On Mon, Aug 19, 2019 at 04:41:11PM -0700, Kees Cook wrote:

> diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
> index 588dd59a5b72..da471fcc5487 100644
> --- a/include/asm-generic/bug.h
> +++ b/include/asm-generic/bug.h
> @@ -10,6 +10,7 @@
>  #define BUGFLAG_WARNING              (1 << 0)
>  #define BUGFLAG_ONCE         (1 << 1)
>  #define BUGFLAG_DONE         (1 << 2)
> +#define BUGFLAG_PRINTK               (1 << 3)
>  #define BUGFLAG_TAINT(taint) ((taint) << 8)
>  #define BUG_GET_TAINT(bug)   ((bug)->flags >> 8)
>  #endif

> diff --git a/lib/bug.c b/lib/bug.c
> index 1077366f496b..6c22e8a6f9de 100644
> --- a/lib/bug.c
> +++ b/lib/bug.c
> @@ -181,6 +181,15 @@ enum bug_trap_type report_bug(unsigned long bugaddr, 
> struct pt_regs *regs)
>               }
>       }
>  
> +     /*
> +      * BUG() and WARN_ON() families don't print a custom debug message
> +      * before triggering the exception handler, so we must add the
> +      * "cut here" line now. WARN() issues its own "cut here" before the
> +      * extra debugging message it writes before triggering the handler.
> +      */
> +     if ((bug->flags & BUGFLAG_PRINTK) == 0)
> +             printk(KERN_DEFAULT CUT_HERE);

I'm not loving that BUGFLAG_PRINTK name, BUGFLAG_CUT_HERE makes more
sense to me.

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