[PATCH v3] rust: place generated init_module() function in .init.text

[PATCH v3] rust: place generated init_module() function in .init.text

[Thomas Bertschinger]

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>
---
 v3: adjust safety comment to be more accurate

 rust/macros/module.rs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/rust/macros/module.rs b/rust/macros/module.rs
index d62d8710d77a..27979e582e4b 100644
--- a/rust/macros/module.rs
+++ b/rust/macros/module.rs
@@ -222,10 +222,15 @@ 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()
             }}
 
-- 
2.43.0

Re: [PATCH v3] rust: place generated init_module() function in .init.text

[Miguel Ojeda]

On Tue, Feb 6, 2024 at 4:39 PM Thomas Bertschinger
<tahbertschinger@gmail.com> 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 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>

Applied to `rust-next` -- thanks everyone!

[ Reworded title to add prefix. ]

Cheers,
Miguel