[PATCH v4 4/4] rust: Add warn_on macro

From: FUJITA Tomonori
Date: Wed Mar 05 2025 - 06:11:46 EST

Add warn_on macro, uses the BUG/WARN feature (lib/bug.c) via assembly
for x86_64/arm64/riscv.

The current Rust code simply wraps BUG() macro but doesn't provide the
proper debug information. The BUG/WARN feature can only be used from
assembly.

This uses the assembly code exported by the C side via ARCH_WARN_ASM
macro. To avoid duplicating the assembly code, this approach follows
the same strategy as the static branch code: it generates the assembly
code for Rust using the C preprocessor at compile time.

Similarly, ARCH_WARN_REACHABLE is also used at compile time to
generate the assembly code; objtool's reachable anotation code. It's
used for only architectures that use objtool.

For now, Loongarch just uses a wrapper for WARN macro.

UML doesn't use the assembly BUG/WARN feature; just wrapping generic
BUG/WARN functions implemented in C works.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@xxxxxxxxx>
---
rust/Makefile | 8 ++
rust/helpers/bug.c | 5 +
rust/kernel/.gitignore | 2 +
rust/kernel/bug.rs | 113 ++++++++++++++++++
rust/kernel/generated_arch_reachable_asm.rs.S | 7 ++
rust/kernel/generated_arch_warn_asm.rs.S | 7 ++
rust/kernel/lib.rs | 1 +
7 files changed, 143 insertions(+)
create mode 100644 rust/kernel/bug.rs
create mode 100644 rust/kernel/generated_arch_reachable_asm.rs.S
create mode 100644 rust/kernel/generated_arch_warn_asm.rs.S

diff --git a/rust/Makefile b/rust/Makefile
index 8fcfd60447bc..a295b65c43f3 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -34,6 +34,9 @@ obj-$(CONFIG_RUST_KERNEL_DOCTESTS) += doctests_kernel_generated.o
obj-$(CONFIG_RUST_KERNEL_DOCTESTS) += doctests_kernel_generated_kunit.o

always-$(subst y,$(CONFIG_RUST),$(CONFIG_JUMP_LABEL)) += kernel/generated_arch_static_branch_asm.rs
+ifndef CONFIG_UML
+always-$(subst y,$(CONFIG_RUST),$(CONFIG_BUG)) += kernel/generated_arch_warn_asm.rs kernel/generated_arch_reachable_asm.rs
+endif

# Avoids running `$(RUSTC)` when it may not be available.
ifdef CONFIG_RUST
@@ -481,5 +484,10 @@ $(obj)/kernel.o: $(src)/kernel/lib.rs $(obj)/build_error.o \
ifdef CONFIG_JUMP_LABEL
$(obj)/kernel.o: $(obj)/kernel/generated_arch_static_branch_asm.rs
endif
+ifndef CONFIG_UML
+ifdef CONFIG_BUG
+$(obj)/kernel.o: $(obj)/kernel/generated_arch_warn_asm.rs $(obj)/kernel/generated_arch_reachable_asm.rs
+endif
+endif

endif # CONFIG_RUST
diff --git a/rust/helpers/bug.c b/rust/helpers/bug.c
index e2d13babc737..a62c96f507d1 100644
--- a/rust/helpers/bug.c
+++ b/rust/helpers/bug.c
@@ -6,3 +6,8 @@ __noreturn void rust_helper_BUG(void)
{
BUG();
}
+
+bool rust_helper_WARN_ON(bool cond)
+{
+ return WARN_ON(cond);
+}
diff --git a/rust/kernel/.gitignore b/rust/kernel/.gitignore
index 6ba39a178f30..f636ad95aaf3 100644
--- a/rust/kernel/.gitignore
+++ b/rust/kernel/.gitignore
@@ -1,3 +1,5 @@
# SPDX-License-Identifier: GPL-2.0

