[RFC] rust: kernel: Add KUnit tests for ARCH_WARN_ASM bug table emission

["Mukesh Kumar Chaurasiya (IBM)" <mkchauras@gmail.com>]

Verify that the __bug_table entry emitted by ARCH_WARN_ASM has a correct
bug_addr displacement — i.e. the arch's trap label reference resolves to
the trap instruction — by calling find_bug() with the exact virtual address
of the trap, mirroring what the real trap handler does.

To support all architectures, each arch that implements ARCH_WARN_ASM now
defines ARCH_WARN_ASM_TRAP_LABEL, a string constant naming the local label
at which the trap instruction is placed:

  x86       "1"     (ud2 at label 1:)
  powerpc   "1"     (twi at label 1:)
  riscv     "1"     (ebreak at label 1:)
  arm64     "14471" (brk placed at 14471: by __BUG_ENTRY_END)
  s390      "0"     (mc at label 0:)

The label is used consistently: in ARCH_WARN_ASM itself, in the
bug_addr back-reference inside __BUG_ENTRY / _EMIT_BUG_ENTRY, and in
the new generated_arch_warn_asm_trap_label.rs template, which the
C preprocessor expands at build time into a .rs file included by the
test's global_asm! block.

global_asm! is used instead of asm! because LLVM eliminates asm! blocks
in dead branches; global_asm! is file-scope and always emitted.
BUG_KUNIT_TRAP_ADDR is defined as a .global symbol in .data on top of a
.dc.a {trap_label}b relocation so the linker resolves the trap address
into it at link time. A Rust static initialized to zero would land in
BSS where relocations are not applied.

The global_asm! string mirrors warn_flags!() exactly — same "/* {size} */"
prefix, same generated_arch_warn_asm.rs and generated_arch_reachable_asm.rs
includes, same operands — plus the .pushsection/.dc.a/.popsection tail.
This keeps the test structurally in sync with the macro it exercises.

The CONFIG_RUST_BUG_WARN_ASM_KUNIT_TEST option depends on GENERIC_BUG
and excludes ARM, LOONGARCH, and UML (which use a different warn_flags!
path without ARCH_WARN_ASM).

Five tests are included in the rust_kernel_bug_warn_asm suite:

  bug_entry_found           - find_bug() returns non-NULL for the trap
                              address, proving bug_addr is correct
  bug_entry_is_warning      - the emitted entry has BUGFLAG_WARNING set
  bug_entry_file            - bug_get_file_line() returns the correct
                              source file (CONFIG_DEBUG_BUGVERBOSE only)
  bug_entry_line            - recorded line number is non-zero, confirming
                              the {line} operand was substituted correctly
                              (CONFIG_DEBUG_BUGVERBOSE only)
  bug_entry_addr_is_in_text - kernel_text_address() confirms the trap
                              address lies in kernel text

Tested (pass:5 fail:0 skip:0) on:
  x86_64  - QEMU x86_64
  powerpc - QEMU pseries ppc64le (pseries_le_defconfig)
  riscv   - QEMU riscv64
  arm64   - QEMU aarch64
  s390    - QEMU s390x

Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
---
Note: I sent this out as a powerpc specific tests earlier. During review
process some people wanted these to be arch independent. Hence sending
this out as an RFC.

Earlier version: https://lore.kernel.org/all/20260915090453.1227034-3-mkchauras@gmail.com/

 arch/arm64/include/asm/asm-bug.h              |   3 +
 arch/powerpc/include/asm/bug.h                |  22 +-
 arch/riscv/include/asm/bug.h                  |   5 +-
 arch/s390/include/asm/bug.h                   |   4 +-
 arch/x86/include/asm/bug.h                    |   5 +-
 rust/Makefile                                 |   2 +-
 rust/kernel/.gitignore                        |   1 +
 rust/kernel/Kconfig.test                      |  15 ++
 rust/kernel/bug.rs                            | 191 ++++++++++++++++++
 .../generated_arch_warn_asm_trap_label.rs.S   |   7 +
 10 files changed, 239 insertions(+), 16 deletions(-)
 create mode 100644 rust/kernel/generated_arch_warn_asm_trap_label.rs.S

diff --git a/arch/arm64/include/asm/asm-bug.h b/arch/arm64/include/asm/asm-bug.h
index a5f13801b784..e0694543115e 100644
--- a/arch/arm64/include/asm/asm-bug.h
+++ b/arch/arm64/include/asm/asm-bug.h
@@ -60,6 +60,9 @@ _BUGVERBOSE_LOCATION(__FILE__, __LINE__)		\
 		".short " flags ";"			\
 		__stringify(__BUG_ENTRY_END)
 
