libbpf: Add various netlink helpers

This change introduces a few helpers to wrap open coded attribute
preparation in netlink.c. It also adds a libbpf_netlink_send_recv() that
is useful to wrap send + recv handling in a generic way. Subsequent patch
will also use this function for sending and receiving a netlink response.
The libbpf_nl_get_link() helper has been removed instead, moving socket
creation into the newly named libbpf_netlink_send_recv().

Every nested attribute's closure must happen using the helper
nlattr_end_nested(), which sets its length properly. NLA_F_NESTED is
enforced using nlattr_begin_nested() helper. Other simple attributes
can be added directly.

The maxsz parameter corresponds to the size of the request structure
which is being filled in, so for instance with req being:

  struct {
	struct nlmsghdr nh;
	struct tcmsg t;
	char buf[4096];
  } req;

Then, maxsz should be sizeof(req).

This change also converts the open coded attribute preparation with these
helpers. Note that the only failure the internal call to nlattr_add()
could result in the nested helper would be -EMSGSIZE, hence that is what
we return to our caller.

The libbpf_netlink_send_recv() call takes care of opening the socket,
sending the netlink message, receiving the response, potentially invoking
callbacks, and return errors if any, and then finally close the socket.
This allows users to avoid identical socket setup code in different places.
The only user of libbpf_nl_get_link() has been converted to make use of it.
__bpf_set_link_xdp_fd_replace() has also been refactored to use it.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
[ Daniel: major patch cleanup ]
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210512103451.989420-2-memxor@gmail.com

1 files changed, 48 insertions, 0 deletions

diff --git a/tools/lib/bpf/nlattr.h b/tools/lib/bpf/nlattr.h
index 6cc3ac91690f..3c780ab6d022 100644
--- a/tools/lib/bpf/nlattr.h
+++ b/tools/lib/bpf/nlattr.h
@@ -10,7 +10,10 @@
 #define __LIBBPF_NLATTR_H
 
 #include <stdint.h>
+#include <string.h>
+#include <errno.h>
 #include <linux/netlink.h>
+
 /* avoid multiple definition of netlink features */
 #define __LINUX_NETLINK_H
 
@@ -103,4 +106,49 @@ int libbpf_nla_parse_nested(struct nlattr *tb[], int maxtype,
 
 int libbpf_nla_dump_errormsg(struct nlmsghdr *nlh);
 
+static inline struct nlattr *nla_data(struct nlattr *nla)
+{
+	return (struct nlattr *)((char *)nla + NLA_HDRLEN);
+}
+
+static inline struct nlattr *nh_tail(struct nlmsghdr *nh)
+{
+	return (struct nlattr *)((char *)nh + NLMSG_ALIGN(nh->nlmsg_len));
+}
+
+static inline int nlattr_add(struct nlmsghdr *nh, size_t maxsz, int type,
+			     const void *data, int len)
+{
+	struct nlattr *nla;
+
+	if (NLMSG_ALIGN(nh->nlmsg_len) + NLA_ALIGN(NLA_HDRLEN + len) > maxsz)
+		return -EMSGSIZE;
+	if (!!data != !!len)
+		return -EINVAL;
+
+	nla = nh_tail(nh);
+	nla->nla_type = type;
+	nla->nla_len = NLA_HDRLEN + len;
+	if (data)
+		memcpy(nla_data(nla), data, len);
+	nh->nlmsg_len = NLMSG_ALIGN(nh->nlmsg_len) + NLA_ALIGN(nla->nla_len);
+	return 0;
+}
+
+static inline struct nlattr *nlattr_begin_nested(struct nlmsghdr *nh,
+						 size_t maxsz, int type)
+{
+	struct nlattr *tail;
+
+	tail = nh_tail(nh);
+	if (nlattr_add(nh, maxsz, type | NLA_F_NESTED, NULL, 0))
+		return NULL;
+	return tail;
+}
+
+static inline void nlattr_end_nested(struct nlmsghdr *nh, struct nlattr *tail)
+{
+	tail->nla_len = (char *)nh_tail(nh) - (char *)tail;
+}
+
 #endif /* __LIBBPF_NLATTR_H */