netlink: add variable-length / auto integers

We currently push everyone to use padding to align 64b values
in netlink. Un-padded nla_put_u64() doesn't even exist any more.

The story behind this possibly start with this thread:
https://lore.kernel.org/netdev/20121204.130914.1457976839967676240.davem@davemloft.net/
where DaveM was concerned about the alignment of a structure
containing 64b stats. If user space tries to access such struct
directly:

	struct some_stats *stats = nla_data(attr);
	printf("A: %llu", stats->a);

lack of alignment may become problematic for some architectures.
These days we most often put every single member in a separate
attribute, meaning that the code above would use a helper like
nla_get_u64(), which can deal with alignment internally.
Even for arches which don't have good unaligned access - access
aligned to 4B should be pretty efficient.
Kernel and well known libraries deal with unaligned input already.

Padded 64b is quite space-inefficient (64b + pad means at worst 16B
per attr vs 32b which takes 8B). It is also more typing:

    if (nla_put_u64_pad(rsp, NETDEV_A_SOMETHING_SOMETHING,
                        value, NETDEV_A_SOMETHING_PAD))

Create a new attribute type which will use 32 bits at netlink
level if value is small enough (probably most of the time?),
and (4B-aligned) 64 bits otherwise. Kernel API is just:

    if (nla_put_uint(rsp, NETDEV_A_SOMETHING_SOMETHING, value))

Calling this new type "just" sint / uint with no specific size
will hopefully also make people more comfortable with using it.
Currently telling people "don't use u8, you may need the bits,
and netlink will round up to 4B, anyway" is the #1 comment
we give to newcomers.

In terms of netlink layout it looks like this:

         0       4       8       12      16
32b:     [nlattr][ u32  ]
64b:     [  pad ][nlattr][     u64      ]
uint(32) [nlattr][ u32  ]
uint(64) [nlattr][     u64      ]

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 67 insertions, 2 deletions

diff --git a/include/net/netlink.h b/include/net/netlink.h
index 8a7cd1170e1f7..aba2b162a2260 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -128,6 +128,8 @@
  *   nla_len(nla)			length of attribute payload
  *
  * Attribute Payload Access for Basic Types:
+ *   nla_get_uint(nla)			get payload for a uint attribute
+ *   nla_get_sint(nla)			get payload for a sint attribute
  *   nla_get_u8(nla)			get payload for a u8 attribute
  *   nla_get_u16(nla)			get payload for a u16 attribute
  *   nla_get_u32(nla)			get payload for a u32 attribute
@@ -183,6 +185,8 @@ enum {
 	NLA_REJECT,
 	NLA_BE16,
 	NLA_BE32,
+	NLA_SINT,
+	NLA_UINT,
 	__NLA_TYPE_MAX,
 };
 
@@ -229,6 +233,7 @@ enum nla_policy_validation {
  *                         nested header (or empty); len field is used if
  *                         nested_policy is also used, for the max attr
  *                         number in the nested policy.
+ *    NLA_SINT, NLA_UINT,
  *    NLA_U8, NLA_U16,
  *    NLA_U32, NLA_U64,
  *    NLA_S8, NLA_S16,
@@ -260,12 +265,14 @@ enum nla_policy_validation {
  *                         while an array has the nested attributes at another
  *                         level down and the attribute types directly in the
  *                         nesting don't matter.
+ *    NLA_UINT,
  *    NLA_U8,
  *    NLA_U16,
  *    NLA_U32,
  *    NLA_U64,
  *    NLA_BE16,
  *    NLA_BE32,
+ *    NLA_SINT,
  *    NLA_S8,
  *    NLA_S16,
  *    NLA_S32,
@@ -280,6 +287,7 @@ enum nla_policy_validation {
  *                         or NLA_POLICY_FULL_RANGE_SIGNED() macros instead.
  *                         Use the NLA_POLICY_MIN(), NLA_POLICY_MAX() and
  *                         NLA_POLICY_RANGE() macros.
+ *    NLA_UINT,
  *    NLA_U8,
  *    NLA_U16,
  *    NLA_U32,
@@ -288,6 +296,7 @@ enum nla_policy_validation {
  *                         to a struct netlink_range_validation that indicates
  *                         the min/max values.
  *                         Use NLA_POLICY_FULL_RANGE().
+ *    NLA_SINT,
  *    NLA_S8,
  *    NLA_S16,
  *    NLA_S32,
@@ -377,9 +386,11 @@ struct nla_policy {
 
