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[PATCH v3] rust: transmute: add `as_bytes` method for `AsBytes` trait

Alexandre Courbot posted 1 patch 2 months, 1 week ago 
rust/kernel/transmute.rs | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
[PATCH v3] rust: transmute: add `as_bytes` method for `AsBytes` trait
Posted by Alexandre Courbot 2 months, 1 week ago 
Every type that implements `AsBytes` should be able to provide its byte
representation. Introduce the `as_bytes` method that returns the
implementer as a stream of bytes, and provide a default implementation
that should be suitable for any type that satisfies `AsBytes`'s safety
requirements.

Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
This is the sister patch of [1], providing an `as_bytes` method for
`AsBytes`.

It is going to be used in Nova, but should also be universally useful -
if anything, it felt a bit strange that `AsBytes` did not provide this
method so far.

[1] https://lore.kernel.org/rust-for-linux/20250624042802.105623-1-christiansantoslima21@gmail.com/
---
Changes in v3:
- Use `ptr::from_ref` instead of `as *const T`.
- Link to v2: https://lore.kernel.org/r/20250725-as_bytes-v2-1-c6584c211a6c@nvidia.com

Changes in v2:
- Use `size_of_val` to provide a default implementation for both `Sized`
  and non-`Sized` types, and remove `AsBytesSized`. (thanks Alice!)
- Link to v1: https://lore.kernel.org/r/20250725-as_bytes-v1-1-6f06a3744f69@nvidia.com
---
 rust/kernel/transmute.rs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
index 1c7d43771a37b90150de86699f114a2ffb84db91..69c46c19a89191d8a2abc5801564cacda232218c 100644
--- a/rust/kernel/transmute.rs
+++ b/rust/kernel/transmute.rs
@@ -47,7 +47,16 @@ macro_rules! impl_frombytes {
 ///
 /// Values of this type may not contain any uninitialized bytes. This type must not have interior
 /// mutability.
-pub unsafe trait AsBytes {}
+pub unsafe trait AsBytes {
+    /// Returns `self` as a slice of bytes.
+    fn as_bytes(&self) -> &[u8] {
+        let data = core::ptr::from_ref(self).cast::<u8>();
+        let len = size_of_val(self);
+
+        // SAFETY: `data` is non-null and valid for `len * sizeof::<u8>()` bytes.
+        unsafe { core::slice::from_raw_parts(data, len) }
+    }
+}
 
 macro_rules! impl_asbytes {
     ($($({$($generics:tt)*})? $t:ty, )*) => {

---
base-commit: 14ae91a81ec8fa0bc23170d4aa16dd2a20d54105
change-id: 20250725-as_bytes-6cbc11f2e8c3

Best regards,
-- 
Alexandre Courbot <acourbot@nvidia.com>
Re: [PATCH v3] rust: transmute: add `as_bytes` method for `AsBytes` trait
Posted by Alice Ryhl 2 months, 1 week ago 
On Sat, Jul 26, 2025 at 4:47 AM Alexandre Courbot <acourbot@nvidia.com> wrote:
>
> Every type that implements `AsBytes` should be able to provide its byte
> representation. Introduce the `as_bytes` method that returns the
> implementer as a stream of bytes, and provide a default implementation
> that should be suitable for any type that satisfies `AsBytes`'s safety
> requirements.
>
> Reviewed-by: Danilo Krummrich <dakr@kernel.org>
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> ---
> This is the sister patch of [1], providing an `as_bytes` method for
> `AsBytes`.
>
> It is going to be used in Nova, but should also be universally useful -
> if anything, it felt a bit strange that `AsBytes` did not provide this
> method so far.
>
> [1] https://lore.kernel.org/rust-for-linux/20250624042802.105623-1-christiansantoslima21@gmail.com/
> ---
> Changes in v3:
> - Use `ptr::from_ref` instead of `as *const T`.
> - Link to v2: https://lore.kernel.org/r/20250725-as_bytes-v2-1-c6584c211a6c@nvidia.com
>
> Changes in v2:
> - Use `size_of_val` to provide a default implementation for both `Sized`
>   and non-`Sized` types, and remove `AsBytesSized`. (thanks Alice!)
> - Link to v1: https://lore.kernel.org/r/20250725-as_bytes-v1-1-6f06a3744f69@nvidia.com
> ---
>  rust/kernel/transmute.rs | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index 1c7d43771a37b90150de86699f114a2ffb84db91..69c46c19a89191d8a2abc5801564cacda232218c 100644
> --- a/rust/kernel/transmute.rs
> +++ b/rust/kernel/transmute.rs
> @@ -47,7 +47,16 @@ macro_rules! impl_frombytes {
>  ///
>  /// Values of this type may not contain any uninitialized bytes. This type must not have interior
>  /// mutability.
> -pub unsafe trait AsBytes {}
> +pub unsafe trait AsBytes {
> +    /// Returns `self` as a slice of bytes.
> +    fn as_bytes(&self) -> &[u8] {
> +        let data = core::ptr::from_ref(self).cast::<u8>();
> +        let len = size_of_val(self);
> +
> +        // SAFETY: `data` is non-null and valid for `len * sizeof::<u8>()` bytes.
> +        unsafe { core::slice::from_raw_parts(data, len) }
> +    }
> +}