+/* Trap instruction is placed at label 14471: by __BUG_ENTRY_END */
+#define ARCH_WARN_ASM_TRAP_LABEL "14471"
+
 #define ARCH_WARN_ASM(file, line, flags, size)		\
 	__BUG_ENTRY_STRING(file, line, flags)		\
 	__stringify(brk BUG_BRK_IMM)
diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
index bf31ee1e902a..f0333c564d4f 100644
--- a/arch/powerpc/include/asm/bug.h
+++ b/arch/powerpc/include/asm/bug.h
@@ -32,6 +32,8 @@
 #endif /* verbose */
 
 #else /* !__ASSEMBLER__ */
+#define ARCH_WARN_ASM_TRAP_LABEL "1"
+
 #ifdef CONFIG_DEBUG_BUGVERBOSE
 #define _EMIT_BUG_ENTRY(bug_entry, trap, file, line, flags)	\
 	".section __bug_table,\"aw\"\n"				\
@@ -45,21 +47,21 @@
 	"		.short " flags "\n"
 #endif
 
-#define BUG_ENTRY(cond_str, insn, flags, ...)			\
-	__asm__ __volatile__(					\
-		"1:	" insn "\n"				\
-		_EMIT_BUG_ENTRY(2, 1b, "%0", "%1", "%2")	\
-		".org 2b+%3\n"					\
-		".previous\n"					\
+#define BUG_ENTRY(cond_str, insn, flags, ...)				\
+	__asm__ __volatile__(						\
+		ARCH_WARN_ASM_TRAP_LABEL ":	" insn "\n"		\
+		_EMIT_BUG_ENTRY(2, 1b, "%0", "%1", "%2")		\
+		".org 2b+%3\n"						\
+		".previous\n"						\
 		: : "i" (WARN_CONDITION_STR(cond_str) __FILE__), "i" (__LINE__),	\
 		  "i" (flags),					\
 		  "i" (sizeof(struct bug_entry)),		\
 		  ##__VA_ARGS__)
 
-#define ARCH_WARN_ASM(file, line, flags, size)			\
-		"1:	twi 31, 0, 0\n"				\
-		_EMIT_BUG_ENTRY(2, 1b, file, line, flags)	\
-		".org 2b+" size "\n"				\
+#define ARCH_WARN_ASM(file, line, flags, size)					\
+		ARCH_WARN_ASM_TRAP_LABEL ":	twi 31, 0, 0\n"			\
+		_EMIT_BUG_ENTRY(2, 1b, file, line, flags)			\
+		".org 2b+" size "\n"						\
 		".previous\n"
 
 #define ARCH_WARN_REACHABLE
diff --git a/arch/riscv/include/asm/bug.h b/arch/riscv/include/asm/bug.h
index 699c0cf3e4ef..def6b0d153f2 100644
--- a/arch/riscv/include/asm/bug.h
+++ b/arch/riscv/include/asm/bug.h
@@ -29,7 +29,8 @@
 
 typedef u32 bug_insn_t;
 
-#define __BUG_ENTRY_ADDR	RISCV_INT " 1b - ."
+#define ARCH_WARN_ASM_TRAP_LABEL	"1"
+#define __BUG_ENTRY_ADDR	RISCV_INT " " ARCH_WARN_ASM_TRAP_LABEL "b - ."
 #define __BUG_ENTRY_FILE(file)	RISCV_INT " " file " - ."
 
 #ifdef CONFIG_DEBUG_BUGVERBOSE
@@ -47,7 +48,7 @@ typedef u32 bug_insn_t;
 #ifdef CONFIG_GENERIC_BUG
 
 #define ARCH_WARN_ASM(file, line, flags, size)			\
-		"1:\n\t"					\
+		ARCH_WARN_ASM_TRAP_LABEL ":\n\t"		\
 			"ebreak\n"				\
 			".pushsection __bug_table,\"aw\"\n\t"	\
 		"2:\n\t"					\
diff --git a/arch/s390/include/asm/bug.h b/arch/s390/include/asm/bug.h
index 2010342c97b1..3c0fa1acc92a 100644
--- a/arch/s390/include/asm/bug.h
+++ b/arch/s390/include/asm/bug.h
@@ -26,9 +26,11 @@
 #define WARN_CONDITION_STR(cond_str) ""
 #endif
 
