Re: [PATCH v3 4/7] kunit: Handle test faults

[David Gow]

On Tue, 19 Mar 2024 at 18:49, Mickaël Salaün <mic@xxxxxxxxxxx> wrote:
>
> Previously, when a kernel test thread crashed (e.g. NULL pointer
> dereference, general protection fault), the KUnit test hanged for 30
> seconds and exited with a timeout error.
>
> Fix this issue by waiting on task_struct->vfork_done instead of the
> custom kunit_try_catch.try_completion, and track the execution state by
> initially setting try_result with -EINTR and only setting it to 0 if
> the test passed.
>
> Fix kunit_generic_run_threadfn_adapter() signature by returning 0
> instead of calling kthread_complete_and_exit().  Because thread's exit
> code is never checked, always set it to 0 to make it clear.
>
> Fix the -EINTR error message, which couldn't be reached until now.
>
> This is tested with a following patch.
>
> Cc: Brendan Higgins <brendanhiggins@xxxxxxxxxx>
> Cc: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Reviewed-by: David Gow <davidgow@xxxxxxxxxx>
> Tested-by: Rae Moar <rmoar@xxxxxxxxxx>
> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx>
> Link: https://lore.kernel.org/r/20240319104857.70783-5-mic@xxxxxxxxxxx
> ---
>
> Changes since v2:
> * s/-EFAULT/-EINTR/ in commit message as spotted by Rae.
> * Add a comment explaining vfork_done as suggested by David.
> * Add David's Reviewed-by.
> * Add Rae's Tested-by.
>
> Changes since v1:
> * Add Kees's Reviewed-by.
> ---
>  include/kunit/try-catch.h |  3 ---
>  lib/kunit/try-catch.c     | 19 ++++++++++++-------
>  2 files changed, 12 insertions(+), 10 deletions(-)
>
> diff --git a/include/kunit/try-catch.h b/include/kunit/try-catch.h
> index c507dd43119d..7c966a1adbd3 100644
> --- a/include/kunit/try-catch.h
> +++ b/include/kunit/try-catch.h
> @@ -14,13 +14,11 @@
>
>  typedef void (*kunit_try_catch_func_t)(void *);
>
> -struct completion;
>  struct kunit;
>
>  /**
>   * struct kunit_try_catch - provides a generic way to run code which might fail.
>   * @test: The test case that is currently being executed.
> - * @try_completion: Completion that the control thread waits on while test runs.
>   * @try_result: Contains any errno obtained while running test case.
>   * @try: The function, the test case, to attempt to run.
>   * @catch: The function called if @try bails out.
> @@ -46,7 +44,6 @@ struct kunit;
>  struct kunit_try_catch {
>         /* private: internal use only. */
>         struct kunit *test;
> -       struct completion *try_completion;
>         int try_result;
>         kunit_try_catch_func_t try;
>         kunit_try_catch_func_t catch;
> diff --git a/lib/kunit/try-catch.c b/lib/kunit/try-catch.c
> index cab8b24b5d5a..7a3910dd78a6 100644
> --- a/lib/kunit/try-catch.c
> +++ b/lib/kunit/try-catch.c
> @@ -18,7 +18,7 @@
>  void __noreturn kunit_try_catch_throw(struct kunit_try_catch *try_catch)
>  {
>         try_catch->try_result = -EFAULT;
> -       kthread_complete_and_exit(try_catch->try_completion, -EFAULT);
> +       kthread_exit(0);

It turns out kthread_exit() is not exported, so this doesn't work if
KUnit is built as a module.

I think the options we have are:
- Add EXPORT_SYMBOL(kthread_exit).
- Keep using out own completion, and kthread_complete_and_exit()
- try_get_module() before spawning the thread, and use
module_put_and_kthread_exit().

I think all of these would be okay, but I could've missed something.

-- David

>  }
>  EXPORT_SYMBOL_GPL(kunit_try_catch_throw);
>
> @@ -26,9 +26,12 @@ static int kunit_generic_run_threadfn_adapter(void *data)
>  {
>         struct kunit_try_catch *try_catch = data;
>
> +       try_catch->try_result = -EINTR;
>         try_catch->try(try_catch->context);
> +       if (try_catch->try_result == -EINTR)
> +               try_catch->try_result = 0;
>
> -       kthread_complete_and_exit(try_catch->try_completion, 0);
> +       return 0;
>  }
>
>  static unsigned long kunit_test_timeout(void)
> @@ -58,13 +61,11 @@ static unsigned long kunit_test_timeout(void)
>
>  void kunit_try_catch_run(struct kunit_try_catch *try_catch, void *context)
>  {
> -       DECLARE_COMPLETION_ONSTACK(try_completion);
>         struct kunit *test = try_catch->test;
>         struct task_struct *task_struct;
>         int exit_code, time_remaining;
>
>         try_catch->context = context;
> -       try_catch->try_completion = &try_completion;
>         try_catch->try_result = 0;
>         task_struct = kthread_create(kunit_generic_run_threadfn_adapter,
>                                      try_catch, "kunit_try_catch_thread");
> @@ -75,8 +76,12 @@ void kunit_try_catch_run(struct kunit_try_catch *try_catch, void *context)
>         }
>         get_task_struct(task_struct);
>         wake_up_process(task_struct);
> -
> -       time_remaining = wait_for_completion_timeout(&try_completion,
> +       /*
> +        * As for a vfork(2), task_struct->vfork_done (pointing to the
> +        * underlying kthread->exited) can be used to wait for the end of a
> +        * kernel thread.
> +        */
> +       time_remaining = wait_for_completion_timeout(task_struct->vfork_done,
>                                                      kunit_test_timeout());
>         if (time_remaining == 0) {
>                 try_catch->try_result = -ETIMEDOUT;
> @@ -92,7 +97,7 @@ void kunit_try_catch_run(struct kunit_try_catch *try_catch, void *context)
>         if (exit_code == -EFAULT)
>                 try_catch->try_result = 0;
>         else if (exit_code == -EINTR)
> -               kunit_err(test, "wake_up_process() was never called\n");
> +               kunit_err(test, "try faulted\n");
>         else if (exit_code == -ETIMEDOUT)
>                 kunit_err(test, "try timed out\n");
>         else if (exit_code)
> --
> 2.44.0
>

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