关于使用netlink message and attributes interface的建议
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author: jonathan
本文档的CopyRight归jonathan所有，可自由转载，转载时请保持文档的完整性。
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1 建议
再写netlink代码(无论是用户空间还是内核空间)其格式最好都按照netlink给出一套规则去做。不仅因为对netlink规则linux给出完整的数据结构和接口函数，而且这套规则符合“名称/属性”标准定义数据的方式，这种方式在rfc中有明确的定义，其灵活可靠。
我在使用过程中基本都是按照这个标准，而且把这种机制也灵活运用到其他项目中。
因此建议大家在写netlink相关项目时候，尽量参考标准netlink机制。
2 注意事项
这里仅简单说一下关于NLA_F_NESTED这个属性。
使用netlink Message and Attributes Interface时，使用nla_parse解析属性时要求每一个属性类型都是唯一的，也就是如下所示：
nlmsghdr | Pad | Family Header | Pad | attri1 | Pad | attri value | Pad | attri2 | Pad | attri value | Pad | attri3 | ...
其实，一个对象的属性本来就是唯一的。但是如果一个属性下面又分了多个属性，怎么办？NLA_F_NESTED粉末登场了。使用NLA_F_NESTED可以达到属性多级嵌套，其数据结构如下：
nlmsghdr | Pad | Family Header | Pad | attri1 | Pad | attri value | attri2 | Pad | attri:NLA_F_NESTED | Pad | attri2_1 | Pad | attri value | Pad | attri2_2 | Pad | ....
通过nla_parse得到attri2的属性值，再通过nla_parse_nested得到嵌套在attri2里面的属性值。
附录：
/* ========================================================================
* Netlink Messages and Attributes Interface (As Seen On TV)
* ------------------------------------------------------------------------
* Messages Interface
* ------------------------------------------------------------------------
*
* Message Format:
* <--- nlmsg_total_size(payload) --->
* <-- nlmsg_msg_size(payload) ->
* +----------+- - -+-------------+- - -+-------- - -
* | nlmsghdr | Pad | Payload | Pad | nlmsghdr
* +----------+- - -+-------------+- - -+-------- - -
* nlmsg_data(nlh)--^ ^
* nlmsg_next(nlh)----------------------+
*
* Payload Format:
* <---------------------- nlmsg_len(nlh) --------------------->
* <------ hdrlen ------> <- nlmsg_attrlen(nlh, hdrlen) ->
* +----------------------+- - -+--------------------------------+
* | Family Header | Pad | Attributes |
* +----------------------+- - -+--------------------------------+
* nlmsg_attrdata(nlh, hdrlen)--^
*
* Data Structures:
* struct nlmsghdr netlink message header
*
* Message Construction:
* nlmsg_new() create a new netlink message
* nlmsg_put() add a netlink message to an skb
* nlmsg_put_answer() callback based nlmsg_put()
* nlmsg_end() finanlize netlink message
* nlmsg_get_pos() return current position in message
* nlmsg_trim() trim part of message
* nlmsg_cancel() cancel message construction
* nlmsg_free() free a netlink message
*
* Message Sending:
* nlmsg_multicast() multicast message to several groups
* nlmsg_unicast() unicast a message to a single socket
* nlmsg_notify() send notification message
*
* Message Length Calculations:
* nlmsg_msg_size(payload) length of message w/o padding
* nlmsg_total_size(payload) length of message w/ padding
* nlmsg_padlen(payload) length of padding at tail
*
* Message Payload Access:
* nlmsg_data(nlh) head of message payload
* nlmsg_len(nlh) length of message payload
* nlmsg_attrdata(nlh, hdrlen) head of attributes data
* nlmsg_attrlen(nlh, hdrlen) length of attributes data
*
* Message Parsing:
* nlmsg_ok(nlh, remaining) does nlh fit into remaining bytes?
* nlmsg_next(nlh, remaining) get next netlink message
* nlmsg_parse() parse attributes of a message
* nlmsg_find_attr() find an attribute in a message
* nlmsg_for_each_msg() loop over all messages
* nlmsg_validate() validate netlink message incl. attrs
* nlmsg_for_each_attr() loop over all attributes
*
* Misc:
* nlmsg_report() report back to application?