+#define ARCH_WARN_ASM_TRAP_LABEL "0"
+
 #define __BUG_ENTRY(format, file, line, flags, size)			\
 		"	.section __bug_table,\"aw\"\n"			\
-		"1:	.long	0b - .		# bug_entry::bug_addr\n"\
+		"1:	.long	" ARCH_WARN_ASM_TRAP_LABEL "b - .	# bug_entry::bug_addr\n" \
 		"	.long	" format " - .	# bug_entry::format\n"	\
 		__BUG_ENTRY_VERBOSE(file, line)				\
 		"	.short	"flags"		# bug_entry::flags\n"	\
diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index 23ab05438269..6cca8880660e 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -62,8 +62,9 @@ extern void __WARN_trap(struct bug_entry *bug, ...);
 #define HAVE_ARCH_BUG_FORMAT_ARGS
 #endif
 
+#define ARCH_WARN_ASM_TRAP_LABEL "1"
 #define __BUG_ENTRY(format, file, line, flags)				\
-	"\t.long 1b - ."	"\t# bug_entry::bug_addr\n"		\
+	"\t.long " ARCH_WARN_ASM_TRAP_LABEL "b - ." "\t# bug_entry::bug_addr\n" \
 	__BUG_ENTRY_FORMAT(format)					\
 	__BUG_ENTRY_VERBOSE(file, line)					\
 	"\t.word " flags	"\t# bug_entry::flags\n"
@@ -101,7 +102,7 @@ do {									\
 	"99:\n"								\
 	"\t.string \"\"\n"						\
 	".popsection\n"							\
-	"1:\t " ASM_UD2 "\n"						\
+	ARCH_WARN_ASM_TRAP_LABEL ":\t " ASM_UD2 "\n"			\
 	_BUG_FLAGS_ASM("99b", file, line, flags, size, "")
 
 #else
diff --git a/rust/Makefile b/rust/Makefile
index da1a7409d984..8f253d516495 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -46,7 +46,7 @@ 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
+always-$(subst y,$(CONFIG_RUST),$(CONFIG_BUG)) += kernel/generated_arch_warn_asm.rs kernel/generated_arch_reachable_asm.rs kernel/generated_arch_warn_asm_trap_label.rs
 endif
 
 # Avoids running `$(RUSTC)` when it may not be available.
diff --git a/rust/kernel/.gitignore b/rust/kernel/.gitignore
index f636ad95aaf3..05bb00bbb83c 100644
--- a/rust/kernel/.gitignore
+++ b/rust/kernel/.gitignore
@@ -3,3 +3,4 @@
 /generated_arch_static_branch_asm.rs
 /generated_arch_warn_asm.rs
 /generated_arch_reachable_asm.rs
+/generated_arch_warn_asm_trap_label.rs
diff --git a/rust/kernel/Kconfig.test b/rust/kernel/Kconfig.test
index e6a5c7a795f0..8cd7c8f91b07 100644
--- a/rust/kernel/Kconfig.test
+++ b/rust/kernel/Kconfig.test
@@ -83,4 +83,19 @@ config RUST_BITFIELD_KUNIT_TEST
 
 	  If unsure, say N.
 
