OpenWrt – Rev 4
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/*
* lib/attr.c Netlink Attributes
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation version 2.1
* of the License.
*
* Copyright (c) 2003-2008 Thomas Graf <tgraf@suug.ch>
*/
#include <netlink-local.h>
#include <netlink/netlink.h>
#include <netlink/utils.h>
#include <netlink/addr.h>
#include <netlink/attr.h>
#include <netlink/msg.h>
#include <linux/socket.h>
/**
* @ingroup msg
* @defgroup attr Attributes
* Netlink Attributes Construction/Parsing Interface
*
* \section attr_sec Netlink Attributes
* Netlink attributes allow for data chunks of arbitary length to be
* attached to a netlink message. Each attribute is encoded with a
* type and length field, both 16 bits, stored in the attribute header
* preceding the attribute data. The main advantage of using attributes
* over packing everything into the family header is that the interface
* stays extendable as new attributes can supersede old attributes while
* remaining backwards compatible. Also attributes can be defined optional
* thus avoiding the transmission of unnecessary empty data blocks.
* Special nested attributes allow for more complex data structures to
* be transmitted, e.g. trees, lists, etc.
*
* While not required, netlink attributes typically follow the family
* header of a netlink message and must be properly aligned to NLA_ALIGNTO:
* @code
* +----------------+- - -+---------------+- - -+------------+- - -+
* | Netlink Header | Pad | Family Header | Pad | Attributes | Pad |
* +----------------+- - -+---------------+- - -+------------+- - -+
* @endcode
*
* The actual attributes are chained together each separately aligned to
* NLA_ALIGNTO. The position of an attribute is defined based on the
* length field of the preceding attributes:
* @code
* +-------------+- - -+-------------+- - -+------
* | Attribute 1 | Pad | Attribute 2 | Pad | ...
* +-------------+- - -+-------------+- - -+------
* nla_next(attr1)------^
* @endcode
*
* The attribute itself consists of the attribute header followed by
* the actual payload also aligned to NLA_ALIGNTO. The function nla_data()
* returns a pointer to the start of the payload while nla_len() returns
* the length of the payload in bytes.
*
* \b Note: Be aware, NLA_ALIGNTO equals to 4 bytes, therefore it is not
* safe to dereference any 64 bit data types directly.
*
* @code
* <----------- nla_total_size(payload) ----------->
* <-------- nla_attr_size(payload) --------->
* +------------------+- - -+- - - - - - - - - +- - -+
* | Attribute Header | Pad | Payload | Pad |
* +------------------+- - -+- - - - - - - - - +- - -+
* nla_data(nla)-------------^
* <- nla_len(nla) ->
* @endcode
*
* @subsection attr_datatypes Attribute Data Types
* A number of basic data types are supported to simplify access and
* validation of netlink attributes. This data type information is
* not encoded in the attribute, both the kernel and userspace part
* are required to share this information on their own.
*
* One of the major advantages of these basic types is the automatic
* validation of each attribute based on an attribute policy. The
* validation covers most of the checks required to safely use
* attributes and thus keeps the individual sanity check to a minimum.
*
* Never access attribute payload without ensuring basic validation
* first, attributes may:
* - not be present even though required
* - contain less actual payload than expected
* - fake a attribute length which exceeds the end of the message
* - contain unterminated character strings
*
* Policies are defined as array of the struct nla_policy. The array is
* indexed with the attribute type, therefore the array must be sized
* accordingly.
* @code
* static struct nla_policy my_policy[ATTR_MAX+1] = {
* [ATTR_FOO] = { .type = ..., .minlen = ..., .maxlen = ... },
* };
*
* err = nla_validate(attrs, attrlen, ATTR_MAX, &my_policy);
* @endcode
*
* Some basic validations are performed on every attribute, regardless of type.
* - If the attribute type exceeds the maximum attribute type specified or
* the attribute type is lesser-or-equal than zero, the attribute will
* be silently ignored.
* - If the payload length falls below the \a minlen value the attribute
* will be rejected.
* - If \a maxlen is non-zero and the payload length exceeds the \a maxlen
* value the attribute will be rejected.
*
*
* @par Unspecific Attribute (NLA_UNSPEC)
* This is the standard type if no type is specified. It is used for
* binary data of arbitary length. Typically this attribute carries
* a binary structure or a stream of bytes.
* @par
* @code
* // In this example, we will assume a binary structure requires to
* // be transmitted. The definition of the structure will typically
* // go into a header file available to both the kernel and userspace
* // side.
