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

On Tue, Sep 22, 2026 at 08:47:35AM +0200, Peter Zijlstra wrote:

On Tue, Sep 22, 2026 at 11:20:00AM +0530, Mukesh Kumar Chaurasiya (IBM) wrote:

quoted

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, 

quoted

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

Not really a fan of that. And I can't really tell what you're doing with
it either. The kunit is in Rust and thus unreadable :-(

I would rather you fix up is_valid_bugaddr(), some architectures seem to
have an always true stub because of the callchains always being from the
break instruction.

Hey Peter,

thanks for the suggestion. I will fix up the is_valid_bugaddr for
powerpc. I am not very good with other archs asm but i can do it for
ppc32 and ppc64.

Regards,
Mukesh