mbuf_tags(9)

NAME

mbuf_tags - a framework for generic packet attributes

SYNOPSIS

#include <sys/mbuf.h>
struct m_tag *
m_tag_alloc(u_int32_t cookie, int type, int len, int wait);
struct m_tag *
m_tag_copy(struct m_tag *t, int how);
int
m_tag_copy_chain(struct mbuf *to,  struct  mbuf  *from,  int
how);
void
m_tag_delete(struct mbuf *m, struct m_tag *t);
void
m_tag_delete_chain(struct mbuf *m, struct m_tag *t);
void
m_tag_delete_nonpersistent(struct mbuf *m);
struct m_tag *
m_tag_find(struct mbuf *m, int type, struct m_tag *start);
struct m_tag *
m_tag_first(struct mbuf *m);
void
m_tag_free(struct m_tag *t);
struct m_tag *
m_tag_get(int type, int len, int wait);
void
m_tag_init(struct mbuf *m);
struct m_tag *
m_tag_locate(struct mbuf *m, u_int32_t cookie, int type,
        struct m_tag *t);
struct m_tag *
m_tag_next(struct mbuf *m, struct m_tag *t);
void
m_tag_prepend(struct mbuf *m, struct m_tag *t);
void
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 addi
tional 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. Typ
ical uses of mbuf tags include the storage of VLAN tags as described in
vlan(4), 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, in
cluding the copying of packet headers using facilities such as

M_COPY_PKTHDR

M_MOVE_PKTHDR(). Any tags associated with an mbuf header

will be automatically freed when the mbuf is freed, although some sub

systems 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 iden

tify a specific module or API. The packet tags are attached to mbuf

packet headers.

The first sizeof(struct m_tag) bytes of a tag contain a

struct m_tag:

struct m_tag {

SLIST_ENTRY(m_tag) m_tag_link; /* List of

packet tags */

u_int16_t m_tag_id; /* Tag ID */

u_int16_t m_tag_len; /* Length of

data */

u_int32_t m_tag_cookie; /* ABI/Mod

ule 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
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

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 OpenBSD does not support cookies, it needs

m_tag_id to be globally unique. To keep compatibility with OpenBSD, a cookie
MTAG_ABI_COMPAT is provided along with some compatibility

functions. When writing an OpenBSD compatible code, one should be care

ful not to take already used tag type. Tag types are defined in

Packet Tag Manipulation Functions

m_tag_alloc(cookie, type, len, wait) Allocate a new tag of type type and cookie cookie with

len bytes of

space following the tag header itself. The wait argu

ment is passed

directly to malloc(9). If successful, m_tag_alloc()

returns a memory buffer of (len + sizeof(struct m_tag)) bytes.

Otherwise, NULL

is returned. A compatibility function m_tag_get() is

also provided.

m_tag_copy(tag, how)

Allocate a copy of tag. The how argument is passed

directly to

m_tag_alloc(). The return values are the same as in

m_tag_alloc().

m_tag_copy_chain(tombuf, frommbuf, how) Allocate and copy all tags from mbuf frommbuf to mbuf

tombuf.

Returns 1 on success, and 0 on failure. In the latter

case, mbuf

tombuf loses all its tags, even previously present.

m_tag_delete(mbuf, tag)

Remove tag from mbuf's list and free it.

m_tag_delete_chain(mbuf, tag) Remove and free a packet tag chain, starting from tag.

If tag is

NULL, all tags are deleted.

m_tag_delete_nonpersistent(mbuf) Traverse mbuf's tags and delete those which do not

have the

MTAG_PERSISTENT flag set.

m_tag_first(mbuf)

Return the first tag associated with mbuf.

m_tag_free(tag)

Free tag using its m_tag_free method. The

m_tag_free_default()

function is used by default.

m_tag_init(mbuf)

Initialize the tag storage for packet mbuf.

m_tag_locate(mbuf, cookie, type, tag) Search for a tag defined by type and cookie in mbuf,

starting from

position specified by 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 m_tag_find() is also provided.

m_tag_next(mbuf, tag)

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

returned.

m_tag_prepend(mbuf, tag)

Add the new tag tag at the head of the tag list for

packet mbuf.

m_tag_unlink(mbuf, tag)

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

CODE REFERENCES

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

HISTORY

The packet tags first appeared in OpenBSD 2.9 and were writ
ten by Angelos D. Keromytis <angelos@openbsd.org>.
BSD November 18, 2004

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See also