SYNOPSIS

In sys/mbuf.h Ft struct m_tag * Fn m_tag_alloc uint32_t cookie int type int len int wait Ft struct m_tag * Fn m_tag_copy struct m_tag *t int how Ft int Fn m_tag_copy_chain struct mbuf *to struct mbuf *from int how Ft void Fn m_tag_delete struct mbuf *m struct m_tag *t Ft void Fn m_tag_delete_chain struct mbuf *m struct m_tag *t Ft void Fn m_tag_delete_nonpersistent struct mbuf *m Ft struct m_tag * Fn m_tag_find struct mbuf *m int type struct m_tag *start Ft struct m_tag * Fn m_tag_first struct mbuf *m Ft void Fn m_tag_free struct m_tag *t Ft struct m_tag * Fn m_tag_get int type int len int wait Ft void Fn m_tag_init struct mbuf *m Ft struct m_tag * Fn m_tag_locate struct mbuf *m uint32_t cookie int type struct m_tag *t Ft struct m_tag * Fn m_tag_next struct mbuf *m struct m_tag *t Ft void Fn m_tag_prepend struct mbuf *m struct m_tag *t Ft void Fn m_tag_unlink struct mbuf *m struct m_tag *t

DESCRIPTION

Mbuf tags allow additional meta-data to be associated with in-flight packets by providing a mechanism for the tagging of additional kernel memory onto packet header mbufs. Tags are maintained in chains off of the mbuf(9) header, and maintained using a series of API calls to allocate, search, and delete tags. Tags are identified using an ID and cookie that uniquely identify a class of data tagged onto the packet, and may contain an arbitrary amount of additional storage. Typical uses of mbuf tags include Mandatory Access Control (MAC) labels as described in mac(9), IPsec policy information as described in ipsec(4), and packet filter tags used by pf(4).

Tags will be maintained across a variety of operations, including the copying of packet headers using facilities such as Fn M_COPY_PKTHDR and Fn M_MOVE_PKTHDR . Any tags associated with an mbuf header will be automatically freed when the mbuf is freed, although some subsystems will wish to delete the tags prior to that time.

Packet tags are used by different kernel APIs to keep track of operations done or scheduled to happen to packets. Each packet tag can be distinguished by its type and cookie. The cookie is used to identify a specific module or API. The packet tags are attached to mbuf packet headers.

The first Fn sizeof struct m_tag bytes of a tag contain a Vt struct m_tag :

struct m_tag {
        SLIST_ENTRY(m_tag)      m_tag_link;     /* List of packet tags */
        uint16_t                m_tag_id;       /* Tag ID */
        uint16_t                m_tag_len;      /* Length of data */
        uint32_t                m_tag_cookie;   /* ABI/Module ID */
        void                    (*m_tag_free)(struct m_tag *);
};

The m_tag_link field is used to link tags together (see queue(3) for more details). The m_tag_id , m_tag_len and m_tag_cookie fields are set to type, length, and cookie, respectively. m_tag_free points to Fn m_tag_free_default . Following this structure are m_tag_len bytes of space that can be used to store tag-specific information. Addressing this data region may be tricky. A safe way is embedding Vt struct m_tag into a private data structure, as follows:

struct foo {
        struct m_tag    tag;
        ...
};
struct foo *p = (struct foo *)m_tag_alloc(...);
struct m_tag *mtag = &p->tag;

Note that Ox does not support cookies, it needs m_tag_id to be globally unique. To keep compatibility with Ox , a cookie MTAG_ABI_COMPAT is provided along with some compatibility functions. When writing an Ox compatible code, one should be careful not to take already used tag type. Tag types are defined in In sys/mbuf.h .

Packet Tag Manipulation Functions

Fn m_tag_alloc cookie type len wait

Allocate a new tag of type Fa type and cookie Fa cookie with len bytes of space following the tag header itself. The Fa wait argument is passed directly to malloc(9). If successful, Fn m_tag_alloc returns a memory buffer of (len + Fn sizeof struct m_tag ) bytes. Otherwise, NULL is returned. A compatibility function Fn m_tag_get is also provided.

Fn m_tag_copy tag how

Allocate a copy of Fa tag . The Fa how argument is passed directly to Fn m_tag_alloc . The return values are the same as in Fn m_tag_alloc .

Fn m_tag_copy_chain tombuf frommbuf how

Allocate and copy all tags from mbuf Fa frommbuf to mbuf Fa tombuf . Returns 1 on success, and 0 on failure. In the latter case, mbuf Fa tombuf loses all its tags, even previously present.

Fn m_tag_delete mbuf tag

Remove Fa tag from Fa mbuf Ns 's list and free it.

Fn m_tag_delete_chain mbuf tag

Remove and free a packet tag chain, starting from Fa tag . If Fa tag is NULL all tags are deleted.

Fn m_tag_delete_nonpersistent mbuf

Traverse Fa mbuf Ns 's tags and delete those which do not have the MTAG_PERSISTENT flag set.

Fn m_tag_first mbuf

Return the first tag associated with Fa mbuf .

Fn m_tag_free tag

Free Fa tag using its m_tag_free method. The Fn m_tag_free_default function is used by default.

Fn m_tag_init mbuf

Initialize the tag storage for packet Fa mbuf .

Fn m_tag_locate mbuf cookie type tag

Search for a tag defined by Fa type and Fa cookie in Fa mbuf , starting from position specified by Fa tag . If the latter is NULL then search through the whole list. Upon success, a pointer to the first found tag is returned. In either case, NULL is returned. A compatibility function Fn m_tag_find is also provided.

Fn m_tag_next mbuf tag

Return tag next to Fa tag in Fa mbuf . If absent, NULL is returned.

Fn m_tag_prepend mbuf tag

Add the new tag Fa tag at the head of the tag list for packet Fa mbuf .

Fn m_tag_unlink mbuf tag

Remove tag Fa tag from the list of tags of packet Fa mbuf .

CODE REFERENCES

The tag-manipulating code is contained in the file sys/kern/uipc_mbuf2.c Inlined functions are defined in In sys/mbuf.h .

HISTORY

The packet tags first appeared in Ox 2.9 and were written by An Angelos D. Keromytis Aq [email protected] .