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

[Boqun Feng]

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?

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

> +                    }};
> +                    #[cfg(not(MODULE))]
> +                    static THIS_MODULE: kernel::ThisModule = unsafe {{
> +                        kernel::ThisModule::from_ptr(core::ptr::null_mut())
> +                    }};
> +
> +                    // Loadable modules need to export the `{{init,cleanup}}_module` identifiers.
> +                    /// # Safety
> +                    ///
> +                    /// This function must not be called after module initialization, because it may be
> +                    /// freed after that completes.
> +                    #[cfg(MODULE)]
> +                    #[doc(hidden)]
> +                    #[no_mangle]
> +                    #[link_section = \".init.text\"]
> +                    pub unsafe extern \"C\" fn init_module() -> core::ffi::c_int {{
> +                        // SAFETY: this function is inaccessible to the outside due to the double
> +                        // module wrapping it. It is called exactly once by the C side via its
> +                        // unique name.
> +                        unsafe {{ __init() }}
> +                    }}
>  
> -            #[cfg(MODULE)]
> -            #[doc(hidden)]
> -            #[no_mangle]
> -            pub extern \"C\" fn cleanup_module() {{
> -                __exit()
> -            }}
> +                    #[cfg(MODULE)]
> +                    #[doc(hidden)]
> +                    #[no_mangle]
> +                    pub extern \"C\" fn cleanup_module() {{
> +                        // SAFETY:
> +                        // - this function is inaccessible to the outside due to the double
> +                        //   module wrapping it. It is called exactly once by the C side via its
> +                        //   unique name,
> +                        // - furthermore it is only called after `init_module` has returned `0`
> +                        //   (which delegates to `__init`).
> +                        unsafe {{ __exit() }}
> +                    }}
>  
> -            // Built-in modules are initialized through an initcall pointer
> -            // and the identifiers need to be unique.
> -            #[cfg(not(MODULE))]
> -            #[cfg(not(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS))]
> -            #[doc(hidden)]
> -            #[link_section = \"{initcall_section}\"]
> -            #[used]
> -            pub static __{name}_initcall: extern \"C\" fn() -> core::ffi::c_int = __{name}_init;
> -
> -            #[cfg(not(MODULE))]
> -            #[cfg(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS)]
> -            core::arch::global_asm!(
> -                r#\".section \"{initcall_section}\", \"a\"
> -                __{name}_initcall:
> -                    .long   __{name}_init - .
> -                    .previous
> -                \"#
> -            );
> +                    // Built-in modules are initialized through an initcall pointer
> +                    // and the identifiers need to be unique.
> +                    #[cfg(not(MODULE))]
> +                    #[cfg(not(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS))]
> +                    #[doc(hidden)]
> +                    #[link_section = \"{initcall_section}\"]
> +                    #[used]
> +                    pub static __{name}_initcall: extern \"C\" fn() -> core::ffi::c_int = __{name}_init;
> +
> +                    #[cfg(not(MODULE))]
> +                    #[cfg(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS)]
> +                    core::arch::global_asm!(
> +                        r#\".section \"{initcall_section}\", \"a\"
> +                        __{name}_initcall:
> +                            .long   __{name}_init - .
> +                            .previous
> +                        \"#
> +                    );
> +
> +                    #[cfg(not(MODULE))]
> +                    #[doc(hidden)]
> +                    #[no_mangle]
> +                    pub extern \"C\" fn __{name}_init() -> core::ffi::c_int {{
> +                        // SAFETY: this function is inaccessible to the outside due to the double
> +                        // module wrapping it. It is called exactly once by the C side via its
> +                        // placement above in the initcall section.
> +                        unsafe {{ __init() }}
> +                    }}
>  
> -            #[cfg(not(MODULE))]
> -            #[doc(hidden)]
> -            #[no_mangle]
> -            pub extern \"C\" fn __{name}_init() -> core::ffi::c_int {{
> -                __init()
> -            }}
> +                    #[cfg(not(MODULE))]
> +                    #[doc(hidden)]
> +                    #[no_mangle]
> +                    pub extern \"C\" fn __{name}_exit() {{
> +                        // SAFETY:
> +                        // - this function is inaccessible to the outside due to the double
> +                        //   module wrapping it. It is called exactly once by the C side via its
> +                        //   unique name,
> +                        // - furthermore it is only called after `__{name}_init` has returned `0`
> +                        //   (which delegates to `__init`).
> +                        unsafe {{ __exit() }}
> +                    }}
>  
> -            #[cfg(not(MODULE))]
> -            #[doc(hidden)]
> -            #[no_mangle]
> -            pub extern \"C\" fn __{name}_exit() {{
> -                __exit()
> -            }}
> +                    /// # Safety
> +                    ///
> +                    /// This function must only be called once.
> +                    unsafe fn __init() -> core::ffi::c_int {{
> +                        match <{type_} as kernel::Module>::init(&THIS_MODULE) {{
> +                            Ok(m) => {{
> +                                // SAFETY:
> +                                // no data race, since `__MOD` can only be accessed by this module and
> +                                // there only `__init` and `__exit` access it. These functions are only
> +                                // called once and `__exit` cannot be called before or during `__init`.
> +                                unsafe {{
> +                                    __MOD = Some(m);
> +                                }}
> +                                return 0;
> +                            }}
> +                            Err(e) => {{
> +                                return e.to_errno();
> +                            }}
> +                        }}
> +                    }}
>  
> -            fn __init() -> core::ffi::c_int {{
> -                match <{type_} as kernel::Module>::init(&THIS_MODULE) {{
> -                    Ok(m) => {{
> +                    /// # Safety
> +                    ///
> +                    /// This function must
> +                    /// - only be called once,
> +                    /// - be called after `__init` has been called and returned `0`.
> +                    unsafe fn __exit() {{
> +                        // SAFETY:
> +                        // no data race, since `__MOD` can only be accessed by this module and there
> +                        // only `__init` and `__exit` access it. These functions are only called once
> +                        // and `__init` was already called.
>                          unsafe {{
> -                            __MOD = Some(m);
> +                            // Invokes `drop()` on `__MOD`, which should be used for cleanup.
> +                            __MOD = None;
>                          }}
> -                        return 0;
>                      }}
> -                    Err(e) => {{
> -                        return e.to_errno();
> -                    }}
> -                }}
> -            }}
>  
> -            fn __exit() {{
> -                unsafe {{
> -                    // Invokes `drop()` on `__MOD`, which should be used for cleanup.
> -                    __MOD = None;
> +                    {modinfo}
>                  }}
>              }}
> -
> -            {modinfo}
>          ",
>          type_ = info.type_,
>          name = info.name,
> 
> base-commit: 9ffe2a730313f27cebd0859ea856247ac59c576c
> -- 
> 2.44.0
> 
>