helpers.c

// 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`"
);