+config RUST_BUG_WARN_ASM_KUNIT_TEST
+	bool "KUnit tests for ARCH_WARN_ASM bug table emission" if !KUNIT_ALL_TESTS
+	depends on GENERIC_BUG && !ARM && !LOONGARCH && !UML
+	default KUNIT_ALL_TESTS
+	help
+	  This option enables KUnit tests that verify ARCH_WARN_ASM emits a
+	  correct __bug_table entry: the bug_addr displacement must resolve
+	  back to the trap instruction so that find_bug() can locate the
+	  entry — exactly as the real trap handler does.
+
+	  Supported on all architectures that implement ARCH_WARN_ASM
+	  (x86, powerpc, riscv, arm64, s390).
+
+	  If unsure, say N.
+
 endif
diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
index 3566f0234ca4..9e9b6d3a68b7 100644
--- a/rust/kernel/bug.rs
+++ b/rust/kernel/bug.rs
@@ -152,3 +152,194 @@ macro_rules! warn_on {
         cond
     }};
 }
+
+// Test-only constants and file static referenced by the global_asm block below.
+//
+// global_asm! is file-scope and always emitted — LLVM cannot eliminate it,
+// unlike asm! inside a function which is subject to dead-code removal.
+//
+// BUG_KUNIT_TRAP_ADDR is declared as a .global symbol entirely inside the
+// global_asm! block so the .dc.a {trap_label}b relocation lands directly on it.
+// A Rust static initialized to zero would end up in BSS; the linker does
+// not apply relocations to BSS, so the address would stay zero at runtime.
+#[cfg(CONFIG_RUST_BUG_WARN_ASM_KUNIT_TEST)]
+mod test_statics {
+    use crate::bindings::{bug_entry, BUGFLAG_WARNING, TAINT_WARN};
+
+    pub(super) const FLAGS: u32 = BUGFLAG_WARNING | (TAINT_WARN << 8);
+    pub(super) const SIZE: usize = core::mem::size_of::<bug_entry>();
+
+    // LINE and BUG_KUNIT_FILE are only referenced by the {file}/{line}
+    // operands in global_asm!, which are only present when
+    // CONFIG_DEBUG_BUGVERBOSE is set (the non-verbose _EMIT_BUG_ENTRY
+    // drops the file/line fields from the bug table entry entirely).
+    #[cfg(CONFIG_DEBUG_BUGVERBOSE)]
+    pub(super) const LINE: u32 = line!();
+
+    // Null-terminated source file name — the assembler references this symbol
+    // for the verbose file pointer in __bug_table, same as warn_flags!.
+    #[cfg(CONFIG_DEBUG_BUGVERBOSE)]
+    const _FILE: &[u8] = file!().as_bytes();
+    #[cfg(CONFIG_DEBUG_BUGVERBOSE)]
+    #[no_mangle]
+    pub(super) static BUG_KUNIT_FILE: [u8; _FILE.len() + 1] = {
+        let mut bytes = [0u8; _FILE.len() + 1];
+        let mut i = 0;
+        while i < _FILE.len() {
+            bytes[i] = _FILE[i];
+            i += 1;
+        }
+        bytes
+    };
+}
+
+// Emit ARCH_WARN_ASM at file scope and capture the trap address.
+//
+// The asm string mirrors warn_flags!() exactly — same "/* {size} */" prefix,
+// same generated_arch_warn_asm.rs and generated_arch_reachable_asm.rs includes,
+// same operands — so the two stay in sync as the macro evolves.  The only
+// addition is the .pushsection/.dc.a/.popsection tail that records the trap
+// address into BUG_KUNIT_TRAP_ADDR at link time.
+//
+// BUG_KUNIT_TRAP_ADDR is a .global symbol in .data whose value is set by a
+// .dc.a relocation to the arch's trap label (from
+// generated_arch_warn_asm_trap_label.rs); .dc.a self-aligns to pointer width
+// so no .balign is needed.
+#[cfg(all(CONFIG_RUST_BUG_WARN_ASM_KUNIT_TEST, CONFIG_DEBUG_BUGVERBOSE))]
+::core::arch::global_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")),
+        "\n\t.pushsection .data\n\t",
+        ".global BUG_KUNIT_TRAP_ADDR\n\t",
+        "BUG_KUNIT_TRAP_ADDR:\n\t",
+        ".dc.a ",
+        include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_warn_asm_trap_label.rs")),
+        "b\n\t",
+        ".popsection\n",
+    ),
+    file  = sym test_statics::BUG_KUNIT_FILE,
+    line  = const test_statics::LINE,
+    flags = const test_statics::FLAGS,
+    size  = const test_statics::SIZE,
+);
+
+#[cfg(all(CONFIG_RUST_BUG_WARN_ASM_KUNIT_TEST, not(CONFIG_DEBUG_BUGVERBOSE)))]
+::core::arch::global_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")),
+        "\n\t.pushsection .data\n\t",
+        ".global BUG_KUNIT_TRAP_ADDR\n\t",
+        "BUG_KUNIT_TRAP_ADDR:\n\t",
+        ".dc.a ",
+        include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_warn_asm_trap_label.rs")),
+        "b\n\t",
+        ".popsection\n",
+    ),
+    flags = const test_statics::FLAGS,
+    size  = const test_statics::SIZE,
+);
+
+#[cfg(CONFIG_RUST_BUG_WARN_ASM_KUNIT_TEST)]
+#[::kernel::macros::kunit_tests(rust_kernel_bug_warn_asm)]
+mod tests {
+    use crate::bindings;
+
+    fn trap_addr() -> usize {
