[PATCH] kcov: reject open when kernel not instrumented

[Mark Rutland <mark.rutland@arm.com>]

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.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: James Morse <james.morse@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Michal Marek <mmarek@suse.com>
Cc: linux-kernel@vger.kernel.org
---
 Makefile      | 2 +-
 kernel/kcov.c | 8 ++++++++
 2 files changed, 9 insertions(+), 1 deletion(-)

As discussed [1] in reply to the arm64 patch.

[1] http://lkml.kernel.org/r/CACT4Y+Z3=juvxBJBXRh5PgE35twFRxg-3iMc-owenONU84x5XQ@mail.gmail.com

Mark.

diff --git a/Makefile b/Makefile
index 0f70de6..e6ef260 100644
--- a/Makefile
+++ b/Makefile
@@ -369,7 +369,7 @@ LDFLAGS_MODULE  =
 CFLAGS_KERNEL	=
 AFLAGS_KERNEL	=
 CFLAGS_GCOV	= -fprofile-arcs -ftest-coverage -fno-tree-loop-im -Wno-maybe-uninitialized
-CFLAGS_KCOV	= -fsanitize-coverage=trace-pc
+CFLAGS_KCOV	= -fsanitize-coverage=trace-pc -DCONFIG_KCOV_CC
 
 
 # Use USERINCLUDE when you must reference the UAPI directories only.
diff --git a/kernel/kcov.c b/kernel/kcov.c
index a02f2dd..df2cafd 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -3,6 +3,7 @@
 #define DISABLE_BRANCH_PROFILING
 #include <linux/compiler.h>
 #include <linux/types.h>
+#include <linux/errno.h>
 #include <linux/file.h>
 #include <linux/fs.h>
 #include <linux/mm.h>
@@ -160,6 +161,13 @@ static int kcov_open(struct inode *inode, struct file *filep)
 {
 	struct kcov *kcov;
 
+	/*
+	 * CONFIG_KCOV was selected, but the compiler does not support the
+	 * options KCOV requires.
+	 */
+	if (!IS_ENABLED(CONFIG_KCOV_CC))
+		return -ENOTSUPP;
+
 	kcov = kzalloc(sizeof(*kcov), GFP_KERNEL);
 	if (!kcov)
 		return -ENOMEM;
-- 
1.9.1