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

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Message-ID: <201908200908.6437DF5@keescook>
Date:   Tue, 20 Aug 2019 09:33:24 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 Tue, Aug 20, 2019 at 12:58:49PM +0200, Christophe Leroy wrote:
> Le 20/08/2019 à 12:06, Peter Zijlstra a écrit :
> > 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.

That's fine -- easy rename. :)

> 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.

I am testing for the lack of the flag (so that only the
CONFIG_GENERIC_BUG with __WARN_FLAGS case needs to set it). I was
thinking of the flag to mean "this reporting flow has already issued
cut-here". It sounds like it would be more logical to have it named
BUGFLAG_NO_CUT_HERE to mean "do not issue a cut-here; it has already
happened"? I will update the patch.

Thanks!

-- 
Kees Cook