc: add Vec::truncate method

Posted by Benno Lossin 9 months, 1 week ago

On Sat Mar 15, 2025 at 12:15 PM CET, Andrew Ballance wrote:
> On Sat, Mar 15, 2025 at 10:09:26AM +0000, Benno Lossin wrote:
>> On Sat Mar 15, 2025 at 3:42 AM CET, Andrew Ballance wrote:
>> > implements the equivalent to the std's Vec::truncate
>> > on the kernel's Vec type.
>> >
>> > Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
>> > ---
>> >  rust/kernel/alloc/kvec.rs | 36 ++++++++++++++++++++++++++++++++++++
>> >  1 file changed, 36 insertions(+)
>> >
>> > diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
>> > index ae9d072741ce..75e9feebb81f 100644
>> > --- a/rust/kernel/alloc/kvec.rs
>> > +++ b/rust/kernel/alloc/kvec.rs
>> > @@ -452,6 +452,42 @@ pub fn reserve(&mut self, additional: usize, flags: Flags) -> Result<(), AllocEr
>> >  
>> >          Ok(())
>> >      }
>> > +
>> > +    /// Shortens the vector, setting the length to `len` and drops the removed values.
>> > +    /// If `len` is greater than or equal to the current length, this does nothing.
>> > +    ///
>> > +    /// This has no effect on the capacity and will not allocate.
>> > +    /// # Examples
>> > +    /// ```
>> > +    /// let mut v = kernel::kvec![1, 2, 3]?;
>> > +    /// v.truncate(1);
>> > +    /// assert_eq!(v.len(), 1);
>> > +    /// assert_eq!(&v, &[1]);
>> > +    ///
>> > +    /// # Ok::<(), Error>(())
>> > +    /// ```
>> > +    pub fn truncate(&mut self, len: usize) {
>> > +        if len >= self.len() {
>> > +            return;
>> > +        }
>> > +
>> > +        // [new_len, len) is guaranteed to be valid because [0, len) is guaranteed to be valid
>> > +        let drop_range = len..self.len();
>> > +
>> > +        // SAFETY:
>> > +        // we can safely ignore the bounds check because we already did our own check
>> > +        let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };
>> 
>> What's this `get_unchecked_mut` method, I don't see it in `rust-next` or
>> `alloc-next`.
>
> Vec derefs into a slice which implements get_uncheked_mut
> https://rust.docs.kernel.org/next/kernel/alloc/kvec/struct.Vec.html#method.get_unchecked_mut

Ah, I forgot about that... Can you change the safety comment to:

    // SAFETY: `drop_range` is a subrange of `[0, len)` by the bounds check above.

---
Cheers,
Benno