rust: Add warn_on macro

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 annotation code. It's
used for only architectures that use objtool.

For now, Loongarch and arm just use 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@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250502094537.231725-5-fujita.tomonori@gmail.com
[ Avoid evaluating the condition twice (a good idea in general,
  but it also matches the C side). Simplify with `as_char_ptr()`
  to avoid a cast. Cast to `ffi` integer types for
  `warn_slowpath_fmt`. Avoid cast for `null()`. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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
@@ -540,5 +543,10 @@ $(obj)/kernel.o: $(src)/kernel/lib.rs $(obj)/build_error.o $(obj)/pin_init.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

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);
+}

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

rust/kernel/bug.rs

@@ -0,0 +1,126 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024, 2025 FUJITA Tomonori <fujita.tomonori@gmail.com>
+
+//! 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), not(CONFIG_ARM)))]
+#[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:
+        // - `file`, `line`, `flags`, and `size` are all compile-time constants or
+        // symbols, preventing any invalid memory access.
+        // - The asm block has no side effects and does not modify any registers
+        // or memory. It is purely for embedding metadata into the ELF section.
+        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), not(CONFIG_ARM)))]
+#[cfg(not(CONFIG_DEBUG_BUGVERBOSE))]
+macro_rules! warn_flags {
+    ($flags:expr) => {
+        const FLAGS: u32 = $crate::bindings::BUGFLAG_WARNING | $flags;
+
+        // SAFETY:
+        // - `flags` and `size` are all compile-time constants, preventing
+        // any invalid memory access.
+        // - The asm block has no side effects and does not modify any registers
+        // or memory. It is purely for embedding metadata into the ELF section.
+        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: It is always safe to call `warn_slowpath_fmt()`
+        // with a valid null-terminated string.
+        unsafe {
+            $crate::bindings::warn_slowpath_fmt(
+                $crate::c_str!(::core::file!()).as_char_ptr(),
+                line!() as $crate::ffi::c_int,
+                $flags as $crate::ffi::c_uint,
+                ::core::ptr::null(),
+            );
+        }
+    };
+}
+
+#[macro_export]
+#[doc(hidden)]
+#[cfg(all(CONFIG_BUG, any(CONFIG_LOONGARCH, CONFIG_ARM)))]
+macro_rules! warn_flags {
+    ($flags:expr) => {
+        // SAFETY: It is always safe to call `WARN_ON()`.
+        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) => {{
+        let cond = $cond;
+        if cond {
+            const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
+
+            $crate::warn_flags!(WARN_ON_FLAGS);
+        }
+        cond
+    }};
+}

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)

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}"))

rust/kernel/lib.rs

@@ -57,6 +57,7 @@ pub mod auxiliary;
 pub mod bits;
 #[cfg(CONFIG_BLOCK)]
 pub mod block;
+pub mod bug;
 #[doc(hidden)]
 pub mod build_assert;
 pub mod clk;