Let's also have an as_bytes_mut() method. I would require the type to
also implement FromBytes as it lets you replace the value with another
set of bytes.

Alice
Re: [PATCH v3] rust: transmute: add `as_bytes` method for `AsBytes` trait
Posted by Alice Ryhl 2 months, 1 week ago 
On Sun, Jul 27, 2025 at 08:52:00AM +0200, Alice Ryhl wrote:
> On Sat, Jul 26, 2025 at 4:47 AM Alexandre Courbot <acourbot@nvidia.com> wrote:
> >
> > Every type that implements `AsBytes` should be able to provide its byte
> > representation. Introduce the `as_bytes` method that returns the
> > implementer as a stream of bytes, and provide a default implementation
> > that should be suitable for any type that satisfies `AsBytes`'s safety
> > requirements.
> >
> > Reviewed-by: Danilo Krummrich <dakr@kernel.org>
> > Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> > ---
> > This is the sister patch of [1], providing an `as_bytes` method for
> > `AsBytes`.
> >
> > It is going to be used in Nova, but should also be universally useful -
> > if anything, it felt a bit strange that `AsBytes` did not provide this
> > method so far.
> >
> > [1] https://lore.kernel.org/rust-for-linux/20250624042802.105623-1-christiansantoslima21@gmail.com/
> > ---
> > Changes in v3:
> > - Use `ptr::from_ref` instead of `as *const T`.
> > - Link to v2: https://lore.kernel.org/r/20250725-as_bytes-v2-1-c6584c211a6c@nvidia.com
> >
> > Changes in v2:
> > - Use `size_of_val` to provide a default implementation for both `Sized`
> >   and non-`Sized` types, and remove `AsBytesSized`. (thanks Alice!)
> > - Link to v1: https://lore.kernel.org/r/20250725-as_bytes-v1-1-6f06a3744f69@nvidia.com
> > ---
> >  rust/kernel/transmute.rs | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> > index 1c7d43771a37b90150de86699f114a2ffb84db91..69c46c19a89191d8a2abc5801564cacda232218c 100644
> > --- a/rust/kernel/transmute.rs
> > +++ b/rust/kernel/transmute.rs
> > @@ -47,7 +47,16 @@ macro_rules! impl_frombytes {
> >  ///
> >  /// Values of this type may not contain any uninitialized bytes. This type must not have interior
> >  /// mutability.
> > -pub unsafe trait AsBytes {}
> > +pub unsafe trait AsBytes {
> > +    /// Returns `self` as a slice of bytes.
> > +    fn as_bytes(&self) -> &[u8] {
> > +        let data = core::ptr::from_ref(self).cast::<u8>();
> > +        let len = size_of_val(self);
> > +
> > +        // SAFETY: `data` is non-null and valid for `len * sizeof::<u8>()` bytes.
> > +        unsafe { core::slice::from_raw_parts(data, len) }
> > +    }
> > +}
> 
> Let's also have an as_bytes_mut() method. I would require the type to
> also implement FromBytes as it lets you replace the value with another
> set of bytes.

s/I would/It would/

FromBytes is needed only for as_bytes_mut(), not for the existing
method.

