Re: [PATCH v3 06/10] rust: debugfs: support for binary large objects

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Oct 22, 2025 at 04:30:40PM +0200, Danilo Krummrich wrote:
> Introduce support for read-only, write-only, and read-write binary files
> in Rust debugfs. This adds:
> 
> - BinaryWriter and BinaryReader traits for writing to and reading from
>   user slices in binary form.
> - New Dir methods: read_binary_file(), write_binary_file(),
>   `read_write_binary_file`.
> - Corresponding FileOps implementations: BinaryReadFile,
>   BinaryWriteFile, BinaryReadWriteFile.
> 
> This allows kernel modules to expose arbitrary binary data through
> debugfs, with proper support for offsets and partial reads/writes.
> 
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Reviewed-by: Matthew Maurer <mmaurer@google.com>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

> +extern "C" fn blob_read<T: BinaryWriter>(
> +    file: *mut bindings::file,
> +    buf: *mut c_char,
> +    count: usize,
> +    ppos: *mut bindings::loff_t,
> +) -> isize {
> +    // SAFETY:
> +    // - `file` is a valid pointer to a `struct file`.
> +    // - The type invariant of `FileOps` guarantees that `private_data` points to a valid `T`.
> +    let this = unsafe { &*((*file).private_data.cast::<T>()) };
> +
> +    // SAFETY:
> +    // `ppos` is a valid `file::Offset` pointer.
> +    // We have exclusive access to `ppos`.
> +    let pos = unsafe { file::Offset::from_raw(ppos) };
> +
> +    let mut writer = UserSlice::new(UserPtr::from_ptr(buf.cast()), count).writer();
> +
> +    let ret = || -> Result<isize> {
> +        let written = this.write_to_slice(&mut writer, pos)?;
> +
> +        Ok(written.try_into()?)

Hmm ... a conversion? Sounds like write_to_slice() has the wrong return
type.

Alice