linux-kernel - Re: [PATCH rust-next 2/2] rust: samples: miscdevice: add lseek samples

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Message-Id: <DC5W7UQHOJSG.310L4WJ0AKZFE@kernel.org>
Date: Tue, 19 Aug 2025 00:05:41 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Ryosuke Yasuoka" <ryasuoka@...hat.com>, <arnd@...db.de>,
 <gregkh@...uxfoundation.org>, <ojeda@...nel.org>, <alex.gaynor@...il.com>,
 <boqun.feng@...il.com>, <gary@...yguo.net>, <bjorn3_gh@...tonmail.com>,
 <a.hindborg@...nel.org>, <aliceryhl@...gle.com>, <tmgross@...ch.edu>,
 <dakr@...nel.org>, <lee@...nel.org>
Cc: <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH rust-next 2/2] rust: samples: miscdevice: add lseek
 samples

On Mon Aug 18, 2025 at 3:58 PM CEST, Ryosuke Yasuoka wrote:
> +    fn llseek(me: Pin<&RustMiscDevice>, file: &File, offset: i64, whence: i32) -> Result<isize> {
> +        dev_info!(me.dev, "LLSEEK Rust Misc Device Sample\n");
> +        let pos: i64;
> +        let eof: i64;
> +
> +        // SAFETY:
> +        // * The file is valid for the duration of this call.
> +        // * f_inode must be valid while the file is valid.
> +        unsafe {
> +            pos = (*file.as_ptr()).f_pos;
> +            eof = (*(*file.as_ptr()).f_inode).i_size;
> +        }

Please include abstractions for writing & reading the file position
instead of using `unsafe`.

---
Cheers,
Benno

> +
> +        let new_pos = match whence {
> +            SEEK_SET => offset,
> +            SEEK_CUR => pos + offset,
> +            SEEK_END => eof + offset,
> +            _ => {
> +                dev_err!(me.dev, "LLSEEK does not recognised: {}.\n", whence);
> +                return Err(EINVAL);
> +            }
> +        };
> +
> +        if new_pos < 0 {
> +            dev_err!(me.dev, "The file offset becomes negative: {}.\n", new_pos);
> +            return Err(EINVAL);
> +        }
> +
> +        // SAFETY: The file is valid for the duration of this call.
> +        let ret: isize = unsafe {
> +            (*file.as_ptr()).f_pos = new_pos;
> +            new_pos as isize
> +        };
> +
> +        Ok(ret)
> +    }
> +
>      fn ioctl(me: Pin<&RustMiscDevice>, _file: &File, cmd: u32, arg: usize) -> Result<isize> {
>          dev_info!(me.dev, "IOCTLing Rust Misc Device Sample\n");
>