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

[Gary Guo <gary@garyguo.net>]

On Sat, 12 Oct 2024 04:01:21 -0300
Christian dos Santos de Lima <christiansantoslima21@gmail.com> 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>
> ---
> 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

missing space after `///`.

> +    unsafe fn from_bytes(slice_of_bytes: &[u8]) -> &Self;

There's no safety section. The only reason for this being unsafe is
length mismatch? I would rather have the impl perform length check and
return an Result type. If there's a case where length is known, people
can use `unwrap_unchecked` instead.

> +    /// 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

I don't think we want a manual implementation like this for each type
(even if it's abstracted behind macros). I'd suggestion take
inspiration from zerocopy's KnownLayout type:
https://docs.rs/zerocopy/latest/zerocopy/trait.KnownLayout.html

Best,
Gary

> +                }
> +            }
> +
> +            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)
> +        }
> +    }
>  }