Re: [PATCHv2] kcov: reject open when kernel not instrumented

[Dmitry Vyukov]

On Wed, Jun 15, 2016 at 7:04 PM, Mark Rutland <mark.rutland@xxxxxxx> 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.
>
> This patch ensures that CC_HAVE_SANCOV_TRACE_PC is defined when the
> toolchain supports -fsanitize-coverage=trace-pc, and is not defined
> otherwise. When CC_HAVE_SANCOV_TRACE_PC is not defined, 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@xxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: James Morse <james.morse@xxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Michal Marek <mmarek@xxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  Makefile      | 2 +-
>  kernel/kcov.c | 9 +++++++++
>  2 files changed, 10 insertions(+), 1 deletion(-)
>
> Since v1 [1]:
> * Use CC_HAVE_SANCOV_TRACE_PC rather than CONFIG_KCOV_CC
>
> [1] lkml.kernel.org/r/1466005756-15626-1-git-send-email-mark.rutland@xxxxxxx
>
> diff --git a/Makefile b/Makefile
> index 0f70de6..3785a63 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 -DCC_HAVE_SANCOV_TRACE_PC
>
>
>  # Use USERINCLUDE when you must reference the UAPI directories only.
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index a02f2dd..0a0b164 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 +161,14 @@ static int kcov_open(struct inode *inode, struct file *filep)
>  {
>         struct kcov *kcov;
>
> +#ifndef CC_HAVE_SANCOV_TRACE_PC
> +       /*
> +        * CONFIG_KCOV was selected, but the compiler does not support the
> +        * options KCOV requires.
> +        */
> +       return -ENOTSUPP;
> +#endif /* CC_HAVE_SANCOV_TRACE_PC */
> +
>         kcov = kzalloc(sizeof(*kcov), GFP_KERNEL);
>         if (!kcov)
>                 return -ENOMEM;


Looks good to me.