rust: module: place generated init_module() function in .init.text

commit 1b6170ff7a203a5e8354f19b7839fe8b897a9c0d upstream.

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 as long as all code called during initialization lives
in `.text`.

However, if a Rust `init_module()` function (that lives in `.text`)
calls a function marked with `__init` (in C) or
`#[link_section = ".init.text"]` (in Rust), then a warning is
generated by modpost because that function lives in `.init.text`.
For example:

WARNING: modpost: fs/bcachefs/bcachefs: section mismatch in reference: init_module+0x6 (section: .text) -> _RNvXCsj7d3tFpT5JS_15bcachefs_moduleNtB2_8BcachefsNtCsjDtqRIL3JAG_6kernel6Module4init (section: .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 `init_module()` as unsafe--now
it may not be called after module initialization completes 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 `__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>
Link: https://lore.kernel.org/r/20240206153806.567055-1-tahbertschinger@gmail.com
[ Reworded title to add prefix. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 6 insertions, 1 deletions

diff --git a/rust/macros/module.rs b/rust/macros/module.rs
index ee9674c50a32..76471161ffa6 100644
--- a/rust/macros/module.rs
+++ b/rust/macros/module.rs
@@ -202,10 +202,15 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
             }};
 
             // 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]
-            pub extern \"C\" fn init_module() -> core::ffi::c_int {{
+            #[link_section = \".init.text\"]
+            pub unsafe extern \"C\" fn init_module() -> core::ffi::c_int {{
                 __init()
             }}