netlink: make nla_nest_start() add NLA_F_NESTED flag

Even if the NLA_F_NESTED flag was introduced more than 11 years ago, most
netlink based interfaces (including recently added ones) are still not
setting it in kernel generated messages. Without the flag, message parsers
not aware of attribute semantics (e.g. wireshark dissector or libmnl's
mnl_nlmsg_fprintf()) cannot recognize nested attributes and won't display
the structure of their contents.

Unfortunately we cannot just add the flag everywhere as there may be
userspace applications which check nlattr::nla_type directly rather than
through a helper masking out the flags. Therefore the patch renames
nla_nest_start() to nla_nest_start_noflag() and introduces nla_nest_start()
as a wrapper adding NLA_F_NESTED. The calls which add NLA_F_NESTED manually
are rewritten to use nla_nest_start().

Except for changes in include/net/netlink.h, the patch was generated using
this semantic patch:

@@ expression E1, E2; @@
-nla_nest_start(E1, E2)
+nla_nest_start_noflag(E1, E2)

@@ expression E1, E2; @@
-nla_nest_start_noflag(E1, E2 | NLA_F_NESTED)
+nla_nest_start(E1, E2)

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netlink.h

@@ -1415,13 +1415,18 @@ static inline void *nla_memdup(const struct nlattr *src, gfp_t gfp)
 }
 
 /**
- * nla_nest_start - Start a new level of nested attributes
+ * nla_nest_start_noflag - Start a new level of nested attributes
  * @skb: socket buffer to add attributes to
  * @attrtype: attribute type of container
  *
- * Returns the container attribute
+ * This function exists for backward compatibility to use in APIs which never
+ * marked their nest attributes with NLA_F_NESTED flag. New APIs should use
+ * nla_nest_start() which sets the flag.
+ *
+ * Returns the container attribute or NULL on error
  */
-static inline struct nlattr *nla_nest_start(struct sk_buff *skb, int attrtype)
+static inline struct nlattr *nla_nest_start_noflag(struct sk_buff *skb,
+						   int attrtype)
 {
 	struct nlattr *start = (struct nlattr *)skb_tail_pointer(skb);
 
@@ -1431,6 +1436,21 @@ static inline struct nlattr *nla_nest_start(struct sk_buff *skb, int attrtype)
 	return start;
 }
 
+/**
+ * nla_nest_start - Start a new level of nested attributes, with NLA_F_NESTED
+ * @skb: socket buffer to add attributes to
+ * @attrtype: attribute type of container
+ *
+ * Unlike nla_nest_start_noflag(), mark the nest attribute with NLA_F_NESTED
+ * flag. This is the preferred function to use in new code.
+ *
+ * Returns the container attribute or NULL on error
+ */
+static inline struct nlattr *nla_nest_start(struct sk_buff *skb, int attrtype)
+{
+	return nla_nest_start_noflag(skb, attrtype | NLA_F_NESTED);
+}
+
 /**
  * nla_nest_end - Finalize nesting of attributes
  * @skb: socket buffer the attributes are stored in