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Subject: Re: [PATCH] kcov: reject open when kernel not instrumented
To: Mark Rutland <mark.rutland@arm.com>
References: <1466005756-15626-1-git-send-email-mark.rutland@arm.com>
Cc: linux-kernel@vger.kernel.org, Alexander Potapenko <glider@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dmitry Vyukov <dvyukov@google.com>, James Morse <james.morse@arm.com>,
        Kees Cook <keescook@chromium.org>
From: Michal Marek <mmarek@suse.com>
Message-ID: <5761806F.5020103@suse.com>
Date: Wed, 15 Jun 2016 18:21:03 +0200
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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.

Thanks,
Michal