Re: [PATCH v2] x86/bug: Handle __WARN_printf() trap in early_fixup_exception()

[Peter Zijlstra]

On Sat, Jan 10, 2026 at 11:47:37AM +0800, Hou Wenlong wrote:
> The commit 5b472b6e5bd9 ("x86_64/bug: Implement __WARN_printf()")
> implemented __WARN_printf(), which changed the mechanism to use UD1
> instead of UD2. However, it only handles the trap in the runtime IDT
> handler, while the early booting IDT handler lacks this handling. As a
> result, the usage of WARN() before the runtime IDT setup can lead to
> kernel crashes. 

Urgh, indeed. Clearly I don't see many early WARNs :/

> Since KMSAN is enabled after the runtime IDT setup, it
> is safe to use handle_bug() directly in early_fixup_exception() to
> address this issue.

I'm not sure I understand this. Should it not be that KMSAN is enabled
*before* early IDT to make this a non-issue?

> Fixes: 5b472b6e5bd9 ("x86_64/bug: Implement __WARN_printf()")
> Signed-off-by: Hou Wenlong <houwenlong.hwl@xxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/traps.h | 2 ++
>  arch/x86/kernel/traps.c      | 2 +-
>  arch/x86/mm/extable.c        | 7 ++-----
>  3 files changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
> index 869b88061801..3f24cc472ce9 100644
> --- a/arch/x86/include/asm/traps.h
> +++ b/arch/x86/include/asm/traps.h
> @@ -25,6 +25,8 @@ extern int ibt_selftest_noendbr(void);
>  void handle_invalid_op(struct pt_regs *regs);
>  #endif
>  
> +noinstr bool handle_bug(struct pt_regs *regs);
> +
>  static inline int get_si_code(unsigned long condition)
>  {
>  	if (condition & DR_STEP)
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index bcf1dedc1d00..aca1eca5daff 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -397,7 +397,7 @@ static inline void handle_invalid_op(struct pt_regs *regs)
>  		      ILL_ILLOPN, error_get_trap_addr(regs));
>  }
>  
> -static noinstr bool handle_bug(struct pt_regs *regs)
> +noinstr bool handle_bug(struct pt_regs *regs)
>  {
>  	unsigned long addr = regs->ip;
>  	bool handled = false;
> diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
> index 2fdc1f1f5adb..6b9ff1c6cafa 100644
> --- a/arch/x86/mm/extable.c
> +++ b/arch/x86/mm/extable.c
> @@ -411,14 +411,11 @@ void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
>  		return;
>  
>  	if (trapnr == X86_TRAP_UD) {
> -		if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
> -			/* Skip the ud2. */
> -			regs->ip += LEN_UD2;
> +		if (handle_bug(regs))
>  			return;
> -		}
>  
>  		/*
> -		 * If this was a BUG and report_bug returns or if this
> +		 * If this was a BUG and handle_bug returns or if this
>  		 * was just a normal #UD, we want to continue onward and
>  		 * crash.
>  		 */
> 
> base-commit: b7dccac786071bba98b0d834c517fd44a22c50f9
> -- 
> 2.31.1
>