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Date: Tue, 29 Sep 2026 18:02:18 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Gary Guo <gary@garyguo.net>
Cc: Christian Brauner <brauner@kernel.org>,
	Alice Ryhl <aliceryhl@google.com>,
	Georgios Androutsopoulos <georgeandrout13@gmail.com>,
	Miguel Ojeda <ojeda@kernel.org>, Jan Kara <jack@suse.cz>,
	Boqun Feng <boqun@kernel.org>,
	=?iso-8859-1?Q?Bj=F6rn?= Roy Baron <bjorn3_gh@protonmail.com>,
	Benno Lossin <lossin@kernel.org>,
	Andreas Hindborg <a.hindborg@kernel.org>,
	Trevor Gross <tmgross@umich.edu>,
	Danilo Krummrich <dakr@kernel.org>,
	Daniel Almeida <daniel.almeida@collabora.com>,
	Tamir Duberstein <tamird@kernel.org>,
	Alexandre Courbot <acourbot@nvidia.com>,
	Onur =?iso-8859-1?Q?=D6zkan?= <work@onurozkan.dev>,
	linux-fsdevel@vger.kernel.org, rust-for-linux@vger.kernel.org,
	linux-kernel@vger.kernel.org, Al Viro <viro@ftp.linux.org.uk>
Subject: Re: [PATCH v2] rust: file: handle fd table teardown in file
 descriptor APIs
Message-ID: <20260929170218.GD989762@ZenIV>
References: <20260923022339.3340694-1-georgeandrout13@gmail.com>
 <arThye_219ruNbM2@google.com>
 <20260925-stellen-brummen-festrede-266af0305ac7@brauner>
 <20260929044843.GA3909609@ZenIV>
 <DLRSQRUVOGT2.10ZY01VEJVHCE@garyguo.net>
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Sender: Al Viro <viro@ftp.linux.org.uk>

On Tue, Sep 29, 2026 at 05:07:40PM +0100, Gary Guo wrote:

> The reproducer that Georgios posted on GitHub is some cleanup job being added to
> task_work, which drops FileDescriptorReservation. And since exit_task_work()
> happens after exit_files(), put_unused_fd in that cleanup observe that
> current->files is NULL.
>
> So it's not from random thread, it's from the current task. And I find that
> particular case of doing per-task cleanup not unrealistic.

FWIW, descriptor reservation ought to be tied to specific files_struct
instance; note that dup_fd() can be called when there are outstanding
reservations and the copy does *NOT* have those reserved.

What rules would you suggest for such delayed put_unused_fd() wrt e.g.
files_struct unsharing?