Re: [PATCH bpf-next v3 5/6] libbpf: Add bpf_get_link_xdp_info() function to get more XDP information

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:

> On Fri, Nov 8, 2019 at 4:01 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>
>> From: Toke Høiland-Jørgensen <toke@redhat.com>
>>
>> Currently, libbpf only provides a function to get a single ID for the XDP
>> program attached to the interface. However, it can be useful to get the
>> full set of program IDs attached, along with the attachment mode, in one
>> go. Add a new getter function to support this, using an extendible
>> structure to carry the information. Express the old bpf_get_link_id()
>> function in terms of the new function.
>>
>> Acked-by: David S. Miller <davem@davemloft.net>
>> Acked-by: Song Liu <songliubraving@fb.com>
>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
>> ---
>>  tools/lib/bpf/libbpf.h   |   10 ++++++
>>  tools/lib/bpf/libbpf.map |    1 +
>>  tools/lib/bpf/netlink.c  |   82 ++++++++++++++++++++++++++++++----------------
>>  3 files changed, 65 insertions(+), 28 deletions(-)
>>
>
> [...]
>
>>
>> -int bpf_get_link_xdp_id(int ifindex, __u32 *prog_id, __u32 flags)
>> +int bpf_get_link_xdp_info(int ifindex, struct xdp_link_info *info,
>> +                         size_t info_size, __u32 flags)
>>  {
>>         struct xdp_id_md xdp_id = {};
>>         int sock, ret;
>>         __u32 nl_pid;
>>         __u32 mask;
>>
>> -       if (flags & ~XDP_FLAGS_MASK)
>> +       if (flags & ~XDP_FLAGS_MASK || info_size < sizeof(*info))
>>                 return -EINVAL;
>
> Well, now it's backwards-incompatible: older program passes smaller
> (but previously perfectly valid) sizeof(struct xdp_link_info) to newer
> version of libbpf. This has to go both ways: smaller struct should be
> supported as long as program doesn't request (using flags) something,
> that can't be put into allowed space.

But there's nothing to be backwards-compatible with? I get that *when*
we extend the size of xdp_link_info, we should still accept the old,
smaller size. But in this case that cannot happen as we're only just
introducing this now?

-Toke