netlink: add infrastructure to expose policies to userspace

Add, and use in generic netlink, helpers to dump out a netlink
policy to userspace, including all the range validation data,
nested policies etc.

This lets userspace discover what the kernel understands.

For families/commands other than generic netlink, the helpers
need to be used directly in an appropriate command, or we can
add some infrastructure (a new netlink family) that those can
register their policies with for introspection. I'm not that
familiar with non-generic netlink, so that's left out for now.

The data exposed to userspace also includes min and max length
for binary/string data, I've done that instead of letting the
userspace tools figure out whether min/max is intended based
on the type so that we can extend this later in the kernel, we
might want to just use the range data for example.

Because of this, I opted to not directly expose the NLA_*
values, even if some of them are already exposed via BPF, as
with min/max length we don't need to have different types here
for NLA_BINARY/NLA_MIN_LEN/NLA_EXACT_LEN, we just make them
all NL_ATTR_TYPE_BINARY with min/max length optionally set.

Similarly, we don't really need NLA_MSECS, and perhaps can
remove it in the future - but not if we encode it into the
userspace API now. It gets mapped to NL_ATTR_TYPE_U64 here.

Note that the exposing here corresponds to the strict policy
interpretation, and NLA_UNSPEC items are omitted entirely.
To get those, change them to NLA_MIN_LEN which behaves in
exactly the same way, but is exposed.

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

3 files changed, 111 insertions, 0 deletions

diff --git a/include/net/netlink.h b/include/net/netlink.h
index 557b67f1db99..c0411f14fb53 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -1933,4 +1933,10 @@ void nla_get_range_unsigned(const struct nla_policy *pt,
 void nla_get_range_signed(const struct nla_policy *pt,
 			  struct netlink_range_validation_signed *range);
 
+int netlink_policy_dump_start(const struct nla_policy *policy,
+			      unsigned int maxtype,
+			      unsigned long *state);
+bool netlink_policy_dump_loop(unsigned long *state);
+int netlink_policy_dump_write(struct sk_buff *skb, unsigned long state);
+
 #endif
diff --git a/include/uapi/linux/genetlink.h b/include/uapi/linux/genetlink.h
index 877f7fa95466..9c0636ec2286 100644
--- a/include/uapi/linux/genetlink.h
+++ b/include/uapi/linux/genetlink.h
@@ -48,6 +48,7 @@ enum {
 	CTRL_CMD_NEWMCAST_GRP,
 	CTRL_CMD_DELMCAST_GRP,
 	CTRL_CMD_GETMCAST_GRP, /* unused */
+	CTRL_CMD_GETPOLICY,
 	__CTRL_CMD_MAX,
 };
 
@@ -62,6 +63,7 @@ enum {
 	CTRL_ATTR_MAXATTR,
 	CTRL_ATTR_OPS,
 	CTRL_ATTR_MCAST_GROUPS,
+	CTRL_ATTR_POLICY,
 	__CTRL_ATTR_MAX,
 };
 
diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
index 0a4d73317759..eac8a6a648ea 100644
--- a/include/uapi/linux/netlink.h
+++ b/include/uapi/linux/netlink.h
@@ -249,4 +249,107 @@ struct nla_bitfield32 {
 	__u32 selector;
 };
 