Alice
Re: [PATCH v3] rust: transmute: add `as_bytes` method for `AsBytes` trait
Posted by Benno Lossin 2 months, 1 week ago 
On Sun Jul 27, 2025 at 12:23 PM CEST, Alice Ryhl wrote:
> On Sun, Jul 27, 2025 at 08:52:00AM +0200, Alice Ryhl wrote:
>> On Sat, Jul 26, 2025 at 4:47 AM Alexandre Courbot <acourbot@nvidia.com> wrote:
>> > diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
>> > index 1c7d43771a37b90150de86699f114a2ffb84db91..69c46c19a89191d8a2abc5801564cacda232218c 100644
>> > --- a/rust/kernel/transmute.rs
>> > +++ b/rust/kernel/transmute.rs
>> > @@ -47,7 +47,16 @@ macro_rules! impl_frombytes {
>> >  ///
>> >  /// Values of this type may not contain any uninitialized bytes. This type must not have interior
>> >  /// mutability.
>> > -pub unsafe trait AsBytes {}
>> > +pub unsafe trait AsBytes {
>> > +    /// Returns `self` as a slice of bytes.
>> > +    fn as_bytes(&self) -> &[u8] {
>> > +        let data = core::ptr::from_ref(self).cast::<u8>();
>> > +        let len = size_of_val(self);
>> > +
>> > +        // SAFETY: `data` is non-null and valid for `len * sizeof::<u8>()` bytes.
>> > +        unsafe { core::slice::from_raw_parts(data, len) }
>> > +    }
>> > +}
>> 
>> Let's also have an as_bytes_mut() method. I would require the type to
>> also implement FromBytes as it lets you replace the value with another
>> set of bytes.
>
> s/I would/It would/
>
> FromBytes is needed only for as_bytes_mut(), not for the existing
> method.

I agree with your suggestion, but it can be an independent patch and
doesn't need to go in via this one, right?

---
Cheers,
Benno
Re: [PATCH v3] rust: transmute: add `as_bytes` method for `AsBytes` trait
Posted by Alexandre Courbot 2 months, 1 week ago 
On Sun Jul 27, 2025 at 9:39 PM JST, Benno Lossin wrote:
> On Sun Jul 27, 2025 at 12:23 PM CEST, Alice Ryhl wrote:
>> On Sun, Jul 27, 2025 at 08:52:00AM +0200, Alice Ryhl wrote:
>>> On Sat, Jul 26, 2025 at 4:47 AM Alexandre Courbot <acourbot@nvidia.com> wrote:
>>> > diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
>>> > index 1c7d43771a37b90150de86699f114a2ffb84db91..69c46c19a89191d8a2abc5801564cacda232218c 100644
>>> > --- a/rust/kernel/transmute.rs
>>> > +++ b/rust/kernel/transmute.rs
>>> > @@ -47,7 +47,16 @@ macro_rules! impl_frombytes {
>>> >  ///
>>> >  /// Values of this type may not contain any uninitialized bytes. This type must not have interior
>>> >  /// mutability.
>>> > -pub unsafe trait AsBytes {}
>>> > +pub unsafe trait AsBytes {
>>> > +    /// Returns `self` as a slice of bytes.
>>> > +    fn as_bytes(&self) -> &[u8] {
>>> > +        let data = core::ptr::from_ref(self).cast::<u8>();
>>> > +        let len = size_of_val(self);
>>> > +
>>> > +        // SAFETY: `data` is non-null and valid for `len * sizeof::<u8>()` bytes.
>>> > +        unsafe { core::slice::from_raw_parts(data, len) }
>>> > +    }
>>> > +}
>>> 
>>> Let's also have an as_bytes_mut() method. I would require the type to
>>> also implement FromBytes as it lets you replace the value with another
>>> set of bytes.
>>
>> s/I would/It would/
>>
>> FromBytes is needed only for as_bytes_mut(), not for the existing
>> method.
>
> I agree with your suggestion, but it can be an independent patch and
> doesn't need to go in via this one, right?

Given where we are in the merge cycle, it seems like we have a couple of
months until that code gets merged anyway, so I don't see any reason to
not send a v4 with Alice's suggestion? The only drawback I see is that I
would have to reset the Reviewed-by tags.
Re: [PATCH v3] rust: transmute: add `as_bytes` method for `AsBytes` trait
Posted by Danilo Krummrich 2 months, 1 week ago 
On 7/28/25 5:39 AM, Alexandre Courbot wrote:
> On Sun Jul 27, 2025 at 9:39 PM JST, Benno Lossin wrote:
>> On Sun Jul 27, 2025 at 12:23 PM CEST, Alice Ryhl wrote:
>>> On Sun, Jul 27, 2025 at 08:52:00AM +0200, Alice Ryhl wrote:
>>>> On Sat, Jul 26, 2025 at 4:47 AM Alexandre Courbot <acourbot@nvidia.com> wrote:
>>>>> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
>>>>> index 1c7d43771a37b90150de86699f114a2ffb84db91..69c46c19a89191d8a2abc5801564cacda232218c 100644
>>>>> --- a/rust/kernel/transmute.rs
>>>>> +++ b/rust/kernel/transmute.rs
>>>>> @@ -47,7 +47,16 @@ macro_rules! impl_frombytes {
>>>>>   ///
>>>>>   /// Values of this type may not contain any uninitialized bytes. This type must not have interior
>>>>>   /// mutability.
>>>>> -pub unsafe trait AsBytes {}
>>>>> +pub unsafe trait AsBytes {
>>>>> +    /// Returns `self` as a slice of bytes.
>>>>> +    fn as_bytes(&self) -> &[u8] {
>>>>> +        let data = core::ptr::from_ref(self).cast::<u8>();
>>>>> +        let len = size_of_val(self);
>>>>> +
>>>>> +        // SAFETY: `data` is non-null and valid for `len * sizeof::<u8>()` bytes.
>>>>> +        unsafe { core::slice::from_raw_parts(data, len) }
>>>>> +    }
>>>>> +}
>>>>
>>>> Let's also have an as_bytes_mut() method. I would require the type to
>>>> also implement FromBytes as it lets you replace the value with another
>>>> set of bytes.
>>>
>>> s/I would/It would/
>>>
>>> FromBytes is needed only for as_bytes_mut(), not for the existing
>>> method.
>>
>> I agree with your suggestion, but it can be an independent patch and
>> doesn't need to go in via this one, right?
> 
> Given where we are in the merge cycle, it seems like we have a couple of
> months until that code gets merged anyway, so I don't see any reason to
> not send a v4 with Alice's suggestion? The only drawback I see is that I
> would have to reset the Reviewed-by tags.