* //
* // Note: Be careful when putting 64 bit data types into a structure.
* // The attribute payload is only aligned to 4 bytes, dereferencing
* // the member may fail.
* struct my_struct {
* int a;
* int b;
* };
*
* // The validation function will not enforce an exact length match to
* // allow structures to grow as required. Note: While it is allowed
* // to add members to the end of the structure, changing the order or
* // inserting members in the middle of the structure will break your
* // binary interface.
* static struct nla_policy my_policy[ATTR_MAX+1] = {
* [ATTR_MY_STRICT] = { .type = NLA_UNSPEC,
* .minlen = sizeof(struct my_struct) },
*
* // The binary structure is appened to the message using nla_put()
* struct my_struct foo = { .a = 1, .b = 2 };
* nla_put(msg, ATTR_MY_STRUCT, sizeof(foo), &foo);
*
* // On the receiving side, a pointer to the structure pointing inside
* // the message payload is returned by nla_get().
* if (attrs[ATTR_MY_STRUCT])
* struct my_struct *foo = nla_get(attrs[ATTR_MY_STRUCT]);
* @endcode
*
* @par Integers (NLA_U8, NLA_U16, NLA_U32, NLA_U64)
* Integers come in different sizes from 8 bit to 64 bit. However, since the
* payload length is aligned to 4 bytes, integers smaller than 32 bit are
* only useful to enforce the maximum range of values.
* @par
* \b Note: There is no difference made between signed and unsigned integers.
* The validation only enforces the minimal payload length required to store
* an integer of specified type.
* @par
* @code
* // Even though possible, it does not make sense to specify .minlen or
* // .maxlen for integer types. The data types implies the corresponding
* // minimal payload length.
* static struct nla_policy my_policy[ATTR_MAX+1] = {
* [ATTR_FOO] = { .type = NLA_U32 },
*
* // Numeric values can be appended directly using the respective
* // nla_put_uxxx() function
* nla_put_u32(msg, ATTR_FOO, 123);
*
* // Same for the receiving side.
* if (attrs[ATTR_FOO])
* uint32_t foo = nla_get_u32(attrs[ATTR_FOO]);
* @endcode
*
* @par Character string (NLA_STRING)
* This data type represents a NUL terminated character string of variable
* length. For binary data streams the type NLA_UNSPEC is recommended.
* @par
* @code
* // Enforce a NUL terminated character string of at most 4 characters
* // including the NUL termination.
* static struct nla_policy my_policy[ATTR_MAX+1] = {
* [ATTR_BAR] = { .type = NLA_STRING, maxlen = 4 },
*
* // nla_put_string() creates a string attribute of the necessary length
* // and appends it to the message including the NUL termination.
* nla_put_string(msg, ATTR_BAR, "some text");
*
* // It is safe to use the returned character string directly if the
* // attribute has been validated as the validation enforces the proper
* // termination of the string.
* if (attrs[ATTR_BAR])
* char *text = nla_get_string(attrs[ATTR_BAR]);
* @endcode
*
* @par Flag (NLA_FLAG)
* This attribute type may be used to indicate the presence of a flag. The
* attribute is only valid if the payload length is zero. The presence of
* the attribute header indicates the presence of the flag.
* @par
* @code
* // This attribute type is special as .minlen and .maxlen have no effect.
* static struct nla_policy my_policy[ATTR_MAX+1] = {
* [ATTR_FLAG] = { .type = NLA_FLAG },
*
* // nla_put_flag() appends a zero sized attribute to the message.
* nla_put_flag(msg, ATTR_FLAG);
*
* // There is no need for a receival function, the presence is the value.
* if (attrs[ATTR_FLAG])
* // flag is present
* @endcode
*
* @par Micro Seconds (NLA_MSECS)
*
* @par Nested Attribute (NLA_NESTED)
* Attributes can be nested and put into a container to create groups, lists
* or to construct trees of attributes. Nested attributes are often used to
* pass attributes to a subsystem where the top layer has no knowledge of the
* configuration possibilities of each subsystem.
* @par
* \b Note: When validating the attributes using nlmsg_validate() or
* nlmsg_parse() it will only affect the top level attributes. Each
* level of nested attributes must be validated seperately using
* nla_parse_nested() or nla_validate().
* @par
* @code
* // The minimal length policy may be used to enforce the presence of at
* // least one attribute.
* static struct nla_policy my_policy[ATTR_MAX+1] = {
* [ATTR_OPTS] = { .type = NLA_NESTED, minlen = NLA_HDRLEN },
*
* // Nested attributes are constructed by enclosing the attributes
* // to be nested with calls to nla_nest_start() respetively nla_nest_end().
