Re: [PATCH v2] rust: place generated init_module() function in .init.text

[Greg KH <greg@kroah.com>]

On Mon, Feb 05, 2024 at 06:25:35PM -0700, Thomas Bertschinger wrote:
> Currently Rust kernel modules have their init code placed in the `.text`
> section of the .ko file. I don't think this causes any real problems
> for Rust modules but it does present a challenge with implementing Rust
> functionality for existing C modules.
> 
> If a Rust `init_module()` function (that lives in `.text`) calls a C
> function annotated with `__init`, then a warning is generated because
> the C function lives in `.init.text`.
> 
> I ran into this while experimenting with converting the bcachefs kernel
> module from C to Rust. The module's `init()`, written in Rust, calls C
> functions like `bch2_vfs_init()` which are placed in `.init.text`.
> 
> This patch places the macro-generated `init_module()` Rust function in
> the `.init.text` section. It also marks that function as unsafe--now it
> may not be called again after returning because it may be freed already.
> 
> Note that this is not enough on its own to actually get all the module
> initialization code in that section. The module author must still add
> the `#[link_section = ".init.text"]` attribute to the Rust `init()` in
> the `impl kernel::Module` block in order to then call C `__init`
> functions. However, this patch enables module authors do so, when
> previously it would not be possible (without warnings).
> 
> Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> ---
> V2: add "unsafe" to the init_module() function
> 
>  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 d62d8710d77a..e54293faf523 100644
> --- a/rust/macros/module.rs
> +++ b/rust/macros/module.rs
> @@ -222,10 +222,14 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
>              }};
>  
>              // Loadable modules need to export the `{{init,cleanup}}_module` identifiers.
> +            /// # Safety
> +            /// This function must only be called once, during module initialization, because it
> +            /// may be freed after it returns.

Oh, and the compiler will catch this if you get it wrong, correct?

If not, you all should work on that, as it is caught in C code :)

thanks,

greg k-h