+        // BUG_KUNIT_TRAP_ADDR is a .global symbol defined in the global_asm!
+        // block above, placed in .data at the exact .dc.a 1b relocation word.
+        // The linker resolves it to the virtual address of the twi instruction
+        // before any Rust code runs, so reading it here is always safe.
+        extern "C" {
+            // .dc.a emits a pointer-width word: 4 bytes on ppc32, 8 on ppc64.
+            // usize matches the native pointer width on both.
+            static BUG_KUNIT_TRAP_ADDR: usize;
+        }
+        // SAFETY: read-only after link time, no concurrent mutation possible.
+        unsafe { BUG_KUNIT_TRAP_ADDR }
+    }
+
+    /// The `__bug_table` entry emitted by `ARCH_WARN_ASM` must be locatable
+    /// via `find_bug()` using the trap instruction's address.  A NULL result
+    /// means the `1b` label reference in `_EMIT_BUG_ENTRY` resolved to the
+    /// wrong address and the real trap handler would not recognise the site.
+    #[test]
+    fn bug_entry_found() {
+        // Non-zero proves the .dc.a relocation was resolved by the linker.
+        assert!(trap_addr() != 0);
+
+        // SAFETY: find_bug() is always safe to call with any address; it
+        // simply walks __bug_table and returns NULL if nothing matches.
+        let entry = unsafe { bindings::find_bug(trap_addr()) };
+        // Non-NULL proves the bug_addr displacement in _EMIT_BUG_ENTRY is correct.
+        assert!(!entry.is_null());
+    }
+
+    /// The emitted entry must be flagged as a warning (not a hard BUG).
+    #[test]
+    fn bug_entry_is_warning() {
+        assert!(trap_addr() != 0);
+        let entry = unsafe { bindings::find_bug(trap_addr()) };
+        assert!(!entry.is_null());
+        // SAFETY: entry is non-null and points to a valid bug_entry.
+        let flags = unsafe { (*entry).flags } as u32;
+        assert!(flags & bindings::BUGFLAG_WARNING != 0);
+    }
+
+    /// With `CONFIG_DEBUG_BUGVERBOSE` the entry must record a non-null file
+    /// pointer pointing back into this source file.
+    #[test]
+    #[cfg(CONFIG_DEBUG_BUGVERBOSE)]
+    fn bug_entry_file() {
+        use kernel::str::CStrExt;
+
+        assert!(trap_addr() != 0);
+        let entry = unsafe { bindings::find_bug(trap_addr()) };
+        assert!(!entry.is_null());
+
+        let mut file_ptr: *const kernel::ffi::c_char = core::ptr::null();
+        let mut line: u32 = 0;
+        // SAFETY: entry is non-null and valid; file_ptr and line are local
+        // variables passed as out-parameters.
+        unsafe { bindings::bug_get_file_line(entry, &mut file_ptr, &mut line) };
+
+        assert!(!file_ptr.is_null());
+        // SAFETY: file_ptr is a non-null, null-terminated C string from BUG_KUNIT_FILE,
+        // valid for the lifetime of this function.
+        let file_str = unsafe { kernel::prelude::CStr::from_char_ptr(file_ptr) }
+            .to_str()
+            .unwrap_or("");
+        assert!(file_str.contains("bug"));
+    }
+
+    /// With `CONFIG_DEBUG_BUGVERBOSE` the recorded line number must be
+    /// non-zero (a zero line would mean the asm operand was not substituted).
+    #[test]
+    #[cfg(CONFIG_DEBUG_BUGVERBOSE)]
+    fn bug_entry_line() {
+        assert!(trap_addr() != 0);
+        let entry = unsafe { bindings::find_bug(trap_addr()) };
+        assert!(!entry.is_null());
+
+        let mut file_ptr: *const kernel::ffi::c_char = core::ptr::null();
+        let mut line: u32 = 0;
+        // SAFETY: entry is non-null and valid.
+        unsafe { bindings::bug_get_file_line(entry, &mut file_ptr, &mut line) };
+
+        assert!(line != 0);
+    }
+
+    /// The trap address stored in `__bug_table` must lie within the kernel
+    /// text segment.  If the label reference in `_EMIT_BUG_ENTRY` resolved
+    /// to data or zero, `kernel_text_address()` would return false.
+    #[test]
+    fn bug_entry_addr_is_in_text() {
+        assert!(trap_addr() != 0);
+        // SAFETY: kernel_text_address() is always safe to call with any addr.
+        let in_text = unsafe { bindings::kernel_text_address(trap_addr()) };
+        assert!(in_text != 0);
+    }
+}
diff --git a/rust/kernel/generated_arch_warn_asm_trap_label.rs.S b/rust/kernel/generated_arch_warn_asm_trap_label.rs.S
new file mode 100644
index 000000000000..d7bed8751f10
--- /dev/null
+++ b/rust/kernel/generated_arch_warn_asm_trap_label.rs.S
@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <linux/bug.h>
+
+// Cut here.
+
+::kernel::concat_literals!(ARCH_WARN_ASM_TRAP_LABEL)
-- 
2.55.0


_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv