Re: [PATCH v2 08/14] kasan: add compiler barriers to KUNIT_EXPECT_KASAN_FAIL

[Marco Elver <elver@google.com>]

On Wed, 13 Jan 2021 at 17:22, Andrey Konovalov <andreyknvl@google.com> wrote:
>
> It might not be obvious to the compiler that the expression must be
> executed between writing and reading to fail_data. In this case, the
> compiler might reorder or optimize away some of the accesses, and
> the tests will fail.
>
> Add compiler barriers around the expression in KUNIT_EXPECT_KASAN_FAIL
> and use READ/WRITE_ONCE() for accessing fail_data fields.
>
> Link: https://linux-review.googlesource.com/id/I046079f48641a1d36fe627fc8827a9249102fd50
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

Reviewed-by: Marco Elver <elver@google.com>

> ---
>  lib/test_kasan.c  | 17 ++++++++++++-----
>  mm/kasan/report.c |  2 +-
>  2 files changed, 13 insertions(+), 6 deletions(-)
>
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index 5c8aa3a5ce93..283feda9882a 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -68,23 +68,30 @@ static void kasan_test_exit(struct kunit *test)
>   * normally auto-disabled. When this happens, this test handler reenables
>   * tag checking. As tag checking can be only disabled or enabled per CPU, this
>   * handler disables migration (preemption).
> + *
> + * Since the compiler doesn't see that the expression can change the fail_data
> + * fields, it can reorder or optimize away the accesses to those fields.
> + * Use READ/WRITE_ONCE() for the accesses and compiler barriers around the
> + * expression to prevent that.
>   */
>  #define KUNIT_EXPECT_KASAN_FAIL(test, expression) do {         \
>         if (IS_ENABLED(CONFIG_KASAN_HW_TAGS))                   \
>                 migrate_disable();                              \
> -       fail_data.report_expected = true;                       \
> -       fail_data.report_found = false;                         \
> +       WRITE_ONCE(fail_data.report_expected, true);            \
> +       WRITE_ONCE(fail_data.report_found, false);              \
>         kunit_add_named_resource(test,                          \
>                                 NULL,                           \
>                                 NULL,                           \
>                                 &resource,                      \
>                                 "kasan_data", &fail_data);      \
> +       barrier();                                              \
>         expression;                                             \
> +       barrier();                                              \
>         KUNIT_EXPECT_EQ(test,                                   \
> -                       fail_data.report_expected,              \
> -                       fail_data.report_found);                \
> +                       READ_ONCE(fail_data.report_expected),   \
> +                       READ_ONCE(fail_data.report_found));     \
>         if (IS_ENABLED(CONFIG_KASAN_HW_TAGS)) {                 \
> -               if (fail_data.report_found)                     \
> +               if (READ_ONCE(fail_data.report_found))          \
>                         hw_enable_tagging();                    \
>                 migrate_enable();                               \
>         }                                                       \
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index e93d7973792e..234f35a84f19 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -331,7 +331,7 @@ static void kasan_update_kunit_status(struct kunit *cur_test)
>         }
>
>         kasan_data = (struct kunit_kasan_expectation *)resource->data;
> -       kasan_data->report_found = true;
> +       WRITE_ONCE(kasan_data->report_found, true);
>         kunit_put_resource(resource);
>  }
>  #endif /* IS_ENABLED(CONFIG_KUNIT) */
> --
> 2.30.0.284.gd98b1dd5eaa7-goog
>