Re: [PATCH v9 06/27] rust: add C helpers

[Kees Cook]

On Fri, Aug 05, 2022 at 05:41:51PM +0200, Miguel Ojeda wrote:
> This source file contains forwarders to C macros and inlined
> functions.

Perhaps:

"Introduce the source file that will contain forwarders to common C
macros as inlined Rust functions. Initially this only contains type
size asserts, but will gain more helpers in subsequent patches."

> 
> Co-developed-by: Alex Gaynor <alex.gaynor@xxxxxxxxx>
> Signed-off-by: Alex Gaynor <alex.gaynor@xxxxxxxxx>
> Co-developed-by: Geoffrey Thomas <geofft@xxxxxxxxxxxxx>
> Signed-off-by: Geoffrey Thomas <geofft@xxxxxxxxxxxxx>
> Co-developed-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxxx>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxxx>
> Co-developed-by: Sven Van Asbroeck <thesven73@xxxxxxxxx>
> Signed-off-by: Sven Van Asbroeck <thesven73@xxxxxxxxx>
> Co-developed-by: Gary Guo <gary@xxxxxxxxxxx>
> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
> Co-developed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> Co-developed-by: Maciej Falkowski <m.falkowski@xxxxxxxxxxx>
> Signed-off-by: Maciej Falkowski <m.falkowski@xxxxxxxxxxx>
> Co-developed-by: Wei Liu <wei.liu@xxxxxxxxxx>
> Signed-off-by: Wei Liu <wei.liu@xxxxxxxxxx>
> Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> ---
>  rust/helpers.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 51 insertions(+)
>  create mode 100644 rust/helpers.c
> 
> diff --git a/rust/helpers.c b/rust/helpers.c
> new file mode 100644
> index 000000000000..b4f15eee2ffd
> --- /dev/null
> +++ b/rust/helpers.c
> @@ -0,0 +1,51 @@
> +// 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);

Given the distaste for ever using BUG()[1], why does this helper exist?

> +
> +/*
> + * 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`"
> +);

-Kees

[1] https://docs.kernel.org/process/deprecated.html#bug-and-bug-on

-- 
Kees Cook