Re: [PATCH v5 00/11] rust: bindings: Auto-generate inline static functions

[Andreas Hindborg <a.hindborg@kernel.org>]

"Alistair Francis" <alistair@alistair23.me> writes:

> The kernel includes a large number of static inline functions that are
> defined in header files. One example is the crypto_shash_descsize()
> function which is defined in hash.h as
>
> ```
> static inline unsigned int crypto_shash_descsize(struct crypto_shash *tfm)
> {
>         return tfm->descsize;
> }
> ```
>
> bindgen is currently unable to generate bindings to these functions as
> they are not publically exposed (they are static after all).
>
> The Rust code currently uses rust_helper_* functions, such as
> rust_helper_alloc_pages() for example to call the static inline
> functions. But this is a hassle as someone needs to write a C helper
> function.
>
> Instead we can use the bindgen wrap-static-fns feature. The feature
> is marked as experimental, but has recently been promoted to
> non-experimental (depending on your version of bindgen).
>
> By supporting wrap-static-fns we automatically generate a C file called
> extern.c that exposes the static inline functions, for example like this
>
> ```
> unsigned int crypto_shash_descsize__extern(struct crypto_shash *tfm) { return crypto_shash_descsize(tfm); }
> ```
>
> The nice part is that this is auto-generated.
>
> We then also get a bindings_generate_static.rs file with the Rust
> binding, like this
>
> ```
> extern "C" {
>     #[link_name = "crypto_shash_descsize__extern"]
>     pub fn crypto_shash_descsize(tfm: *mut crypto_shash) -> core::ffi::c_uint;
> }
> ```
>
> So now we can use the static inline functions just like normal
> functions.
>
> There are a bunch of static inline functions that don't work though, because
> the C compiler fails to build extern.c:
>  * functions with inline asm generate "operand probably does not match constraints"
>    errors (rip_rel_ptr() for example)
>  * functions with bit masks (u32_encode_bits() and friends) result in
>    "call to ‘__bad_mask’ declared with attribute error: bad bitfield mask"
>    errors
>
> As well as that any static inline function that calls a function that has been
> kconfig-ed out will fail to link as the function being called isn't built
> (mdio45_ethtool_gset_npage for example)
>
> Due to these failures we use a allow-list system (where functions must
> be manually enabled).
>
> This series adds support for bindgen generating wrappers for inline statics and
> then converts the existing helper functions to this new method. This doesn't
> work for C macros, so we can't reamove all of the helper functions, but we
> can remove most.
>
> v5:
>  - Rebase on latest rust-next on top of v6.13-rc6
>  - Allow support for LTO inlining (not in this series, see
>    https://github.com/alistair23/linux/commit/e6b847324b4f5e904e007c0e288c88d2483928a8)

Thanks! Since Gary just sent v2 of the LTO patch [1], could you rebase
on that and list it as a dependency? If you are using b4 there is a nice
feature for that [2].


Best regards,
Andreas Hindborg


[1] https://lore.kernel.org/all/20250105194054.545201-1-gary@garyguo.net/
[2] https://b4.docs.kernel.org/en/latest/contributor/prep.html#working-with-series-dependencies