linux-kernel - Re: [PATCH 7/7] bug: Move WARN_ON() "cut here" into exception handler

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Message-ID: <20190820100638.GK2332@hirez.programming.kicks-ass.net>
Date:   Tue, 20 Aug 2019 12:06:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        Drew Davenport <ddavenport@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Feng Tang <feng.tang@...el.com>,
        Petr Mladek <pmladek@...e.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        YueHaibing <yuehaibing@...wei.com>, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/7] bug: Move WARN_ON() "cut here" into exception handler

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.