Subject: Re: [PATCH 5/5] bpf: sockmap, sockhash: test looking up fds

From: Jakub Sitnicki <[email-masked]>
Date: Wed, 11 Mar 2020 14:52:39 +0100

On Tue, Mar 10, 2020 at 06:47 PM CET, Lorenz Bauer wrote:
> Make sure that looking up an element from the map succeeds,
> and that the fd is valid by using it an fcntl call.
>
> Signed-off-by: Lorenz Bauer <[email-masked]>
> ---
>  .../selftests/bpf/prog_tests/sockmap_listen.c | 26 ++++++++++++++-----
>  1 file changed, 20 insertions(+), 6 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/sockmap_listen.c b/tools/testing/selftests/bpf/prog_tests/sockmap_listen.c
> index 52aa468bdccd..929e1e77ecc6 100644
> --- a/tools/testing/selftests/bpf/prog_tests/sockmap_listen.c
> +++ b/tools/testing/selftests/bpf/prog_tests/sockmap_listen.c
> @@ -453,7 +453,7 @@ static void test_lookup_after_delete(int family, int sotype, int mapfd)
>  	xclose(s);
>  }
>
> -static void test_lookup_32_bit_value(int family, int sotype, int mapfd)
> +static void test_lookup_fd(int family, int sotype, int mapfd)
>  {
>  	u32 key, value32;
>  	int err, s;
> @@ -466,7 +466,7 @@ static void test_lookup_32_bit_value(int family, int sotype, int mapfd)
>  			       sizeof(value32), 1, 0);
>  	if (mapfd < 0) {
>  		FAIL_ERRNO("map_create");
> -		goto close;
> +		goto close_sock;
>  	}
>
>  	key = 0;
> @@ -475,11 +475,25 @@ static void test_lookup_32_bit_value(int family, int sotype, int mapfd)
>
>  	errno = 0;
>  	err = bpf_map_lookup_elem(mapfd, &key, &value32);
> -	if (!err || errno != ENOSPC)
> -		FAIL_ERRNO("map_lookup: expected ENOSPC");
> +	if (err) {
> +		FAIL_ERRNO("map_lookup");
> +		goto close_map;
> +	}
>
> +	if ((int)value32 == s) {
> +		FAIL("return value is identical");
> +		goto close;
> +	}
> +
> +	err = fcntl(value32, F_GETFD);
> +	if (err == -1)
> +		FAIL_ERRNO("fcntl");

I would call getsockopt()/getsockname() to assert that the FD lookup
succeeded.  We want to know not only that it's an FD (-EBADFD case), but
also that it's associated with a socket (-ENOTSOCK).

We can go even further, and compare socket cookies to ensure we got an
FD for the expected socket.

Also, I'm wondering if we could keep the -ENOSPC case test-covered by
temporarily dropping NET_ADMIN capability.

> +
> +close:
> +	xclose(value32);
> +close_map:
>  	xclose(mapfd);
> -close:
> +close_sock:
>  	xclose(s);
>  }
>
> @@ -1456,7 +1470,7 @@ static void test_ops(struct test_sockmap_listen *skel, struct bpf_map *map,
>  		/* lookup */
>  		TEST(test_lookup_after_insert),
>  		TEST(test_lookup_after_delete),
> -		TEST(test_lookup_32_bit_value),
> +		TEST(test_lookup_fd),
>  		/* update */
>  		TEST(test_update_existing),
>  		/* races with insert/delete */