[PATCH v2 00/27] Allow inlining C helpers into Rust when using LTO

Alice Ryhl
2026-01-05T12:42:40Z

This patch series adds __rust_helper to every single rust helper. The
patches do not depend on each other, so maintainers please go ahead and
pick up any patches relevant to your subsystem! Or provide your Acked-by
so that Miguel can pick them up.

These changes were generated by adding __rust_helper and running
ClangFormat. Unrelated formatting changes were removed manually.

Why is __rust_helper needed?
============================

Currently, C helpers cannot be inlined into Rust even when using LTO
because LLVM detects slightly different options on the codegen units.

* LLVM doesn't want to inline functions compiled with
`-fno-delete-null-pointer-checks` with code compiled without. The C
CGUs all have this enabled and Rust CGUs don't. Inlining is okay since
this is one of the hardening features that does not change the ABI,
and we shouldn't have null pointer dereferences in these helpers.

* LLVM doesn't want to inline functions with different list of builtins. C
side has `-fno-builtin-wcslen`; `wcslen` is not a Rust builtin, so
they should be compatible, but LLVM does not perform inlining due to
attributes mismatch.

* clang and Rust doesn't have the exact target string. Clang generates
`+cmov,+cx8,+fxsr` but Rust doesn't enable them (in fact, Rust will
complain if `-Ctarget-feature=+cmov,+cx8,+fxsr` is used). x86-64
always enable these features, so they are in fact the same target
string, but LLVM doesn't understand this and so inlining is inhibited.
This can be bypassed with `--ignore-tti-inline-compatible`, but this
is a hidden option.

(This analysis was written by Gary Guo.)

How is this fixed?
==================

To fix this we need to add __always_inline to all helpers when compiling
with LTO. However, it should not be added when running bindgen as
bindgen will ignore functions marked inline. To achieve this, we are
using a #define called __rust_helper that is defined differently
depending on whether bindgen is running or not.

Note that __rust_helper is currently always #defined to nothing.
Changing it to __always_inline will happen separately in another patch
series.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Changes in v2:
- Pick up Reviewed-by: Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>
- Update formatting in mm and slab patches.
- Add new helpers in files uaccess, pwm, and time.
- Drop any patches that have been merged.
- Link to v1: https://lore.kernel.org/r/20251202-define-rust-helper-v1-0-a2e13cbc17a6@google.com

---
Alice Ryhl (27):
rust: barrier: add __rust_helper to helpers
rust: blk: add __rust_helper to helpers
rust: bug: add __rust_helper to helpers
rust: clk: add __rust_helper to helpers
rust: completion: add __rust_helper to helpers
rust: cpu: add __rust_helper to helpers
rust: cred: add __rust_helper to helpers
rust: err: add __rust_helper to helpers
rust: jump_label: add __rust_helper to helpers
rust: maple_tree: add __rust_helper to helpers
rust: mm: add __rust_helper to helpers
rust: of: add __rust_helper to helpers
rust: processor: add __rust_helper to helpers
rust: pwm: add __rust_helper to helpers
rust: rbtree: add __rust_helper to helpers
rust: rcu: add __rust_helper to helpers
rust: refcount: add __rust_helper to helpers
rust: security: add __rust_helper to helpers
rust: slab: add __rust_helper to helpers
rust: sync: add __rust_helper to helpers
rust: task: add __rust_helper to helpers
rust: time: add __rust_helper to helpers
rust: uaccess: add __rust_helper to helpers
rust: usb: add __rust_helper to helpers
rust: wait: add __rust_helper to helpers
rust: workqueue: add __rust_helper to helpers
rust: xarray: add __rust_helper to helpers

rust/helpers/barrier.c | 6 +++---
rust/helpers/blk.c | 4 ++--
rust/helpers/bug.c | 4 ++--
rust/helpers/build_bug.c | 2 +-
rust/helpers/clk.c | 24 +++++++++++++-----------
rust/helpers/completion.c | 2 +-
rust/helpers/cpu.c | 2 +-
rust/helpers/cred.c | 4 ++--
rust/helpers/err.c | 6 +++---
rust/helpers/jump_label.c | 2 +-
rust/helpers/maple_tree.c | 3 ++-
rust/helpers/mm.c | 20 ++++++++++----------
rust/helpers/mutex.c | 13 +++++++------
rust/helpers/of.c | 2 +-
rust/helpers/page.c | 9 +++++----
rust/helpers/processor.c | 2 +-
rust/helpers/pwm.c | 6 +++---
rust/helpers/rbtree.c | 9 +++++----
rust/helpers/rcu.c | 4 ++--
rust/helpers/refcount.c | 10 +++++-----
rust/helpers/security.c | 26 +++++++++++++++-----------
rust/helpers/signal.c | 2 +-
rust/helpers/slab.c | 4 ++--
rust/helpers/spinlock.c | 13 +++++++------
rust/helpers/sync.c | 4 ++--
rust/helpers/task.c | 24 ++++++++++++------------
rust/helpers/time.c | 14 +++++++-------
rust/helpers/uaccess.c | 10 ++++++----
rust/helpers/usb.c | 3 ++-
rust/helpers/vmalloc.c | 2 +-
rust/helpers/wait.c | 2 +-
rust/helpers/workqueue.c | 8 +++++---
rust/helpers/xarray.c | 10 +++++-----
33 files changed, 136 insertions(+), 120 deletions(-)
---
base-commit: 9ace4753a5202b02191d54e9fdf7f9e3d02b85eb
change-id: 20251202-define-rust-helper-f7b531813007

Best regards,
--
Alice Ryhl <aliceryhl@google.com>