I'd make it a series and add as_bytes_mut() as a separate patch.
Re: [PATCH v3] rust: transmute: add `as_bytes` method for `AsBytes` trait
Posted by Benno Lossin 2 months, 1 week ago 
On Mon Jul 28, 2025 at 5:39 AM CEST, Alexandre Courbot wrote:
> On Sun Jul 27, 2025 at 9:39 PM JST, Benno Lossin wrote:
>> On Sun Jul 27, 2025 at 12:23 PM CEST, Alice Ryhl wrote:
>>> On Sun, Jul 27, 2025 at 08:52:00AM +0200, Alice Ryhl wrote:
>>>> On Sat, Jul 26, 2025 at 4:47 AM Alexandre Courbot <acourbot@nvidia.com> wrote:
>>>> > diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
>>>> > index 1c7d43771a37b90150de86699f114a2ffb84db91..69c46c19a89191d8a2abc5801564cacda232218c 100644
>>>> > --- a/rust/kernel/transmute.rs
>>>> > +++ b/rust/kernel/transmute.rs
>>>> > @@ -47,7 +47,16 @@ macro_rules! impl_frombytes {
>>>> >  ///
>>>> >  /// Values of this type may not contain any uninitialized bytes. This type must not have interior
>>>> >  /// mutability.
>>>> > -pub unsafe trait AsBytes {}
>>>> > +pub unsafe trait AsBytes {
>>>> > +    /// Returns `self` as a slice of bytes.
>>>> > +    fn as_bytes(&self) -> &[u8] {
>>>> > +        let data = core::ptr::from_ref(self).cast::<u8>();
>>>> > +        let len = size_of_val(self);
>>>> > +
>>>> > +        // SAFETY: `data` is non-null and valid for `len * sizeof::<u8>()` bytes.
>>>> > +        unsafe { core::slice::from_raw_parts(data, len) }
>>>> > +    }
>>>> > +}
>>>> 
>>>> Let's also have an as_bytes_mut() method. I would require the type to
>>>> also implement FromBytes as it lets you replace the value with another
>>>> set of bytes.
>>>
>>> s/I would/It would/
>>>
>>> FromBytes is needed only for as_bytes_mut(), not for the existing
>>> method.
>>
>> I agree with your suggestion, but it can be an independent patch and
>> doesn't need to go in via this one, right?
>
> Given where we are in the merge cycle, it seems like we have a couple of
> months until that code gets merged anyway, so I don't see any reason to
> not send a v4 with Alice's suggestion? The only drawback I see is that I
> would have to reset the Reviewed-by tags.

Yeah sounds good.

