linux-kernel - Re: [PATCH] kcov: reject open when kernel not instrumented

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Message-ID: <20160615163449.GA15332@leverpostej>
Date:	Wed, 15 Jun 2016 17:34:50 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Michal Marek <mmarek@...e.com>
Cc:	linux-kernel@...r.kernel.org,
	Alexander Potapenko <glider@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	James Morse <james.morse@....com>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] kcov: reject open when kernel not instrumented

On Wed, Jun 15, 2016 at 06:21:03PM +0200, Michal Marek wrote:
> Dne 15.6.2016 v 17:49 Mark Rutland napsal(a):
> > If the toolchain does not support -fsanitize-coverage=trace-pc, we blat
> > this option from CFLAGS_KCOV, and build the kernel without
> > instrumentation, even if CONFIG_KCOV was selected. However, we still
> > build the rest of the kcov infrastructure, and expose a kcov file under
> > debugfs. This can be confusing, as the kernel will appear to support
> > kcov, yet will never manage to sample any trace PC values. While we do
> > note this fact at build time, this may be missed, and a user may not
> > have access to build logs.
> > 
> > This patch adds an artificial CONFIG symbol, CONFIG_KCOV_CC, that is
> > only set when the toolchain supports -fsanitize-coverage=trace-pc, and
> > hence the kernel is built with instrumentation. When this is not the
> > case, the kernel will return -ENOTSUPP if userspace attempts to open the
> > kcov debugfs file, indicating that kcov functionality is unavailable.
> 
> Hi Mark,
> 
> please use a define outside the CONFIG_ namespace, because it is not a
> config option that one can find in .config or a Kconfig file. We already
> have CC_HAVE_ASM_GOTO, CC_USING_FENTRY and similar in the kernel.

Sure thing. I guess I should also not use IS_ENABLED, and will fall back
to the usual ifdefferty.

Dmitry, Alexander, any preference for the name? Is
CC_HAVE_SANCOV_TRACE_PC ok?

I guess we should also clean up arm64's current use of CONFIG_AS_LSE
(which I guess should be something like AS_HAVE_LSE per the
CC_HAVE_ASM_GOTO example). I can cook up a patch for that too.

Thanks,
Mark.