*
* ------------------------------------------------------------------------
* Attributes Interface
* ------------------------------------------------------------------------
*
* Attribute Format:
* <------- nla_total_size(payload) ------->
* <---- nla_attr_size(payload) ----->
* +----------+- - -+- - - - - - - - - +- - -+-------- - -
* | Header | Pad | Payload | Pad | Header
* +----------+- - -+- - - - - - - - - +- - -+-------- - -
* <- nla_len(nla) -> ^
* nla_data(nla)----^ |
* nla_next(nla)-----------------------------'
*
* Data Structures:
* struct nlattr netlink attribute header
*
* Attribute Construction:
* nla_reserve(skb, type, len) reserve room for an attribute
* nla_reserve_nohdr(skb, len) reserve room for an attribute w/o hdr
* nla_put(skb, type, len, data) add attribute to skb
* nla_put_nohdr(skb, len, data) add attribute w/o hdr
* nla_append(skb, len, data) append data to skb
*
* Attribute Construction for Basic Types:
* nla_put_u8(skb, type, value) add u8 attribute to skb
* nla_put_u16(skb, type, value) add u16 attribute to skb
* nla_put_u32(skb, type, value) add u32 attribute to skb
* nla_put_u64(skb, type, value) add u64 attribute to skb
* nla_put_string(skb, type, str) add string attribute to skb
* nla_put_flag(skb, type) add flag attribute to skb
* nla_put_msecs(skb, type, jiffies) add msecs attribute to skb
*
* Exceptions Based Attribute Construction:
* NLA_PUT(skb, type, len, data) add attribute to skb
* NLA_PUT_U8(skb, type, value) add u8 attribute to skb
* NLA_PUT_U16(skb, type, value) add u16 attribute to skb
* NLA_PUT_U32(skb, type, value) add u32 attribute to skb
* NLA_PUT_U64(skb, type, value) add u64 attribute to skb
* NLA_PUT_STRING(skb, type, str) add string attribute to skb
* NLA_PUT_FLAG(skb, type) add flag attribute to skb
* NLA_PUT_MSECS(skb, type, jiffies) add msecs attribute to skb
*
* The meaning of these functions is equal to their lower case
* variants but they jump to the label nla_put_failure in case
* of a failure.
*
* Nested Attributes Construction:
* nla_nest_start(skb, type) start a nested attribute
* nla_nest_end(skb, nla) finalize a nested attribute
* nla_nest_compat_start(skb, type, start a nested compat attribute
* len, data)
* nla_nest_compat_end(skb, type) finalize a nested compat attribute
* nla_nest_cancel(skb, nla) cancel nested attribute construction
*
* Attribute Length Calculations:
* nla_attr_size(payload) length of attribute w/o padding
* nla_total_size(payload) length of attribute w/ padding
* nla_padlen(payload) length of padding
*
* Attribute Payload Access:
* nla_data(nla) head of attribute payload
* nla_len(nla) length of attribute payload
*
* Attribute Payload Access for Basic Types:
* 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
* nla_get_u64(nla) get payload for a u64 attribute
* nla_get_flag(nla) return 1 if flag is true
* nla_get_msecs(nla) get payload for a msecs attribute
*
* Attribute Misc:
* nla_memcpy(dest, nla, count) copy attribute into memory
* nla_memcmp(nla, data, size) compare attribute with memory area
* nla_strlcpy(dst, nla, size) copy attribute to a sized string
* nla_strcmp(nla, str) compare attribute with string
*
* Attribute Parsing:
* nla_ok(nla, remaining) does nla fit into remaining bytes?
* nla_next(nla, remaining) get next netlink attribute
* nla_validate() validate a stream of attributes
* nla_validate_nested() validate a stream of nested attributes
* nla_find() find attribute in stream of attributes
* nla_find_nested() find attribute in nested attributes
* nla_parse() parse and validate stream of attrs
* nla_parse_nested() parse nested attribuets
* nla_parse_nested_compat() parse nested compat attributes
* nla_for_each_attr() loop over all attributes
* nla_for_each_nested() loop over the nested attributes
*=========================================================================
*/
/**
* Standard attribute types to specify validation policy
*/
enum {
NLA_UNSPEC,
NLA_U8,
NLA_U16,
NLA_U32,
NLA_U64,
NLA_STRING,
NLA_FLAG,
NLA_MSECS,
NLA_NESTED,
NLA_NESTED_COMPAT,
NLA_NUL_STRING,
NLA_BINARY,
__NLA_TYPE_MAX,
};
#define NLA_TYPE_MAX (__NLA_TYPE_MAX - 1)
/**
* struct nla_policy - attribute validation policy
* @type: Type of attribute or NLA_UNSPEC
* @len: Type specific length of payload
*
* Policies are defined as arrays of this struct, the array must be
* accessible by attribute type up to the highest identifier to be expected.
*
* Meaning of `len' field:
* NLA_STRING Maximum length of string
* NLA_NUL_STRING Maximum length of string (excluding NUL)
* NLA_FLAG Unused
* NLA_BINARY Maximum length of attribute payload
* NLA_NESTED_COMPAT Exact length of structure payload
* All other Exact length of attribute payload
*
* Example:
* static struct nla_policy my_policy[ATTR_MAX+1] __read_mostly = {
* [ATTR_FOO] = { .type = NLA_U16 },
* [ATTR_BAR] = { .type = NLA_STRING, .len = BARSIZ },
* [ATTR_BAZ] = { .len = sizeof(struct mystruct) },
* };
*/
struct nla_policy {
u16 type;
u16 len;
};
/**
* struct nl_info - netlink source information
* @nlh: Netlink message header of original request
* @pid: Netlink PID of requesting application
*/
struct nl_info {
struct nlmsghdr *nlh;
struct net *nl_net;
u32 pid;
};
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