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

[Dmitry Vyukov]

On Wed, Jun 15, 2016 at 5:49 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 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@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 | 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@xxxxxxxxxxxxxx
>
> 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;


Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>

Thanks!