Re: [PATCH v2 2/3] selftests/memfd_secret: add vmsplice() test

Miklos Szeredi
2024-03-26T15:10:27Z

On Tue, Mar 26, 2024 at 3:32 PM David Hildenbrand <david@redhat.com> wrote:
>
> Let's add a simple reproducer for a scenario where GUP-fast could succeed
> on secretmem folios, making vmsplice() succeed instead of failing. The
> reproducer is based on a reproducer [1] by Miklos Szeredi.
>
> We want to perform two tests: vmsplice() when a fresh page was just
> faulted in, and vmsplice() on an existing page after munmap() that
> would drain certain LRU caches/batches in the kernel.
>
> In an ideal world, we could use fallocate(FALLOC_FL_PUNCH_HOLE) /
> MADV_REMOVE to remove any existing page. As that is currently not
> possible, run the test before any other tests that would allocate memory
> in the secretmem fd.
>
> Perform the ftruncate() only once, and check the return value.
>
> [1] https://lkml.kernel.org/r/CAJfpegt3UCsMmxd0taOY11Uaw5U=eS1fE5dn0wZX3HF0oy8-oQ@mail.gmail.com
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> tools/testing/selftests/mm/memfd_secret.c | 51 ++++++++++++++++++++++-
> 1 file changed, 49 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/memfd_secret.c b/tools/testing/selftests/mm/memfd_secret.c
> index 9b298f6a04b3..9a0597310a76 100644
> --- a/tools/testing/selftests/mm/memfd_secret.c
> +++ b/tools/testing/selftests/mm/memfd_secret.c
> @@ -20,6 +20,7 @@
> #include <unistd.h>
> #include <errno.h>
> #include <stdio.h>
> +#include <fcntl.h>
>
> #include "../kselftest.h"
>
> @@ -83,6 +84,45 @@ static void test_mlock_limit(int fd)
> pass("mlock limit is respected\n");
> }
>
> +static void test_vmsplice(int fd, const char *desc)
> +{
> + ssize_t transferred;
> + struct iovec iov;
> + int pipefd[2];
> + char *mem;
> +
> + if (pipe(pipefd)) {
> + fail("pipe failed: %s\n", strerror(errno));
> + return;
> + }
> +
> + mem = mmap(NULL, page_size, prot, mode, fd, 0);
> + if (mem == MAP_FAILED) {
> + fail("Unable to mmap secret memory\n");
> + goto close_pipe;
> + }
> +
> + /*
> + * vmsplice() may use GUP-fast, which must also fail. Prefault the
> + * page table, so GUP-fast could find it.
> + */
> + memset(mem, PATTERN, page_size);

Shouldn't the non-prefault case be tested as well?

Thanks,
Miklos