/generated_arch_static_branch_asm.rs
+/generated_arch_warn_asm.rs
+/generated_arch_reachable_asm.rs
diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
new file mode 100644
index 000000000000..a010033be5a2
--- /dev/null
+++ b/rust/kernel/bug.rs
@@ -0,0 +1,113 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024, 2025 FUJITA Tomonori <fujita.tomonori@xxxxxxxxx>
+
+//! Support for BUG and WARN functionality.
+//!
+//! C header: [`include/asm-generic/bug.h`](srctree/include/asm-generic/bug.h)
+
+#[macro_export]
+#[doc(hidden)]
+#[cfg(all(CONFIG_BUG, not(CONFIG_UML), not(CONFIG_LOONGARCH)))]
+#[cfg(CONFIG_DEBUG_BUGVERBOSE)]
+macro_rules! warn_flags {
+ ($flags:expr) => {
+ const FLAGS: u32 = $crate::bindings::BUGFLAG_WARNING | $flags;
+ const _FILE: &[u8] = file!().as_bytes();
+ // Plus one for null-terminator.
+ static FILE: [u8; _FILE.len() + 1] = {
+ let mut bytes = [0; _FILE.len() + 1];
+ let mut i = 0;
+ while i < _FILE.len() {
+ bytes[i] = _FILE[i];
+ i += 1;
+ }
+ bytes
+ };
+ // SAFETY: Just an FFI call.
+ unsafe {
+ $crate::asm!(concat!(
+ "/* {size} */",
+ include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_warn_asm.rs")),
+ include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_reachable_asm.rs")));
+ file = sym FILE,
+ line = const line!(),
+ flags = const FLAGS,
+ size = const ::core::mem::size_of::<$crate::bindings::bug_entry>(),
+ );
+ }
+ }
+}
+
+#[macro_export]
+#[doc(hidden)]
+#[cfg(all(CONFIG_BUG, not(CONFIG_UML), not(CONFIG_LOONGARCH)))]
+#[cfg(not(CONFIG_DEBUG_BUGVERBOSE))]
+macro_rules! warn_flags {
+ ($flags:expr) => {
+ const FLAGS: u32 = $crate::bindings::BUGFLAG_WARNING | $flags;
+ // SAFETY: Just an FFI call.
+ unsafe {
+ $crate::asm!(
+ concat!(
+ "/* {size} */",
+ include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_warn_asm.rs")),
+ include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_reachable_asm.rs")));
+ flags = const FLAGS,
+ size = const ::core::mem::size_of::<$crate::bindings::bug_entry>(),
+ );
+ }
+ }
+}
+
+#[macro_export]
+#[doc(hidden)]
+#[cfg(all(CONFIG_BUG, CONFIG_UML))]
+macro_rules! warn_flags {
+ ($flags:expr) => {
+ // SAFETY: Just an FFI call.
+ unsafe {
+ $crate::bindings::warn_slowpath_fmt(
+ $crate::c_str!(::core::file!()).as_ptr() as *const ::core::ffi::c_uchar,
+ line!() as i32,
+ $flags as u32,
+ ::core::ptr::null() as *const ::core::ffi::c_uchar,
+ );
+ }
+ };
+}
+
+#[macro_export]
+#[doc(hidden)]
+#[cfg(all(CONFIG_BUG, CONFIG_LOONGARCH))]
+macro_rules! warn_flags {
+ ($flags:expr) => {
+ // SAFETY: Just an FFI call.
+ unsafe { $crate::bindings::WARN_ON(true) }
+ };
+}
+
+#[macro_export]
+#[doc(hidden)]
+#[cfg(not(CONFIG_BUG))]
+macro_rules! warn_flags {
+ ($flags:expr) => {};
+}
+
+#[doc(hidden)]
+pub const fn bugflag_taint(value: u32) -> u32 {
+ value << 8
+}
+
+/// Report a warning if `cond` is true and return the condition's evaluation result.
+#[macro_export]
+macro_rules! warn_on {
+ ($cond:expr) => {{
+ if $cond {
+ const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
+
+ $crate::warn_flags!(WARN_ON_FLAGS);
+ }
+ $cond
+ }};
+}
diff --git a/rust/kernel/generated_arch_reachable_asm.rs.S b/rust/kernel/generated_arch_reachable_asm.rs.S
new file mode 100644
index 000000000000..3886a9ad3a99
--- /dev/null
+++ b/rust/kernel/generated_arch_reachable_asm.rs.S
@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <linux/bug.h>
+
+// Cut here.
+
+::kernel::concat_literals!(ARCH_WARN_REACHABLE)
diff --git a/rust/kernel/generated_arch_warn_asm.rs.S b/rust/kernel/generated_arch_warn_asm.rs.S
new file mode 100644
index 000000000000..409eb4c2d3a1
--- /dev/null
+++ b/rust/kernel/generated_arch_warn_asm.rs.S
@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <linux/bug.h>
+
+// Cut here.
+
+::kernel::concat_literals!(ARCH_WARN_ASM("{file}", "{line}", "{flags}", "{size}"))
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 496ed32b0911..c0720846b249 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -38,6 +38,7 @@
pub mod alloc;
#[cfg(CONFIG_BLOCK)]
pub mod block;
+pub mod bug;
#[doc(hidden)]
pub mod build_assert;
pub mod cred;
--
2.43.0