Re: [PATCH v2] rust: transmute: Add implementation for FromBytes trait

[Dirk Behme <dirk.behme@gmail.com>]

On 12.10.24 09:01, Christian dos Santos de Lima wrote:
> Add implementation and documentation for FromBytes trait.
> 
> Add new feature block in order to allow using ToBytes
> and bound to from_bytes_mut function. I'm adding this feature
> because is possible create a value with disallowed bit pattern
> and as_byte_mut could create such value by mutating the array and
> acessing the original value. So adding ToBytes this can be avoided.
> 
> Link: https://github.com/Rust-for-Linux/linux/issues/1119
> Signed-off-by: Christian dos Santos de Lima <christiansantoslima21@gmail.com>


Applying this on top of rust-next which has

https://github.com/Rust-for-Linux/linux/commit/ce1c54fdff7c4556b08f5b875a331d8952e8b6b7

I'm getting

error[E0405]: cannot find trait `AsBytes` in this scope
    --> rust/kernel/uaccess.rs:360:21
     |
360 |     pub fn write<T: AsBytes>(&mut self, value: &T) -> Result {
     |                     ^^^^^^^ not found in this scope

error: aborting due to 1 previous error

Additionally, could you review the two examples? They don't build. And 
they look identical? Is this intended?

And at several places there seem to miss a space after //.

./scripts/checkpatch.pl --codespell reports

WARNING: 'acessing' may be misspelled - perhaps 'accessing'?
#12:
acessing the original value. So adding ToBytes this can be avoided.
^^^^^^^^

WARNING: 'initiliazed' may be misspelled - perhaps 'initialized'?
#167: FILE: rust/kernel/transmute.rs:103:
+        //Safety: Guarantee that all values are initiliazed
                                                  ^^^^^^^^^^^

WARNING: 'initiliazed' may be misspelled - perhaps 'initialized'?
#179: FILE: rust/kernel/transmute.rs:111:
+    //Safety: Guarantee that all values are initiliazed
                                              ^^^^^^^^^^^

WARNING: 'initiliazed' may be misspelled - perhaps 'initialized'?
#184: FILE: rust/kernel/transmute.rs:116:
+        //Safety: Guarantee that all values are initiliazed
                                                  ^^^^^^^^^^^

