linux-kernel - Re: Build breakage caused by the use of UDB

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Message-ID: <20251218104659.GT3911114@noisy.programming.kicks-ass.net>
Date: Thu, 18 Dec 2025 11:46:59 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Uros Bizjak <ubizjak@...il.com>
Cc: Jean Delvare <jdelvare@...e.de>, LKML <linux-kernel@...r.kernel.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>, ardb@...nel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org
Subject: Re: Build breakage caused by the use of UDB

On Thu, Dec 18, 2025 at 11:38:53AM +0100, Peter Zijlstra wrote:
> On Thu, Dec 18, 2025 at 11:26:13AM +0100, Uros Bizjak wrote:
> 
> > I see... OTOH, a trivial solution would be to just use "ud2" mnemonic,
> > as proposed in [1].
> > 
> > [1] https://lore.kernel.org/lkml/20250909100038.1709394-1-ubizjak@gmail.com/
> 
> Yes, there is that. I suppose I worry about all the other _ASM_BYTES
> users, but since they don't actually show miscompilation, perhaps I
> should not worry.
> 
> Let me do a build with just using ud2 instead.

Yep, this builds. Let me stick a changelog on with Fixes and Cc stable
and stick it in x86/urgent.

---

diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index d561a8443c13..9b4e04690e1a 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -15,7 +15,7 @@ extern void __WARN_trap(struct bug_entry *bug, ...);
 /*
  * Despite that some emulators terminate on UD2, we use it for WARN().
  */
-#define ASM_UD2		_ASM_BYTES(0x0f, 0x0b)
+#define ASM_UD2		__ASM_FORM(ud2)
 #define INSN_UD2	0x0b0f
 #define LEN_UD2		2