linux-kernel - Re: [PATCH 2/3] rust: file: add f_pos and set_f

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Message-ID: <CAH5fLggMgmg-3AH1JcA6hrNLvEoM8f8a=c99v7gSa+4q3diWjA@mail.gmail.com>
Date: Fri, 27 Sep 2024 08:48:41 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Arnd Bergmann <arnd@...db.de>, 
	Miguel Ojeda <ojeda@...nel.org>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, rust-for-linux@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] rust: file: add f_pos and set_f_pos

On Fri, Sep 27, 2024 at 12:08 AM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Thu, Sep 26, 2024 at 02:58:56PM +0000, Alice Ryhl wrote:
> > Add accessors for the file position. Most of the time, you should not
> > use these methods directly, and you should instead use a guard for the
> > file position to prove that you hold the fpos lock. However, under
> > limited circumstances, files are allowed to choose a different locking
> > strategy for their file position. These accessors can be used to handle
> > that case.
> >
> > For now, these accessors are the only way to access the file position
> > within the llseek and read_iter callbacks.
>
> You really should not do that within ->read_iter().  If your method
> does that, it has the wrong signature.
>
> If nothing else, it should be usable for preadv(2), so what file position
> are you talking about?

Sorry, you are right. In read_iter we can access the file position via
the kiocb. It only makes sense for seek.

Alice