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

[Boqun Feng]

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.

Reported-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
 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 {{
-- 
2.45.2