Re: [PATCH bpf] libbpf: Fix bpf_xdp_query() in old kernels

[Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>]

On Mon, Feb 27, 2023 at 2:50 PM Yonghong Song <yhs@xxxxxx> wrote:
>
> Commit 04d58f1b26a4("libbpf: add API to get XDP/XSK supported features")
> added feature_flags to struct bpf_xdp_query_opts. If a user uses
> bpf_xdp_query_opts with feature_flags member, the bpf_xdp_query()
> will check whether 'netdev' family exists or not in the kernel.
> If it does not exist, the bpf_xdp_query() will return -ENOENT.
>
> But 'netdev' family does not exist in old kernels as it is
> introduced in the same patch set as Commit 04d58f1b26a4.
> So old kernel with newer libbpf won't work properly with
> bpf_xdp_query() api call.
>
> To fix this issue, if the return value of
> libbpf_netlink_resolve_genl_family_id() is -ENOENT, bpf_xdp_query()
> will just return 0, skipping the rest of xdp feature query.
> This preserves backward compatibility.
>
> Fixes: 04d58f1b26a4 ("libbpf: add API to get XDP/XSK supported features")
> Cc: Lorenzo Bianconi <lorenzo@xxxxxxxxxx>
> Signed-off-by: Yonghong Song <yhs@xxxxxx>
> ---
>  tools/lib/bpf/netlink.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/netlink.c b/tools/lib/bpf/netlink.c
> index 1653e7a8b0a1..4c1b3502f88d 100644
> --- a/tools/lib/bpf/netlink.c
> +++ b/tools/lib/bpf/netlink.c
> @@ -468,8 +468,11 @@ int bpf_xdp_query(int ifindex, int xdp_flags, struct bpf_xdp_query_opts *opts)
>                 return 0;
>
>         err = libbpf_netlink_resolve_genl_family_id("netdev", sizeof("netdev"), &id);
> -       if (err < 0)
> +       if (err < 0) {
> +               if (err == -ENOENT)
> +                       return 0;
>                 return libbpf_err(err);
> +       }
>

As I mentioned in another thread, I'm a bit worried of this early
return, because query_opts might be extended and then we'll forget
about this early return. So I did these changes and pushed to
bpf-next:

diff --git a/tools/lib/bpf/netlink.c b/tools/lib/bpf/netlink.c
index 4c1b3502f88d..84dd5fa14905 100644
--- a/tools/lib/bpf/netlink.c
+++ b/tools/lib/bpf/netlink.c
@@ -469,8 +469,10 @@ int bpf_xdp_query(int ifindex, int xdp_flags,
struct bpf_xdp_query_opts *opts)

        err = libbpf_netlink_resolve_genl_family_id("netdev",
sizeof("netdev"), &id);
        if (err < 0) {
-               if (err == -ENOENT)
-                       return 0;
+               if (err == -ENOENT) {
+                       opts->feature_flags = 0;
+                       goto skip_feature_flags;
+               }
                return libbpf_err(err);
        }

@@ -492,6 +494,7 @@ int bpf_xdp_query(int ifindex, int xdp_flags,
struct bpf_xdp_query_opts *opts)

        opts->feature_flags = md.flags;

+skip_feature_flags:
        return 0;
 }

>         memset(&req, 0, sizeof(req));
>         req.nh.nlmsg_len = NLMSG_LENGTH(GENL_HDRLEN);
> --
> 2.30.2
>