+/*
+ * policy descriptions - it's specific to each family how this is used
+ * Normally, it should be retrieved via a dump inside another attribute
+ * specifying where it applies.
+ */
+
+/**
+ * enum netlink_attribute_type - type of an attribute
+ * @NL_ATTR_TYPE_INVALID: unused
+ * @NL_ATTR_TYPE_FLAG: flag attribute (present/not present)
+ * @NL_ATTR_TYPE_U8: 8-bit unsigned attribute
+ * @NL_ATTR_TYPE_U16: 16-bit unsigned attribute
+ * @NL_ATTR_TYPE_U32: 32-bit unsigned attribute
+ * @NL_ATTR_TYPE_U64: 64-bit unsigned attribute
+ * @NL_ATTR_TYPE_S8: 8-bit signed attribute
+ * @NL_ATTR_TYPE_S16: 16-bit signed attribute
+ * @NL_ATTR_TYPE_S32: 32-bit signed attribute
+ * @NL_ATTR_TYPE_S64: 64-bit signed attribute
+ * @NL_ATTR_TYPE_BINARY: binary data, min/max length may be specified
+ * @NL_ATTR_TYPE_STRING: string, min/max length may be specified
+ * @NL_ATTR_TYPE_NUL_STRING: NUL-terminated string,
+ *	min/max length may be specified
+ * @NL_ATTR_TYPE_NESTED: nested, i.e. the content of this attribute
+ *	consists of sub-attributes. The nested policy and maxtype
+ *	inside may be specified.
+ * @NL_ATTR_TYPE_NESTED_ARRAY: nested array, i.e. the content of this
+ *	attribute contains sub-attributes whose type is irrelevant
+ *	(just used to separate the array entries) and each such array
+ *	entry has attributes again, the policy for those inner ones
+ *	and the corresponding maxtype may be specified.
+ * @NL_ATTR_TYPE_BITFIELD32: &struct nla_bitfield32 attribute
+ */
+enum netlink_attribute_type {
+	NL_ATTR_TYPE_INVALID,
+
+	NL_ATTR_TYPE_FLAG,
+
+	NL_ATTR_TYPE_U8,
+	NL_ATTR_TYPE_U16,
+	NL_ATTR_TYPE_U32,
+	NL_ATTR_TYPE_U64,
+
+	NL_ATTR_TYPE_S8,
+	NL_ATTR_TYPE_S16,
+	NL_ATTR_TYPE_S32,
+	NL_ATTR_TYPE_S64,
+
+	NL_ATTR_TYPE_BINARY,
+	NL_ATTR_TYPE_STRING,
+	NL_ATTR_TYPE_NUL_STRING,
+
+	NL_ATTR_TYPE_NESTED,
+	NL_ATTR_TYPE_NESTED_ARRAY,
+
+	NL_ATTR_TYPE_BITFIELD32,
+};
+
+/**
+ * enum netlink_policy_type_attr - policy type attributes
+ * @NL_POLICY_TYPE_ATTR_UNSPEC: unused
+ * @NL_POLICY_TYPE_ATTR_TYPE: type of the attribute,
+ *	&enum netlink_attribute_type (U32)
+ * @NL_POLICY_TYPE_ATTR_MIN_VALUE_S: minimum value for signed
+ *	integers (S64)
+ * @NL_POLICY_TYPE_ATTR_MAX_VALUE_S: maximum value for signed
+ *	integers (S64)
+ * @NL_POLICY_TYPE_ATTR_MIN_VALUE_U: minimum value for unsigned
+ *	integers (U64)
+ * @NL_POLICY_TYPE_ATTR_MAX_VALUE_U: maximum value for unsigned
+ *	integers (U64)
+ * @NL_POLICY_TYPE_ATTR_MIN_LENGTH: minimum length for binary
+ *	attributes, no minimum if not given (U32)
+ * @NL_POLICY_TYPE_ATTR_MAX_LENGTH: maximum length for binary
+ *	attributes, no maximum if not given (U32)
+ * @NL_POLICY_TYPE_ATTR_POLICY_IDX: sub policy for nested and
+ *	nested array types (U32)
+ * @NL_POLICY_TYPE_ATTR_POLICY_MAXTYPE: maximum sub policy
+ *	attribute for nested and nested array types, this can
+ *	in theory be < the size of the policy pointed to by
+ *	the index, if limited inside the nesting (U32)
+ * @NL_POLICY_TYPE_ATTR_BITFIELD32_MASK: valid mask for the
+ *	bitfield32 type (U32)
+ * @NL_POLICY_TYPE_ATTR_PAD: pad attribute for 64-bit alignment
+ */
+enum netlink_policy_type_attr {
+	NL_POLICY_TYPE_ATTR_UNSPEC,
+	NL_POLICY_TYPE_ATTR_TYPE,
+	NL_POLICY_TYPE_ATTR_MIN_VALUE_S,
+	NL_POLICY_TYPE_ATTR_MAX_VALUE_S,
+	NL_POLICY_TYPE_ATTR_MIN_VALUE_U,
+	NL_POLICY_TYPE_ATTR_MAX_VALUE_U,
+	NL_POLICY_TYPE_ATTR_MIN_LENGTH,
+	NL_POLICY_TYPE_ATTR_MAX_LENGTH,
+	NL_POLICY_TYPE_ATTR_POLICY_IDX,
+	NL_POLICY_TYPE_ATTR_POLICY_MAXTYPE,
+	NL_POLICY_TYPE_ATTR_BITFIELD32_MASK,
+	NL_POLICY_TYPE_ATTR_PAD,
+
+	/* keep last */
+	__NL_POLICY_TYPE_ATTR_MAX,
+	NL_POLICY_TYPE_ATTR_MAX = __NL_POLICY_TYPE_ATTR_MAX - 1
+};
+
 #endif /* _UAPI__LINUX_NETLINK_H */