Merge branch 'net-introduce-nlmsg_payload-helper'

Breno Leitao says:

====================
net: Introduce nlmsg_payload helper

In the current codebase, there are multiple instances where the
structure size is checked before assigning it to a Netlink message. This
check is crucial for ensuring that the structure is correctly mapped
onto the Netlink message, providing a layer of security.

To streamline this process, Jakub Kicinski suggested creating a helper
function, `nlmsg_payload`, which verifies if the structure fits within
the message. If it does, the function returns the data; otherwise, it
returns NULL. This approach simplifies the code and reduces redundancy.

This patchset introduces the `nlmsg_payload` helper and updates several
parts of the code to use it. Further updates will follow in subsequent
patchsets.

v1: https://lore.kernel.org/20250411-nlmsg-v1-0-ddd4e065cb15@debian.org
====================

Link: https://patch.msgid.link/20250414-nlmsg-v2-0-3d90cb42c6af@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/include/net/netlink.h b/include/net/netlink.h
index 29e0db940382..82e07e272290 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -612,6 +612,22 @@ static inline int nlmsg_len(const struct nlmsghdr *nlh)
 }
 
 /**
+ * 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
  * @hdrlen: length of family specific header