Re: [PATCH v2 08/12] x86/bug: Add BUG_FORMAT basics

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 25, 2025 at 08:27:50AM -0800, Linus Torvalds wrote:
> On Tue, 25 Nov 2025 at 07:17, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > Ard came up with this glorious hack :-)
> 
> "glorious".
> 
> Hmm. I _really_ don't love how it now uses that odd label across
> macros, it's not screaming "obvious" to me.

True.

> How bad would it be to just not use a NULL pointer at all, and instead
> make the "no format" be "empty format"?

Certainly, how does this look?

I verified it works on normal C code and that it compiles and the object
data looks right for Rust code.

---
diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index d0a96ff5c02c..4b5ab56903bb 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -86,7 +86,7 @@ extern void __WARN_trap(struct bug_entry *bug, ...);
 #ifdef CONFIG_DEBUG_BUGVERBOSE_DETAILED
 #define WARN_CONDITION_STR(cond_str) cond_str
 #else
-#define WARN_CONDITION_STR(cond_str) NULL
+#define WARN_CONDITION_STR(cond_str) ""
 #endif
 
 #define _BUG_FLAGS(cond_str, ins, flags, extra)				\
@@ -103,8 +103,12 @@ do {									\
 } while (0)
 
 #define ARCH_WARN_ASM(file, line, flags, size)				\
+	".pushsection .rodata.str1.1, \"aMS\", @progbits, 1\n"		\
+	"99:\n"								\
+	"\t.string \"\"\n"						\
+	".popsection\n"							\
 	"1:\t " ASM_UD2 "\n"						\
-	_BUG_FLAGS_ASM("0", file, line, flags, size, "")
+	_BUG_FLAGS_ASM("99b", file, line, flags, size, "")
 
 #else
 
diff --git a/lib/bug.c b/lib/bug.c
index 581a66b88c5c..edd9041f89f3 100644
--- a/lib/bug.c
+++ b/lib/bug.c
@@ -144,7 +144,17 @@ static const char *bug_get_format(struct bug_entry *bug)
 	const char *format = NULL;
 #ifdef HAVE_ARCH_BUG_FORMAT
 #ifdef CONFIG_GENERIC_BUG_RELATIVE_POINTERS
-	format = (const char *)&bug->format_disp + bug->format_disp;
+	/*
+	 * Allow an architecture to:
+	 *  - relative encode NULL (difficult vs KASLR);
+	 *  - use a literal 0 (there are no valid objects inside
+	 *    the __bug_table itself to refer to after all);
+	 *  - use an empty string.
+	 */
+	if (bug->format_disp)
+		format = (const char *)&bug->format_disp + bug->format_disp;
+	if (format && format[0] == '\0')
+		format = NULL;
 #else
 	format = bug->format;
 #endif