linux-kernel - Re: [PATCH v11 2/3] rust: add parameter support to the `module!` macro

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Message-ID: <aBnQpWJeiRKIMJ5D@google.com>
Date: Tue, 6 May 2025 09:04:37 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: 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>, 
	Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Luis Chamberlain <mcgrof@...nel.org>, Danilo Krummrich <dakr@...nel.org>, 
	Nicolas Schier <nicolas.schier@...ux.dev>, Trevor Gross <tmgross@...ch.edu>, 
	Adam Bratschi-Kaye <ark.email@...il.com>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org, 
	Petr Pavlu <petr.pavlu@...e.com>, Sami Tolvanen <samitolvanen@...gle.com>, 
	Daniel Gomez <da.gomez@...sung.com>, Simona Vetter <simona.vetter@...ll.ch>, 
	Greg KH <gregkh@...uxfoundation.org>, Fiona Behrens <me@...enk.dev>, 
	Daniel Almeida <daniel.almeida@...labora.com>, linux-modules@...r.kernel.org
Subject: Re: [PATCH v11 2/3] rust: add parameter support to the `module!` macro

On Mon, May 05, 2025 at 11:55:33AM +0200, Andreas Hindborg wrote:
> "Alice Ryhl" <aliceryhl@...gle.com> writes:
> 
> > On Fri, May 02, 2025 at 02:16:35PM +0200, Andreas Hindborg wrote:
> > It would be a use-after-free to
> > access it during module teardown. For example, what if I access this
> > static during its own destructor? Or during the destructor of another
> > module parameter?
> 
> Yes, that is a problem.
> 
> We can get around it for now by just not calling `free` for now. We only
> support simple types that do not need drop. I think we would have to
> seal the `ModuleParam` trait for this.
> 
> For a proper solution, we could
>  - Require a token to read the parameter.
>  - Synchronize on a module private field and return an option from the
>    parameter getter. This would require module exit to run before param
>    free. I think this is the case, but I did not check.
>  - Use a `Revocable` and revoke the parameter in `free`.
> 
> Any other ideas or comments on the outlined solutions?

I think the simplest you can do right now is

trait ModuleParam: Copy

so that it can't contain any non-trivial values. That way you don't need
Drop either.

Long term, I think we need a way to detect whether it's safe to access
module globals. The exact same problem applies to the existing global
for the module itself - except it's worse there because we can't access
that one during init either.

Alice