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

From: Kees Cook <keescook@chromium.org>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Drew Davenport <ddavenport@chromium.org>,
	Arnd Bergmann <arnd@arndb.de>,
	"Steven Rostedt (VMware)" <rostedt@goodmis.org>,
	Feng Tang <feng.tang@intel.com>, Petr Mladek <pmladek@suse.com>,
	Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
	Borislav Petkov <bp@suse.de>, YueHaibing <yuehaibing@huawei.com>,
	linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 7/7] bug: Move WARN_ON() "cut here" into exception handler
Date: Tue, 20 Aug 2019 09:33:24 -0700	[thread overview]
Message-ID: <201908200908.6437DF5@keescook> (raw)
In-Reply-To: <06ba33fd-27cc-3816-1cdf-70616b1782dd@c-s.fr>

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