 #define __NLA_IS_UINT_TYPE(tp)					\
 	(tp == NLA_U8 || tp == NLA_U16 || tp == NLA_U32 ||	\
-	 tp == NLA_U64 || tp == NLA_BE16 || tp == NLA_BE32)
+	 tp == NLA_U64 || tp == NLA_UINT ||			\
+	 tp == NLA_BE16 || tp == NLA_BE32)
 #define __NLA_IS_SINT_TYPE(tp)						\
-	(tp == NLA_S8 || tp == NLA_S16 || tp == NLA_S32 || tp == NLA_S64)
+	(tp == NLA_S8 || tp == NLA_S16 || tp == NLA_S32 || tp == NLA_S64 || \
+	 tp == NLA_SINT)
 
 #define __NLA_ENSURE(condition) BUILD_BUG_ON_ZERO(!(condition))
 #define NLA_ENSURE_UINT_TYPE(tp)			\
@@ -1358,6 +1369,22 @@ static inline int nla_put_u32(struct sk_buff *skb, int attrtype, u32 value)
 }
 
 /**
+ * nla_put_uint - Add a variable-size unsigned int to a socket buffer
+ * @skb: socket buffer to add attribute to
+ * @attrtype: attribute type
+ * @value: numeric value
+ */
+static inline int nla_put_uint(struct sk_buff *skb, int attrtype, u64 value)
+{
+	u64 tmp64 = value;
+	u32 tmp32 = value;
+
+	if (tmp64 == tmp32)
+		return nla_put_u32(skb, attrtype, tmp32);
+	return nla_put(skb, attrtype, sizeof(u64), &tmp64);
+}
+
+/**
  * nla_put_be32 - Add a __be32 netlink attribute to a socket buffer
  * @skb: socket buffer to add attribute to
  * @attrtype: attribute type
@@ -1512,6 +1539,22 @@ static inline int nla_put_s64(struct sk_buff *skb, int attrtype, s64 value,
 }
 
 /**
+ * nla_put_sint - Add a variable-size signed int to a socket buffer
+ * @skb: socket buffer to add attribute to
+ * @attrtype: attribute type
+ * @value: numeric value
+ */
+static inline int nla_put_sint(struct sk_buff *skb, int attrtype, s64 value)
+{
+	s64 tmp64 = value;
+	s32 tmp32 = value;
+
+	if (tmp64 == tmp32)
+		return nla_put_s32(skb, attrtype, tmp32);
+	return nla_put(skb, attrtype, sizeof(s64), &tmp64);
+}
+
+/**
  * nla_put_string - Add a string netlink attribute to a socket buffer
  * @skb: socket buffer to add attribute to
  * @attrtype: attribute type
@@ -1668,6 +1711,17 @@ static inline u64 nla_get_u64(const struct nlattr *nla)
 }
 
 /**
+ * nla_get_uint - return payload of uint attribute
+ * @nla: uint netlink attribute
+ */
+static inline u64 nla_get_uint(const struct nlattr *nla)
+{
+	if (nla_len(nla) == sizeof(u32))
+		return nla_get_u32(nla);
+	return nla_get_u64(nla);
+}
+
+/**
  * nla_get_be64 - return payload of __be64 attribute
  * @nla: __be64 netlink attribute
  */
@@ -1730,6 +1784,17 @@ static inline s64 nla_get_s64(const struct nlattr *nla)
 }
 
 /**
+ * nla_get_sint - return payload of uint attribute
+ * @nla: uint netlink attribute
+ */
+static inline s64 nla_get_sint(const struct nlattr *nla)
+{
+	if (nla_len(nla) == sizeof(s32))
+		return nla_get_s32(nla);
+	return nla_get_s64(nla);
+}
+
+/**
  * nla_get_flag - return payload of flag attribute
  * @nla: flag netlink attribute
  */