linux-kernel - Re: [PATCH 00/13] rust: bindings: Auto-generate inline static functions

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Message-ID: <3d898bd0-315d-4e3d-a500-5605e9047946@de.bosch.com>
Date: Thu, 7 Nov 2024 08:01:21 +0100
From: Dirk Behme <dirk.behme@...bosch.com>
To: Alistair Francis <alistair23@...il.com>, <alex.gaynor@...il.com>,
	<benno.lossin@...ton.me>, <gary@...yguo.net>, <linux-kernel@...r.kernel.org>,
	<ojeda@...nel.org>, <rust-for-linux@...r.kernel.org>,
	<bjorn3_gh@...tonmail.com>, <alistair.francis@....com>, <me@...enk.dev>,
	<a.hindborg@...nel.org>, <tmgross@...ch.edu>, <boqun.feng@...il.com>,
	<aliceryhl@...gle.com>
Subject: Re: [PATCH 00/13] rust: bindings: Auto-generate inline static
 functions

On 07.11.2024 03:08, Alistair Francis wrote:
> 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 (dependig 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.

Maybe this should be re-done against rust-next which at least contains

https://github.com/Rust-for-Linux/linux/commit/d4d7c05f9656fd8e14c6977a54986264eb296ec8

which seems to conflict with the removal of some helpers file in this 
series.

Thanks!

Dirk