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

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Date:   Sat, 24 Aug 2019 10:17:39 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Christophe Leroy <christophe.leroy@....fr>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Drew Davenport <ddavenport@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, 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 Tue, Aug 20, 2019 at 09:14:29PM -0400, Steven Rostedt wrote:
> On Tue, 20 Aug 2019 12:58:49 +0200
> Christophe Leroy <christophe.leroy@....fr> wrote:
> 
> > >> 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.
> > >   
> > 
> > Actually it would be BUGFLAG_NO_CUT_HERE then, otherwise all arches not 
> > using the generic macros will have to add the flag to get the "cut here" 
> > line.
> >
> 
> Perhaps they all should be audited to see if they don't have the same
> problem?

As far as I could tell, all the other combinations end up either using
the slow path bug helpers or the common exception handler.
warn-with-a-fmt-string is the only case that does the "early cut here".

-- 
Kees Cook