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

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Message-ID: <20240926220821.GP3550746@ZenIV>
Date: Thu, 26 Sep 2024 23:08:21 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Alice Ryhl <aliceryhl@...gle.com>
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 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?