linux-kernel - Re: [PATCHv3] kcov: reject open when kernel not instrumented

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Date:	Fri, 17 Jun 2016 16:53:19 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Alexander Potapenko <glider@...gle.com>
Cc:	Kees Cook <keescook@...omium.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	James Morse <james.morse@....com>,
	Michal Marek <mmarek@...e.com>
Subject: Re: [PATCHv3] kcov: reject open when kernel not instrumented

On Fri, Jun 17, 2016 at 05:46:38PM +0200, Alexander Potapenko wrote:
> On Fri, Jun 17, 2016 at 5:42 PM, Kees Cook <keescook@...omium.org> wrote:
> > On Fri, Jun 17, 2016 at 2:39 AM, Mark Rutland <mark.rutland@....com> wrote:
> >> 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.
> >
> > Do you want to refuse to build if the compiler doesn't support the
> > flag? I finally figured out how to do this, I think, for
> > -fstack-protector:
> >
> > http://git.kernel.org/cgit/linux/kernel/git/kees/linux.git/commit/?h=kbuild/stackprotector&id=600c1bd5f8647a8470dc2fc5a8697e3eafb5fd52
> >
> > If you wanted, the CONFIG_KCOV test could live under the same
> > prepare-compiler-check target.
> >
> > -Kees
> >
> I suspect the intention here is not to abort the build, but to have an
> opportunity to report at run time that the kernel has been built
> without kcov support.

My concern was that it was easy to miss the build-time warning, and
having the kcov file visible and accessible didn't make clear to the
user that the feature won't actually work.

I'm happy for a failed kcov_open or anything stronger than that (e.g.
the kcov file not bring present, or the build being aborted).

Thanks,
Mark.