Best regards

Dirk

> ---
> changes in v2:
>       - Rollback the implementation for the macro in the repository and add implementation of functions in trait
> ---
>   rust/kernel/lib.rs       |   2 +
>   rust/kernel/transmute.rs | 120 ++++++++++++++++++++++++++++-----------
>   2 files changed, 88 insertions(+), 34 deletions(-)
> 
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index dc37aef6a008..5215f5744e12 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -18,6 +18,8 @@
>   #![feature(lint_reasons)]
>   #![feature(new_uninit)]
>   #![feature(unsize)]
> +#![feature(portable_simd)]
> +#![feature(trivial_bounds)]
>   
>   // Ensure conditional compilation based on the kernel configuration works;
>   // otherwise we may silently break things like initcall handling.
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index 1c7d43771a37..bce42cc7265e 100644
> --- a/rust/kernel/transmute.rs
> +++ b/rust/kernel/transmute.rs
> @@ -2,6 +2,7 @@
>   
>   //! Traits for transmuting types.
>   
> +use core::simd::ToBytes;
>   /// Types for which any bit pattern is valid.
>   ///
>   /// Not all types are valid for all values. For example, a `bool` must be either zero or one, so
> @@ -9,15 +10,58 @@
>   ///
>   /// It's okay for the type to have padding, as initializing those bytes has no effect.
>   ///
> +/// # Example
> +///
> +/// This example is how to use the FromBytes trait
> +/// ```
> +/// // Initialize a slice of bytes
> +/// let foo = &[1, 2, 3, 4];
> +///
> +/// //Use the function implemented by trait in integer type
> +/// let result = u8::from_bytes(foo);
> +///
> +/// assert_eq!(*result, 0x4030201);
> +/// ```
>   /// # Safety
>   ///
>   /// All bit-patterns must be valid for this type. This type must not have interior mutability.
> -pub unsafe trait FromBytes {}
> +pub unsafe trait FromBytes {
> +    ///Get an imutable slice of bytes and converts to a reference to Self
> +    unsafe fn from_bytes(slice_of_bytes: &[u8]) -> &Self;
> +    /// Get a mutable slice of bytes and converts to a reference to Self
> +    ///
> +    /// # Safety
> +    ///
> +    ///  Bound ToBytes in order to avoid use with disallowed bit patterns
> +    unsafe fn from_bytes_mut(slice_of_bytes: &mut [u8]) -> &mut Self
> +    where
> +        Self: ToBytes;
> +}
>   
> +//Get a reference of slice of bytes and converts into a reference of integer or a slice with a defined size
>   macro_rules! impl_frombytes {
>       ($($({$($generics:tt)*})? $t:ty, )*) => {
>           // SAFETY: Safety comments written in the macro invocation.
> -        $(unsafe impl$($($generics)*)? FromBytes for $t {})*
> +        $(unsafe impl$($($generics)*)? FromBytes for $t {
> +            unsafe fn from_bytes(slice_of_bytes: &[u8]) -> &Self
> +            {
> +                unsafe {
> +                    let slice_ptr = slice_of_bytes.as_ptr() as *const Self;
> +                    &*slice_ptr
> +                }
> +            }
> +
> +            unsafe fn from_bytes_mut(slice_of_bytes: &mut [u8]) -> &mut Self
> +            where
> +                Self: ToBytes,
> +            {
> +                unsafe {
> +                    let slice_ptr = slice_of_bytes.as_mut_ptr() as *mut Self;
> +                    &mut *slice_ptr
> +                }
> +
> +            }
> +        })*
>       };
>   }
>   
> @@ -28,44 +72,52 @@ macro_rules! impl_frombytes {
>   
>       // SAFETY: If all bit patterns are acceptable for individual values in an array, then all bit
>       // patterns are also acceptable for arrays of that type.
> -    {<T: FromBytes>} [T],
>       {<T: FromBytes, const N: usize>} [T; N],
>   }
>   
> -/// Types that can be viewed as an immutable slice of initialized bytes.
> +/// Get a reference of slice of bytes and converts into a reference of an array of integers
>   ///
> -/// If a struct implements this trait, then it is okay to copy it byte-for-byte to userspace. This
> -/// means that it should not have any padding, as padding bytes are uninitialized. Reading
> -/// uninitialized memory is not just undefined behavior, it may even lead to leaking sensitive
> -/// information on the stack to userspace.
> +/// Types for which any bit pattern is valid.
>   ///
> -/// The struct should also not hold kernel pointers, as kernel pointer addresses are also considered
> -/// sensitive. However, leaking kernel pointers is not considered undefined behavior by Rust, so
> -/// this is a correctness requirement, but not a safety requirement.
> +/// Not all types are valid for all values. For example, a `bool` must be either zero or one, so
> +/// reading arbitrary bytes into something that contains a `bool` is not okay.
>   ///
> -/// # Safety
> +/// It's okay for the type to have padding, as initializing those bytes has no effect.
>   ///
> -/// Values of this type may not contain any uninitialized bytes. This type must not have interior
> -/// mutability.
> -pub unsafe trait AsBytes {}
> -
> -macro_rules! impl_asbytes {
> -    ($($({$($generics:tt)*})? $t:ty, )*) => {
> -        // SAFETY: Safety comments written in the macro invocation.
> -        $(unsafe impl$($($generics)*)? AsBytes for $t {})*
> -    };
> -}
> -
> -impl_asbytes! {
> -    // SAFETY: Instances of the following types have no uninitialized portions.
> -    u8, u16, u32, u64, usize,
> -    i8, i16, i32, i64, isize,
> -    bool,
> -    char,
> -    str,
> +/// # Example
> +///
> +/// This example is how to use the FromBytes trait
> +/// ```
> +/// // Initialize a slice of bytes
> +/// let foo = &[1, 2, 3, 4];
> +///
> +/// //Use the function implemented by trait in integer type
> +/// let result = <[u32]>::from_bytes(slice_of_bytes);
> +///
> +/// assert_eq!(*result, 0x4030201);
> +/// ```
> +// SAFETY: If all bit patterns are acceptable for individual values in an array, then all bit
> +// patterns are also acceptable for arrays of that type.
> +unsafe impl<T: FromBytes> FromBytes for [T] {
> +    unsafe fn from_bytes(slice_of_bytes: &[u8]) -> &Self {
> +        //Safety: Guarantee that all values are initiliazed
> +        unsafe {
> +            let slice_ptr = slice_of_bytes.as_ptr() as *const T;
> +            let slice_len = slice_of_bytes.len() / core::mem::size_of::<T>();
> +            core::slice::from_raw_parts(slice_ptr, slice_len)
> +        }
> +    }
>   
> -    // SAFETY: If individual values in an array have no uninitialized portions, then the array
> -    // itself does not have any uninitialized portions either.
> -    {<T: AsBytes>} [T],
> -    {<T: AsBytes, const N: usize>} [T; N],
> +    //Safety: Guarantee that all values are initiliazed
> +    unsafe fn from_bytes_mut(slice_of_bytes: &mut [u8]) -> &mut Self
> +    where
> +        Self: ToBytes,
> +    {
> +        //Safety: Guarantee that all values are initiliazed
> +        unsafe {
> +            let slice_ptr = slice_of_bytes.as_mut_ptr() as *mut T;
> +            let slice_len = slice_of_bytes.len() / core::mem::size_of::<T>();
> +            core::slice::from_raw_parts_mut(slice_ptr, slice_len)
> +        }
> +    }
>   }