[PATCH] bug: Add the condition string to the CONFIG_DEBUG_BUGVERBOSE=y output

[Ingo Molnar <mingo@kernel.org>]

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> How about going a different route instead? Right now we have that 
> CONFIG_DEBUG_BUGVERBOSE thing which adds the file name and line 
> number information. That has been very good.
> 
> But maybe that should be extended to also always take the 
> compile-time '#condition' string?
> 
> So then all warnings would have the warning condition string 
> (assuming you end up enabling DEBUG_BUGVERBOSE, of course, which I 
> think everybody pretty much does). With no extra code.

So something like the patch below?

Testcase:

  @@ -8508,6 +8508,8 @@ void __init sched_init(void)
          unsigned long ptr = 0;
          int i;
 
  +       WARN_ON_ONCE(ptr == 0 && 1);
  +

Before:

  WARNING: CPU: 0 PID: 0 at kernel/sched/core.c:8511 sched_init+0x20/0x410

After:

  WARNING: CPU: 0 PID: 0 at [ptr == 0 && 1] kernel/sched/core.c:8511 sched_init+0x20/0x410
                            ^^^^^^^^^^^^^^^

I concatenated the condition string with the file string, so didn't 
have to extend the 'struct bug_entry' backend, and could be shared with 
the regular WARN() and BUG*() code as well without modifying its 
output.

The .text impact is zero, as hoped for:

       text       data        bss         dec        hex    filename
   29523998    7926322    1389904    38840224    250a7a0    vmlinux.before
   29523998    8024626    1389904    38938528    25227a0    vmlinue.after

So this does have the debugging advantages of SCHED_WARN_ON() and the 
code generation benefits of WARN_ON_ONCE().

Note that the patch has still the maturity of a Labradoodle puppy: it 
won't build on the majority of non-x86 architectures, has only been 
built and booted once, etc. - so it's not signed off on.

Thanks,

	Ingo

===================>
From: Ingo Molnar <mingo@kernel.org>
Date: Tue, 25 Mar 2025 12:18:44 +0100
Subject: [PATCH] bug: Add the condition string to the CONFIG_DEBUG_BUGVERBOSE=y output

       text       data        bss         dec        hex    filename
   29523998    7926322    1389904    38840224    250a7a0    vmlinux.before
   29523998    8024626    1389904    38938528    25227a0    vmlinue.after

Before:

  WARNING: CPU: 0 PID: 0 at kernel/sched/core.c:8511 sched_init+0x20/0x410

After:

  WARNING: CPU: 0 PID: 0 at [ptr == 0 && 1] kernel/sched/core.c:8511 sched_init+0x20/0x410
                            ^^^^^^^^^^^^^^^
Not-Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/Z-KRD3ODxT9f8Yjw@gmail.com
---
 arch/x86/include/asm/bug.h | 14 +++++++-------
 include/asm-generic/bug.h  |  7 ++++---
 kernel/sched/core.c        |  2 ++
 3 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index f0e9acf72547..e966199c8ef7 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -39,7 +39,7 @@
 
 #ifdef CONFIG_DEBUG_BUGVERBOSE
 
-#define _BUG_FLAGS(ins, flags, extra)					\
+#define _BUG_FLAGS(cond_str, ins, flags, extra)				\
 do {									\
 	asm_inline volatile("1:\t" ins "\n"				\
 		     ".pushsection __bug_table,\"aw\"\n"		\
@@ -50,14 +50,14 @@ do {									\
 		     "\t.org 2b+%c3\n"					\
 		     ".popsection\n"					\
 		     extra						\
-		     : : "i" (__FILE__), "i" (__LINE__),		\
+		     : : "i" (cond_str __FILE__), "i" (__LINE__),		\
 			 "i" (flags),					\
 			 "i" (sizeof(struct bug_entry)));		\
 } while (0)
 
 #else /* !CONFIG_DEBUG_BUGVERBOSE */
 
-#define _BUG_FLAGS(ins, flags, extra)					\
+#define _BUG_FLAGS(cond_str, ins, flags, extra)				\
 do {									\
 	asm_inline volatile("1:\t" ins "\n"				\
 		     ".pushsection __bug_table,\"aw\"\n"		\
@@ -74,7 +74,7 @@ do {									\
 
 #else
 
-#define _BUG_FLAGS(ins, flags, extra)  asm volatile(ins)
+#define _BUG_FLAGS(cond_str, ins, flags, extra)  asm volatile(ins)
 
 #endif /* CONFIG_GENERIC_BUG */
 
@@ -82,7 +82,7 @@ do {									\
 #define BUG()							\
 do {								\
 	instrumentation_begin();				\
-	_BUG_FLAGS(ASM_UD2, 0, "");				\
+	_BUG_FLAGS("", ASM_UD2, 0, "");				\
 	__builtin_unreachable();				\
 } while (0)
 
@@ -92,11 +92,11 @@ do {								\
  * were to trigger, we'd rather wreck the machine in an attempt to get the
  * message out than not know about it.
  */
-#define __WARN_FLAGS(flags)					\
+#define __WARN_FLAGS(cond_str, flags)				\
 do {								\
 	__auto_type __flags = BUGFLAG_WARNING|(flags);		\
 	instrumentation_begin();				\
-	_BUG_FLAGS(ASM_UD2, __flags, ANNOTATE_REACHABLE(1b));	\
+	_BUG_FLAGS(cond_str, ASM_UD2, __flags, ANNOTATE_REACHABLE(1b)); \
 	instrumentation_end();					\
 } while (0)
 
diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index 387720933973..c8e7126bc26e 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -100,17 +100,18 @@ extern __printf(1, 2) void __warn_printk(const char *fmt, ...);
 		instrumentation_end();					\
 	} while (0)
 #else
-#define __WARN()		__WARN_FLAGS(BUGFLAG_TAINT(TAINT_WARN))
+#define __WARN()		__WARN_FLAGS("", BUGFLAG_TAINT(TAINT_WARN))
 #define __WARN_printf(taint, arg...) do {				\
 		instrumentation_begin();				\
 		__warn_printk(arg);					\
-		__WARN_FLAGS(BUGFLAG_NO_CUT_HERE | BUGFLAG_TAINT(taint));\
+		__WARN_FLAGS("", BUGFLAG_NO_CUT_HERE | BUGFLAG_TAINT(taint));\
 		instrumentation_end();					\
 	} while (0)
 #define WARN_ON_ONCE(condition) ({				\
 	int __ret_warn_on = !!(condition);			\
 	if (unlikely(__ret_warn_on))				\
-		__WARN_FLAGS(BUGFLAG_ONCE |			\
+		__WARN_FLAGS("["#condition"] ",			\
+			     BUGFLAG_ONCE |			\
 			     BUGFLAG_TAINT(TAINT_WARN));	\
 	unlikely(__ret_warn_on);				\
 })
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 87540217fc09..71bf94bf68f8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8508,6 +8508,8 @@ void __init sched_init(void)
 	unsigned long ptr = 0;
 	int i;
 
+	WARN_ON_ONCE(ptr == 0 && 1);
+
 	/* Make sure the linker didn't screw up */
 #ifdef CONFIG_SMP
 	BUG_ON(!sched_class_above(&stop_sched_class, &dl_sched_class));