linux-kernel - Re: [PATCH v5 4/4] rust: samples: Add debugfs sample

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Message-Id: <DA2Z09MJZSKV.35D1CB1MQ4EJD@kernel.org>
Date: Thu, 22 May 2025 22:26:54 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>, "Greg Kroah-Hartman"
 <gregkh@...uxfoundation.org>
Cc: "Danilo Krummrich" <dakr@...nel.org>, "Matthew Maurer"
 <mmaurer@...gle.com>, "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor"
 <alex.gaynor@...il.com>, "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>, "Rafael J. Wysocki" <rafael@...nel.org>, "Sami
 Tolvanen" <samitolvanen@...gle.com>, "Timur Tabi" <ttabi@...dia.com>,
 <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v5 4/4] rust: samples: Add debugfs sample

On Thu May 22, 2025 at 7:40 PM CEST, Alice Ryhl wrote:
> On Thu, May 22, 2025 at 04:15:46PM +0200, Greg Kroah-Hartman wrote:
>> No, sorry, but debugfs files are "create and forget" type of things.
>> The caller has NO reference back to the file at all in the C version,
>> let's not add that functionality back to the rust side after I spent a
>> long time removing it from the C code :)
>> 
>> If you really want to delete a debugfs file that you have created in the
>> past, then look it up and delete it with the call that is present for
>> that.
>> 
>> The only thing I think that might be worth "keeping" in some form, as an
>> object reference as discussed, is a debugfs directory.
>
> That could work if we don't have any Rust value for files at all. The
> problem is that if we do have such values, then code like this:
>
> let my_file = dir.create_file("my_file_name");
> dir.delete_file("my_file_name");
> my_file.do_something();

I might have misunderstood something, but "deleting a debugfs file" is
not the same as freeing the representing object (is that a dentry?). So
you could still call `do_something`, it just wouldn't do anything.

Or did I misunderstand?

---
Cheers,
Benno

> would be a UAF on the last line. We have to design the Rust API to avoid
> such UAF, which is why I suggested the ghost objects; the delete_file()
> call leaves my_file in a valid but useless state. And as a ghost object,
> the .do_something() call becomes a no-op since the file is now missing
> from the filesystem.
>
> Alice