[v3,0/5] Check Rust signatures at compile time

Message

Alice Ryhl March 3, 2025, 8:45 a.m. UTC

Rust has two different tools for generating function declarations to
call across the FFI boundary:

* bindgen. Generates Rust declarations from a C header.
* cbindgen. Generates C headers from Rust declarations.

However, we only use bindgen in the kernel. This means that when C code
calls a Rust function by name, its signature must be duplicated in both
Rust code and a C header, and the signature needs to be kept in sync
manually.

Introducing cbindgen as a mandatory dependency to build the kernel would
be a rather complex and large change, so we do not consider that at this
time. Instead, to eliminate this manual checking, introduce a new macro
that verifies at compile time that the two function declarations use the
same signature. The idea is to run the C declaration through bindgen,
and then have rustc verify that the function pointers have the same
type.

The signature must still be written twice, but at least you can no
longer get it wrong. If the signatures don't match, you will get errors
that look like this:

error[E0308]: `if` and `else` have incompatible types
  --> <linux>/rust/kernel/print.rs:22:22
   |
21 | #[export]
   | --------- expected because of this
22 | unsafe extern "C" fn rust_fmt_argument(
   |                      ^^^^^^^^^^^^^^^^^ expected `u8`, found `i8`
   |
   = note: expected fn item `unsafe extern "C" fn(*mut u8, *mut u8, *mut c_void) -> *mut u8 {bindings::rust_fmt_argument}`
              found fn item `unsafe extern "C" fn(*mut i8, *mut i8, *const c_void) -> *mut i8 {print::rust_fmt_argument}`

It is unfortunate that the error message starts out by saying "`if` and
`else` have incompatible types", but I believe the rest of the error
message is reasonably clear and not too confusing.

The main commit of this series is "rust: add #[export] macro".

Signed-off-by: Alice Ryhl <[e-mail]>
---
Changes in v3:
- Reword commit message about cbindgen to remove cargo comment.
- Add # token to quote! macro and mention allow(unused_mut) warning.
- Use quote! macro for #[no_mangle] in #[export].
- Reword "since" in `export` last line of docs.
- Drop extern from drm_panic_qr function declarations.
- Add comment about drm_panic_qr_max_data_size being unsafe.
- Add comment to drm/drm_panic.h include.
- Pick up tags to commit trailers.
- Link to v2: https://lore.kernel.org/r/[e-mail]

Changes in v2:
- Various improvements to documentation.
- Split out quote! changes into its own commit.
- Link to v1: https://lore.kernel.org/r/[e-mail]

---
Alice Ryhl (5):
      rust: fix signature of rust_fmt_argument
      rust: macros: support additional tokens in quote!
      rust: add #[export] macro
      print: use new #[export] macro for rust_fmt_argument
      panic_qr: use new #[export] macro

 drivers/gpu/drm/drm_panic.c     |  5 -----
 drivers/gpu/drm/drm_panic_qr.rs | 15 +++++++++++----
 include/drm/drm_panic.h         |  7 +++++++
 include/linux/sprintf.h         |  3 +++
 lib/vsprintf.c                  |  3 ---
 rust/bindings/bindings_helper.h |  5 +++++
 rust/kernel/prelude.rs          |  2 +-
 rust/kernel/print.rs            | 10 +++++-----
 rust/macros/export.rs           | 29 +++++++++++++++++++++++++++++
 rust/macros/helpers.rs          | 19 ++++++++++++++++++-
 rust/macros/lib.rs              | 24 ++++++++++++++++++++++++
 rust/macros/quote.rs            | 27 +++++++++++++++++++++++++--
 12 files changed, 128 insertions(+), 21 deletions(-)
---
base-commit: a64dcfb451e254085a7daee5fe51bf22959d52d3
change-id: 20250227-export-macro-9aa9f1016d8c

Best regards,