Re: [PATCH v2 4/9] selftests/harness: Fix interleaved scheduling leading to race conditions

[Kees Cook <keescook@chromium.org>]

On Mon, Apr 29, 2024 at 03:09:26PM +0200, Mickaël Salaün wrote:
> Fix a race condition when running several FIXTURE_TEARDOWN() managing
> the same resource.  This fixes a race condition in the Landlock file
> system tests when creating or unmounting the same directory.
> 
> Using clone3() with CLONE_VFORK guarantees that the child and grandchild
> test processes are sequentially scheduled.  This is implemented with a
> new clone3_vfork() helper replacing the fork() call.
> 
> This avoids triggering this error in __wait_for_test():
>   Test ended in some other way [127]
> 
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: David S. Miller <davem@davemloft.net>
> Cc: Günther Noack <gnoack@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Mark Brown <broonie@kernel.org>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: Will Drewry <wad@chromium.org>
> Fixes: 41cca0542d7c ("selftests/harness: Fix TEST_F()'s vfork handling")
> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> Link: https://lore.kernel.org/r/20240429130931.2394118-5-mic@digikod.net
> ---
>  tools/testing/selftests/kselftest_harness.h | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/kselftest_harness.h b/tools/testing/selftests/kselftest_harness.h
> index 55699a762c45..9f04638707ae 100644
> --- a/tools/testing/selftests/kselftest_harness.h
> +++ b/tools/testing/selftests/kselftest_harness.h
> @@ -66,6 +66,8 @@
>  #include <sys/wait.h>
>  #include <unistd.h>
>  #include <setjmp.h>
> +#include <syscall.h>
> +#include <linux/sched.h>
>  
>  #include "kselftest.h"
>  
> @@ -80,6 +82,17 @@
>  #  define TH_LOG_ENABLED 1
>  #endif
>  
> +/* Wait for the child process to end but without sharing memory mapping. */
> +static pid_t __attribute__((__unused__)) clone3_vfork(void)

Why "unused"?

> +{
> +	struct clone_args args = {
> +		.flags = CLONE_VFORK,
> +		.exit_signal = SIGCHLD,
> +	};
> +
> +	return syscall(__NR_clone3, &args, sizeof(args));
> +}
> +
>  /**
>   * TH_LOG()
>   *
> @@ -1183,7 +1196,7 @@ void __run_test(struct __fixture_metadata *f,
>  	fflush(stdout);
>  	fflush(stderr);
>  
> -	t->pid = fork();
> +	t->pid = clone3_vfork();
>  	if (t->pid < 0) {
>  		ksft_print_msg("ERROR SPAWNING TEST CHILD\n");
>  		t->exit_code = KSFT_FAIL;

Regardless, yup, looks good.

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook