Re: [PATCH bpf-next] selftest/bpf: check invalid length in test_xdp_update_frags

[Yonghong Song <yhs@fb.com>]

On 2/4/22 5:58 AM, Lorenzo Bianconi wrote:
> Update test_xdp_update_frags adding a test for a buffer size
> set to (MAX_SKB_FRAGS + 2) * PAGE_SIZE. The kernel is supposed
> to return -ENOMEM.
> 
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> ---
>   .../bpf/prog_tests/xdp_adjust_frags.c         | 37 ++++++++++++++++++-
>   1 file changed, 36 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_adjust_frags.c b/tools/testing/selftests/bpf/prog_tests/xdp_adjust_frags.c
> index 134d0ac32f59..61d5b585eb15 100644
> --- a/tools/testing/selftests/bpf/prog_tests/xdp_adjust_frags.c
> +++ b/tools/testing/selftests/bpf/prog_tests/xdp_adjust_frags.c
> @@ -5,11 +5,12 @@
>   void test_xdp_update_frags(void)
>   {
>   	const char *file = "./test_xdp_update_frags.o";
> +	int err, prog_fd, max_skb_frags, buf_size, num;
>   	struct bpf_program *prog;
>   	struct bpf_object *obj;
> -	int err, prog_fd;
>   	__u32 *offset;
>   	__u8 *buf;
> +	FILE *f;
>   	LIBBPF_OPTS(bpf_test_run_opts, topts);
>   
>   	obj = bpf_object__open(file);
> @@ -99,6 +100,40 @@ void test_xdp_update_frags(void)
>   	ASSERT_EQ(buf[7621], 0xbb, "xdp_update_frag buf[7621]");
>   
>   	free(buf);
> +
> +	/* test_xdp_update_frags: unsupported buffer size */
> +	f = fopen("/proc/sys/net/core/max_skb_frags", "r");
> +	if (!ASSERT_OK_PTR(f, "max_skb_frag file pointer"))
> +		goto out;

In kernel, the nr_frags checking is against MAX_SKB_FRAGS,
but if /proc/sys/net/core/max_skb_flags is 2 or more less
than MAX_SKB_FRAGS, the test won't fail, right?

> +
> +	num = fscanf(f, "%d", &max_skb_frags);
> +	fclose(f);
> +
> +	if (!ASSERT_EQ(num, 1, "max_skb_frags read failed"))
> +		goto out;
> +
> +	/* xdp_buff linear area size is always set to 4096 in the
> +	 * bpf_prog_test_run_xdp routine.
> +	 */
> +	buf_size = 4096 + (max_skb_frags + 1) * sysconf(_SC_PAGE_SIZE);
> +	buf = malloc(buf_size);
> +	if (!ASSERT_OK_PTR(buf, "alloc buf"))
> +		goto out;
> +
> +	memset(buf, 0, buf_size);
> +	offset = (__u32 *)buf;
> +	*offset = 16;
> +	buf[*offset] = 0xaa;
> +	buf[*offset + 15] = 0xaa;
> +
> +	topts.data_in = buf;
> +	topts.data_out = buf;
> +	topts.data_size_in = buf_size;
> +	topts.data_size_out = buf_size;
> +
> +	err = bpf_prog_test_run_opts(prog_fd, &topts);
> +	ASSERT_EQ(err, -ENOMEM, "unsupported buffer size");
> +	free(buf);
>   out:
>   	bpf_object__close(obj);
>   }