netlink: Introduce nlmsg_payload helper

Create a new helper function, nlmsg_payload(), to simplify checking and
retrieving Netlink message payloads.

This reduces boilerplate code for users who need to verify the message
length before accessing its data.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250414-nlmsg-v2-1-3d90cb42c6af@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/include/net/netlink.h b/include/net/netlink.h
index 29e0db9403820cc3779c5d9808499ab5bdd31ae9..82e07e272290a970faba014444ab57d04f6ef91c 100644 (file)
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -611,6 +611,22 @@ static inline int nlmsg_len(const struct nlmsghdr *nlh)
        return nlh->nlmsg_len - NLMSG_HDRLEN;
 }
 
+/**
+ * nlmsg_payload - message payload if the data fits in the len
+ * @nlh: netlink message header
+ * @len: struct length
+ *
+ * Returns: The netlink message payload/data if the length is sufficient,
+ * otherwise NULL.
+ */
+static inline void *nlmsg_payload(const struct nlmsghdr *nlh, size_t len)
+{
+       if (nlh->nlmsg_len < nlmsg_msg_size(len))
+               return NULL;
+
+       return nlmsg_data(nlh);
+}
+
 /**
  * nlmsg_attrdata - head of attributes data
  * @nlh: netlink message header