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

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Message-ID: <CAH5fLghiEqqccH-0S9-GD7pJaNuVpuo_NecMMmGVF+zR7Xs_dA@mail.gmail.com>
Date: Thu, 23 Oct 2025 12:21:24 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: gregkh@...uxfoundation.org, rafael@...nel.org, ojeda@...nel.org, 
	alex.gaynor@...il.com, boqun.feng@...il.com, gary@...yguo.net, 
	bjorn3_gh@...tonmail.com, lossin@...nel.org, a.hindborg@...nel.org, 
	tmgross@...ch.edu, mmaurer@...gle.com, rust-for-linux@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 06/10] rust: debugfs: support for binary large objects

On Thu, Oct 23, 2025 at 12:09 PM Danilo Krummrich <dakr@...nel.org> wrote:
>
> 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@...uxfoundation.org>
> >> Reviewed-by: Matthew Maurer <mmaurer@...gle.com>
> >> Signed-off-by: Danilo Krummrich <dakr@...nel.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?

Yes, you're right, I think usize is the right value. The cast is
unfortunate, but it can't really be avoided. In practice it should
never fail because slice lengths always fit in an isize, but isize
isn't the right type.

Alice