Re: [PATCH bpf-next v2 3/4] libbpf: Add function to set link XDP fd while specifying old fd

From: Toke Høiland-Jørgensen <redacted>

This adds a new function to set the XDP fd while specifying the old fd to
replace, using the newly added IFLA_XDP_EXPECTED_FD netlink parameter.

Signed-off-by: Toke Høiland-Jørgensen <redacted>
---
 tools/lib/bpf/libbpf.h   |    2 ++
 tools/lib/bpf/libbpf.map |    1 +
 tools/lib/bpf/netlink.c  |   22 +++++++++++++++++++++-
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index d38d7a629417..b5ca4f741e28 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h

@@ -445,6 +445,8 @@ struct xdp_link_info {
 };

 LIBBPF_API int bpf_set_link_xdp_fd(int ifindex, int fd, __u32 flags);
+LIBBPF_API int bpf_set_link_xdp_fd_replace(int ifindex, int fd, int old_fd,
+                                          __u32 flags);

 LIBBPF_API int bpf_get_link_xdp_id(int ifindex, __u32 *prog_id, __u32 flags);
 LIBBPF_API int bpf_get_link_xdp_info(int ifindex, struct xdp_link_info *info,
                                     size_t info_size, __u32 flags);

diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 5129283c0284..154f1d94fa63 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map

@@ -244,4 +244,5 @@ LIBBPF_0.0.8 {
                bpf_link__pin_path;
                bpf_link__unpin;
                bpf_program__set_attach_target;
+               bpf_set_link_xdp_fd_replace;
 } LIBBPF_0.0.7;

diff --git a/tools/lib/bpf/netlink.c b/tools/lib/bpf/netlink.c
index 431bd25c6cdb..39bd0ead1546 100644
--- a/tools/lib/bpf/netlink.c
+++ b/tools/lib/bpf/netlink.c

@@ -132,7 +132,8 @@ static int bpf_netlink_recv(int sock, __u32 nl_pid, int seq,
        return ret;
 }

-int bpf_set_link_xdp_fd(int ifindex, int fd, __u32 flags)
+static int __bpf_set_link_xdp_fd_replace(int ifindex, int fd, int old_fd,
+                                        __u32 flags)
 {
        int sock, seq = 0, ret;
        struct nlattr *nla, *nla_xdp;

@@ -178,6 +179,14 @@ int bpf_set_link_xdp_fd(int ifindex, int fd, __u32 flags)
                nla->nla_len += nla_xdp->nla_len;
        }

+       if (flags & XDP_FLAGS_EXPECT_FD) {
+               nla_xdp = (struct nlattr *)((char *)nla + nla->nla_len);
+               nla_xdp->nla_type = IFLA_XDP_EXPECTED_FD;
+               nla_xdp->nla_len = NLA_HDRLEN + sizeof(int);
+               memcpy((char *)nla_xdp + NLA_HDRLEN, &old_fd, sizeof(old_fd));
+               nla->nla_len += nla_xdp->nla_len;
+       }
+
        req.nh.nlmsg_len += NLA_ALIGN(nla->nla_len);

        if (send(sock, &req, req.nh.nlmsg_len, 0) < 0) {

@@ -191,6 +200,17 @@ int bpf_set_link_xdp_fd(int ifindex, int fd, __u32 flags)
        return ret;
 }

+int bpf_set_link_xdp_fd_replace(int ifindex, int fd, int old_fd, __u32 flags)
+{
+       return __bpf_set_link_xdp_fd_replace(ifindex, fd, old_fd,
+                                            flags | XDP_FLAGS_EXPECT_FD);
+}
+
+int bpf_set_link_xdp_fd(int ifindex, int fd, __u32 flags)
+{
+       return __bpf_set_link_xdp_fd_replace(ifindex, fd, -1, flags);
+}
+
 static int __dump_link_nlmsg(struct nlmsghdr *nlh,
                             libbpf_dump_nlmsg_t dump_link_nlmsg, void *cookie)
 {