[LTP] [PATCH v1] security/dirtyc0w_shmem: Fix remaining cases where UFFD_FEATURE_MINOR_SHMEM is absent

[Martin Doucha]

Hi,

On 25. 11. 22 11:52, David Hildenbrand wrote:
> When UFFD_FEATURE_MINOR_SHMEM is not defined, we still have to make the
> checkpoint happy, otherwise our parent process will run into a timeout.
> Further, we have to test if UFFD_FEATURE_MINOR_SHMEM is really returned by
> the UFFD_API ioctl: if the kernel knows about the feature but doesn't
> support it, it will be masked off.
> 
> Reported-by: Martin Doucha <mdoucha@suse.cz>
> Cc: Petr Vorel <pvorel@suse.cz>
> Cc: Cyril Hrubis <chrubis@suse.cz>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>   .../dirtyc0w_shmem/dirtyc0w_shmem_child.c        | 16 ++++++++++++----
>   1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/testcases/kernel/security/dirtyc0w_shmem/dirtyc0w_shmem_child.c b/testcases/kernel/security/dirtyc0w_shmem/dirtyc0w_shmem_child.c
> index cb2e9df0c..c117c6f39 100644
> --- a/testcases/kernel/security/dirtyc0w_shmem/dirtyc0w_shmem_child.c
> +++ b/testcases/kernel/security/dirtyc0w_shmem/dirtyc0w_shmem_child.c
> @@ -24,12 +24,12 @@
>   #include <linux/userfaultfd.h>
>   #endif
>   
> -#ifdef UFFD_FEATURE_MINOR_SHMEM
> -
>   #define TST_NO_DEFAULT_MAIN
>   #include "tst_test.h"
>   #include "tst_safe_macros.h"
>   #include "tst_safe_pthread.h"
> +
> +#ifdef UFFD_FEATURE_MINOR_SHMEM
>   #include "lapi/syscalls.h"
>   
>   #define TMP_DIR "tmp_dirtyc0w_shmem"
> @@ -162,6 +162,10 @@ retry:
>   			"Could not create userfault file descriptor");
>   	}
>   
> +	if (!(uffdio_api.features & UFFD_FEATURE_MINOR_SHMEM))
> +		tst_brk(TCONF,
> +			"System does not have userfaultfd minor fault support for shmem");
> +
>   	uffdio_register.range.start = (unsigned long) map;
>   	uffdio_register.range.len = page_size;
>   	uffdio_register.mode = UFFDIO_REGISTER_MODE_MINOR;
> @@ -236,6 +240,10 @@ int main(void)
>   	return 0;
>   }
>   #else /* UFFD_FEATURE_MINOR_SHMEM */
> -#include "tst_test.h"
> -TST_TEST_TCONF("System does not have userfaultfd minor fault support for shmem");
> +int main(void)
> +{
> +	tst_reinit();
> +	TST_CHECKPOINT_WAKE(0);
> +	tst_brk(TCONF, "System does not have userfaultfd minor fault support for shmem");
> +}
>   #endif /* UFFD_FEATURE_MINOR_SHMEM */

This would work as a workaround but I'd recommend adding the necessary 
structures and constants to include/lapi/userfaultfd.h instead. Then you 
won't need this conditional compilation at all.

I also recommend looking at the fuzzy sync library we use for race 
conditions:
https://github.com/linux-test-project/ltp/wiki/C-Test-API#133-reproducing-race-conditions

The original dirtyc0w test was written before this library but using it 
generally makes race condition reproducers faster, simpler and more 
reliable.

-- 
Martin Doucha   mdoucha@suse.cz
QA Engineer for Software Maintenance
SUSE LINUX, s.r.o.
CORSO IIa
Krizikova 148/34
186 00 Prague 8
Czech Republic