Re: [RFC PATCH] rust: Provide correct provenance when constructing THIS_MODULE

[Gary Guo <gary@garyguo.net>]

On Wed, 28 Aug 2024 11:01:29 -0700
Boqun Feng <boqun.feng@gmail.com> wrote:

> Currently while defining `THIS_MODULE` symbol in `module!()`, the
> pointer used to contruct `ThisModule` is derived from an immutable
> reference of `__this_module`, which means the pointer doesn't have
> the provenance for writting, and that means any write to that pointer
> is UB regardless of data races or not. However, the usage of
> `THIS_MODULE` includes passing this pointer to functions that may write
> to it (probably in unsafe code), and this will create soundness issues.
> 
> One way to fix this is using `addr_of_mut!()` but that requires the
> unstable feature "const_mut_refs". So instead of `addr_of_mut()!`,
> an extern static `Opaque` is used here: since `Opaque<T>` is transparent
> to `T`, an extern static `Opaque` will just wrap the C symbol (defined
> in a C compile unit) in an `Opaque`, which provides a pointer with
> writable provenance via `Opaque::get()`. This fix the potential UBs
> because of pointer provenance unmatched.

`const_mut_refs` is getting stablised so we should be able to use it
soon. I am slightly inclined to use `addr_of_mut!()` over `Opaque` in
this case so we can use it directly from bindgen.

That said, the current approach also LGTM.

Reviewed-by: Gary Guo <gary@garyguo.net>

> 
> Reported-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
>  rust/macros/module.rs | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/macros/module.rs b/rust/macros/module.rs
> index 571ffa2e189c..aef3b132f32b 100644
> --- a/rust/macros/module.rs
> +++ b/rust/macros/module.rs
> @@ -217,7 +217,11 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
>              // freed until the module is unloaded.
>              #[cfg(MODULE)]
>              static THIS_MODULE: kernel::ThisModule = unsafe {{
> -                kernel::ThisModule::from_ptr(&kernel::bindings::__this_module as *const _ as *mut _)
> +                extern \"C\" {{
> +                    static __this_module: kernel::types::Opaque<kernel::bindings::module>;
> +                }}
> +
> +                kernel::ThisModule::from_ptr(__this_module.get())
>              }};
>              #[cfg(not(MODULE))]
>              static THIS_MODULE: kernel::ThisModule = unsafe {{