Re: [PATCH v2] rust: macros: fix soundness issue in `module!` macro

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Apr 01, 2024 at 10:01:34PM +0000, Benno Lossin wrote:
> On 01.04.24 23:10, Boqun Feng wrote:
> > On Mon, Apr 01, 2024 at 06:52:50PM +0000, Benno Lossin wrote:
> > [...]
> >> +            // Double nested modules, since then nobody can access the public items inside.
> >> +            mod __module_init {{
> >> +                mod __module_init {{
> >> +                    use super::super::{type_};
> >> +
> >> +                    /// The \"Rust loadable module\" mark.
> >> +                    //
> >> +                    // This may be best done another way later on, e.g. as a new modinfo
> >> +                    // key or a new section. For the moment, keep it simple.
> >> +                    #[cfg(MODULE)]
> >> +                    #[doc(hidden)]
> >> +                    #[used]
> >> +                    static __IS_RUST_MODULE: () = ();
> >> +
> >> +                    static mut __MOD: Option<{type_}> = None;
> >> +
> >> +                    // SAFETY: `__this_module` is constructed by the kernel at load time and will not be
> >> +                    // 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 _)
> > 
> > While we're at it, probably we want the following as well? I.e. using
> > `Opaque` and extern block, because __this_module is certainly something
> > interior mutable and !Unpin.
> > 
> > diff --git a/rust/macros/module.rs b/rust/macros/module.rs
> > index 293beca0a583..8aa4eed6578c 100644
> > --- a/rust/macros/module.rs
> > +++ b/rust/macros/module.rs
> > @@ -219,7 +219,11 @@ mod __module_init {{
> >                       // 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 {{
> > 
> > Thoughts?
> 
> I am not sure we need it. Bindgen generates
> 
>      extern "C" {
>          pub static mut __this_module: module;
>      }
> 
> And the `mut` should take care of the "it might be modified by other
> threads".

Hmm.. but there could a C thread modifies some field of __this_module
while Rust code uses it, e.g. struct module has a list_head in it, which
could be used by C code to put another module next to it.

> The only thing that sticks out to me is the borrow, it should probably
> be using `addr_of_mut!` instead. Then we don't need to re-import it
> again manually.
> 
> I think it should be a separate patch though.
> 

Yes, agreed.

Regards,
Boqun

> -- 
> Cheers,
> Benno
> 
> > 
> > Note this requires `Opaque::get` to be `const`, which I will send out
> > shortly, I think it's a good change regardless of the usage here.
> > 
> > Regards,
> > Boqun
> > 
>