netlink: allow NLA_NESTED to specify nested policy to validate

Now that we have a validation_data pointer, and the len field in
the policy is unused for NLA_NESTED, we can allow using them both
to have nested validation. This can be nice in code, although we
still have to use nla_parse_nested() or similar which would also
take a policy; however, it also serves as documentation in the
policy without requiring a look at the code.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netlink.h

@@ -200,8 +200,10 @@ enum {
  *    NLA_NUL_STRING       Maximum length of string (excluding NUL)
  *    NLA_FLAG             Unused
  *    NLA_BINARY           Maximum length of attribute payload
- *    NLA_NESTED           Don't use `len' field -- length verification is
- *                         done by checking len of nested header (or empty)
+ *    NLA_NESTED           Length verification is done by checking len of
+ *                         nested header (or empty); len field is used if
+ *                         validation_data is also used, for the max attr
+ *                         number in the nested policy.
  *    NLA_U8, NLA_U16,
  *    NLA_U32, NLA_U64,
  *    NLA_S8, NLA_S16,
@@ -224,6 +226,10 @@ enum {
  *    NLA_REJECT           This attribute is always rejected and validation data
  *                         may point to a string to report as the error instead
  *                         of the generic one in extended ACK.
+ *    NLA_NESTED           Points to a nested policy to validate, must also set
+ *                         `len' to the max attribute number.
+ *                         Note that nla_parse() will validate, but of course not
+ *                         parse, the nested sub-policies.
  *    All other            Unused
  *
  * Example:
@@ -247,6 +253,9 @@ struct nla_policy {
 #define NLA_POLICY_ETH_ADDR		NLA_POLICY_EXACT_LEN(ETH_ALEN)
 #define NLA_POLICY_ETH_ADDR_COMPAT	NLA_POLICY_EXACT_LEN_WARN(ETH_ALEN)
 
+#define NLA_POLICY_NESTED(maxattr, policy) \
+	{ .type = NLA_NESTED, .validation_data = policy, .len = maxattr }
+
 /**
  * struct nl_info - netlink source information
  * @nlh: Netlink message header of original request