rust: add C helpers

// SPDX-License-Identifier: GPL-2.0

/*

 * Non-trivial C macros cannot be used in Rust. Similarly, inlined C functions

 * cannot be called either. This file explicitly creates functions ("helpers")

 * that wrap those so that they can be called from Rust.

 *

 * Even though Rust kernel modules should never use directly the bindings, some

 * of these helpers need to be exported because Rust generics and inlined

 * functions may not get their code generated in the crate where they are

 * defined. Other helpers, called from non-inline functions, may not be

 * exported, in principle. However, in general, the Rust compiler does not

 * guarantee codegen will be performed for a non-inline function either.

 * Therefore, this file exports all the helpers. In the future, this may be

 * revisited to reduce the number of exports after the compiler is informed

 * about the places codegen is required.

 *

 * All symbols are exported as GPL-only to guarantee no GPL-only feature is

 * accidentally exposed.

 */

#include <linux/bug.h>

#include <linux/build_bug.h>

__noreturn void rust_helper_BUG(void)

{

	BUG();

}

EXPORT_SYMBOL_GPL(rust_helper_BUG);

/*

 * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type

 * as the Rust `usize` type, so we can use it in contexts where Rust

 * expects a `usize` like slice (array) indices. `usize` is defined to be

 * the same as C's `uintptr_t` type (can hold any pointer) but not

 * necessarily the same as `size_t` (can hold the size of any single

 * object). Most modern platforms use the same concrete integer type for

 * both of them, but in case we find ourselves on a platform where

 * that's not true, fail early instead of risking ABI or

 * integer-overflow issues.

 *

 * If your platform fails this assertion, it means that you are in

 * danger of integer-overflow bugs (even if you attempt to remove

 * `--size_t-is-usize`). It may be easiest to change the kernel ABI on

 * your platform such that `size_t` matches `uintptr_t` (i.e., to increase

 * `size_t`, because `uintptr_t` has to be at least as big as `size_t`).

 */

static_assert(

	sizeof(size_t) == sizeof(uintptr_t) &&

	__alignof__(size_t) == __alignof__(uintptr_t),

	"Rust code expects C `size_t` to match Rust `usize`"

);