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

["Danilo Krummrich" <dakr@kernel.org>]

On Thu Oct 23, 2025 at 10:26 AM CEST, Alice Ryhl wrote:
> 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.

write_to_slice() returns the number of bytes written as usize, which seems
correct, no?