---
Cheers,
Benno
Re: [PATCH v3] rust: transmute: add `as_bytes` method for `AsBytes` trait
Posted by Alice Ryhl 2 months, 1 week ago 
On Sun, Jul 27, 2025 at 2:39 PM Benno Lossin <lossin@kernel.org> wrote:
>
> On Sun Jul 27, 2025 at 12:23 PM CEST, Alice Ryhl wrote:
> > On Sun, Jul 27, 2025 at 08:52:00AM +0200, Alice Ryhl wrote:
> >> On Sat, Jul 26, 2025 at 4:47 AM Alexandre Courbot <acourbot@nvidia.com> wrote:
> >> > diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> >> > index 1c7d43771a37b90150de86699f114a2ffb84db91..69c46c19a89191d8a2abc5801564cacda232218c 100644
> >> > --- a/rust/kernel/transmute.rs
> >> > +++ b/rust/kernel/transmute.rs
> >> > @@ -47,7 +47,16 @@ macro_rules! impl_frombytes {
> >> >  ///
> >> >  /// Values of this type may not contain any uninitialized bytes. This type must not have interior
> >> >  /// mutability.
> >> > -pub unsafe trait AsBytes {}
> >> > +pub unsafe trait AsBytes {
> >> > +    /// Returns `self` as a slice of bytes.
> >> > +    fn as_bytes(&self) -> &[u8] {
> >> > +        let data = core::ptr::from_ref(self).cast::<u8>();
> >> > +        let len = size_of_val(self);
> >> > +
> >> > +        // SAFETY: `data` is non-null and valid for `len * sizeof::<u8>()` bytes.
> >> > +        unsafe { core::slice::from_raw_parts(data, len) }
> >> > +    }
> >> > +}
> >>
> >> Let's also have an as_bytes_mut() method. I would require the type to
> >> also implement FromBytes as it lets you replace the value with another
> >> set of bytes.
> >
> > s/I would/It would/
> >
> > FromBytes is needed only for as_bytes_mut(), not for the existing
> > method.
>
> I agree with your suggestion, but it can be an independent patch and
> doesn't need to go in via this one, right?

Sure, that's fine.

Alice
Re: [PATCH v3] rust: transmute: add `as_bytes` method for `AsBytes` trait
Posted by Benno Lossin 2 months, 1 week ago 
On Sat Jul 26, 2025 at 4:47 AM CEST, Alexandre Courbot wrote:
> Every type that implements `AsBytes` should be able to provide its byte
> representation. Introduce the `as_bytes` method that returns the
> implementer as a stream of bytes, and provide a default implementation
> that should be suitable for any type that satisfies `AsBytes`'s safety
> requirements.
>
> Reviewed-by: Danilo Krummrich <dakr@kernel.org>
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

Reviewed-by: Benno Lossin <lossin@kernel.org>

> ---
> This is the sister patch of [1], providing an `as_bytes` method for
> `AsBytes`.
>
> It is going to be used in Nova, but should also be universally useful -
> if anything, it felt a bit strange that `AsBytes` did not provide this
> method so far.
>
> [1] https://lore.kernel.org/rust-for-linux/20250624042802.105623-1-christiansantoslima21@gmail.com/
> ---
> Changes in v3:
> - Use `ptr::from_ref` instead of `as *const T`.
> - Link to v2: https://lore.kernel.org/r/20250725-as_bytes-v2-1-c6584c211a6c@nvidia.com
>
> Changes in v2:
> - Use `size_of_val` to provide a default implementation for both `Sized`
>   and non-`Sized` types, and remove `AsBytesSized`. (thanks Alice!)
> - Link to v1: https://lore.kernel.org/r/20250725-as_bytes-v1-1-6f06a3744f69@nvidia.com
> ---
>  rust/kernel/transmute.rs | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index 1c7d43771a37b90150de86699f114a2ffb84db91..69c46c19a89191d8a2abc5801564cacda232218c 100644
> --- a/rust/kernel/transmute.rs
> +++ b/rust/kernel/transmute.rs
> @@ -47,7 +47,16 @@ macro_rules! impl_frombytes {
>  ///
>  /// Values of this type may not contain any uninitialized bytes. This type must not have interior
>  /// mutability.
> -pub unsafe trait AsBytes {}
> +pub unsafe trait AsBytes {
> +    /// Returns `self` as a slice of bytes.
> +    fn as_bytes(&self) -> &[u8] {
> +        let data = core::ptr::from_ref(self).cast::<u8>();
> +        let len = size_of_val(self);
> +
> +        // SAFETY: `data` is non-null and valid for `len * sizeof::<u8>()` bytes.
> +        unsafe { core::slice::from_raw_parts(data, len) }
> +    }
> +}
>  
>  macro_rules! impl_asbytes {
>      ($($({$($generics:tt)*})? $t:ty, )*) => {
>
> ---
> base-commit: 14ae91a81ec8fa0bc23170d4aa16dd2a20d54105

Normally I'd expect this to be `rust-next` or an `-rc` tag... I did find
this in one of the trees that I fetch, so no worries, but maybe in the
future use one of those? Thanks!

---
Cheers,
Benno

> change-id: 20250725-as_bytes-6cbc11f2e8c3
>
> Best regards,

Patchew 2.1-devel-b67e4c2

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