Re: [PATCH net] bpf: don't zero out the info struct in bpf_obj_get_info_by_fd()

[Daniel Borkmann <daniel@iogearbox.net>]

On 07/26/2017 12:16 AM, Jakub Kicinski wrote:
> The buffer passed to bpf_obj_get_info_by_fd() should be initialized
> to zeros.  Kernel will enforce that to guarantee we can safely extend
> info structures in the future.
>
> Making the bpf_obj_get_info_by_fd() call in libbpf perform the zeroing
> is problematic, however, since some members of the info structures
> may need to be initialized by the callers (for instance pointers
> to buffers to which kernel is to dump translated and jited images).
>
> Remove the zeroing and fix up the in-tree callers before any kernel
> has been released with this code.
>
> As Daniel points out this seems to be the intended operation anyway,
> since commit 95b9afd3987f ("bpf: Test for bpf ID") is itself setting
> the buffer pointers before calling bpf_obj_get_info_by_fd().
>
> Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
> ---
> I have a small patch to add checking if kernel actually populated
> the instruction dumps which I will post after this ends up in net-next.
>
>   tools/lib/bpf/bpf.c                      | 1 -
>   tools/testing/selftests/bpf/test_progs.c | 8 ++++++--
>   2 files changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> index 412a7c82995a..256f571f2ab5 100644
> --- a/tools/lib/bpf/bpf.c
> +++ b/tools/lib/bpf/bpf.c
> @@ -314,7 +314,6 @@ int bpf_obj_get_info_by_fd(int prog_fd, void *info, __u32 *info_len)
>   	int err;
>
>   	bzero(&attr, sizeof(attr));
> -	bzero(info, *info_len);
>   	attr.info.bpf_fd = prog_fd;
>   	attr.info.info_len = *info_len;
>   	attr.info.info = ptr_to_u64(info);
> diff --git a/tools/testing/selftests/bpf/test_progs.c b/tools/testing/selftests/bpf/test_progs.c
> index 5855cd3d3d45..1f7dd35551b9 100644
> --- a/tools/testing/selftests/bpf/test_progs.c
> +++ b/tools/testing/selftests/bpf/test_progs.c
> @@ -340,6 +340,7 @@ static void test_bpf_obj_id(void)
>
>   		/* Check getting prog info */
>   		info_len = sizeof(struct bpf_prog_info) * 2;
> +		bzero(&prog_infos[i], info_len);
>   		prog_infos[i].jited_prog_insns = ptr_to_u64(jited_insns);
>   		prog_infos[i].jited_prog_len = sizeof(jited_insns);
>   		prog_infos[i].xlated_prog_insns = ptr_to_u64(xlated_insns);
> @@ -369,6 +370,7 @@ static void test_bpf_obj_id(void)
>
>   		/* Check getting map info */
>   		info_len = sizeof(struct bpf_map_info) * 2;
> +		bzero(&map_infos[i], info_len);
>   		err = bpf_obj_get_info_by_fd(map_fds[i], &map_infos[i],
>   					     &info_len);
>   		if (CHECK(err ||
> @@ -394,7 +396,7 @@ static void test_bpf_obj_id(void)
>   	nr_id_found = 0;
>   	next_id = 0;
>   	while (!bpf_prog_get_next_id(next_id, &next_id)) {
> -		struct bpf_prog_info prog_info;
> +		struct bpf_prog_info prog_info = {};
>   		int prog_fd;
>
>   		info_len = sizeof(prog_info);
> @@ -418,6 +420,8 @@ static void test_bpf_obj_id(void)
>   		nr_id_found++;
>
>   		err = bpf_obj_get_info_by_fd(prog_fd, &prog_info, &info_len);
> +		prog_infos[i].jited_prog_insns = 0;
> +		prog_infos[i].xlated_prog_insns = 0;

Can you elaborate why this one above is needed?

>   		CHECK(err || info_len != sizeof(struct bpf_prog_info) ||
>   		      memcmp(&prog_info, &prog_infos[i], info_len),
>   		      "get-prog-info(next_id->fd)",
> @@ -436,7 +440,7 @@ static void test_bpf_obj_id(void)
>   	nr_id_found = 0;
>   	next_id = 0;
>   	while (!bpf_map_get_next_id(next_id, &next_id)) {
> -		struct bpf_map_info map_info;
> +		struct bpf_map_info map_info = {};
>   		int map_fd;
>
>   		info_len = sizeof(map_info);
>