* struct nlattr *opts = nla_nest_start(msg, ATTR_OPTS);
* nla_put_u32(msg, ATTR_FOO, 123);
* nla_put_string(msg, ATTR_BAR, "some text");
* nla_nest_end(msg, opts);
*
* // Various methods exist to parse nested attributes, the easiest being
* // nla_parse_nested() which also allows validation in the same step.
* if (attrs[ATTR_OPTS]) {
* struct nlattr *nested[ATTR_MAX+1];
*
* nla_parse_nested(nested, ATTR_MAX, attrs[ATTR_OPTS], &policy);
*
* if (nested[ATTR_FOO])
* uint32_t foo = nla_get_u32(nested[ATTR_FOO]);
* }
* @endcode
*
* @subsection attr_exceptions Exception Based Attribute Construction
* Often a large number of attributes are added to a message in a single
* function. In order to simplify error handling, a second set of
* construction functions exist which jump to a error label when they
* fail instead of returning an error code. This second set consists
* of macros which are named after their error code based counterpart
* except that the name is written all uppercase.
*
* All of the macros jump to the target \c nla_put_failure if they fail.
* @code
* void my_func(struct nl_msg *msg)
* {
* NLA_PUT_U32(msg, ATTR_FOO, 10);
* NLA_PUT_STRING(msg, ATTR_BAR, "bar");
*
* return 0;
*
* nla_put_failure:
* return -NLE_NOMEM;
* }
* @endcode
*
* @subsection attr_examples Examples
* @par Example 1.1 Constructing a netlink message with attributes.
* @code
* struct nl_msg *build_msg(int ifindex, struct nl_addr *lladdr, int mtu)
* {
* struct nl_msg *msg;
* struct nlattr *info, *vlan;
* struct ifinfomsg ifi = {
* .ifi_family = AF_INET,
* .ifi_index = ifindex,
* };
*
* // Allocate a new netlink message, type=RTM_SETLINK, flags=NLM_F_ECHO
* if (!(msg = nlmsg_alloc_simple(RTM_SETLINK, NLM_F_ECHO)))
* return NULL;
*
* // Append the family specific header (struct ifinfomsg)
* if (nlmsg_append(msg, &ifi, sizeof(ifi), NLMSG_ALIGNTO) < 0)
* goto nla_put_failure
*
* // Append a 32 bit integer attribute to carry the MTU
* NLA_PUT_U32(msg, IFLA_MTU, mtu);
*
* // Append a unspecific attribute to carry the link layer address
* NLA_PUT_ADDR(msg, IFLA_ADDRESS, lladdr);
*
* // Append a container for nested attributes to carry link information
* if (!(info = nla_nest_start(msg, IFLA_LINKINFO)))
* goto nla_put_failure;
*
* // Put a string attribute into the container
* NLA_PUT_STRING(msg, IFLA_INFO_KIND, "vlan");
*
* // Append another container inside the open container to carry
* // vlan specific attributes
* if (!(vlan = nla_nest_start(msg, IFLA_INFO_DATA)))
* goto nla_put_failure;
*
* // add vlan specific info attributes here...
*
* // Finish nesting the vlan attributes and close the second container.
* nla_nest_end(msg, vlan);
*
* // Finish nesting the link info attribute and close the first container.
* nla_nest_end(msg, info);
*
* return msg;
*
* // If any of the construction macros fails, we end up here.
* nla_put_failure:
* nlmsg_free(msg);
* return NULL;
* }
* @endcode
*
* @par Example 2.1 Parsing a netlink message with attributes.
* @code
* int parse_message(struct nl_msg *msg)
* {
* // The policy defines two attributes: a 32 bit integer and a container
* // for nested attributes.
* struct nla_policy attr_policy[ATTR_MAX+1] = {
* [ATTR_FOO] = { .type = NLA_U32 },
* [ATTR_BAR] = { .type = NLA_NESTED },
* };
* struct nlattr *attrs[ATTR_MAX+1];
* int err;
*
* // The nlmsg_parse() function will make sure that the message contains
* // enough payload to hold the header (struct my_hdr), validates any
* // attributes attached to the messages and stores a pointer to each
* // attribute in the attrs[] array accessable by attribute type.
* if ((err = nlmsg_parse(nlmsg_hdr(msg), sizeof(struct my_hdr), attrs,
* ATTR_MAX, attr_policy)) < 0)
* goto errout;
*
* if (attrs[ATTR_FOO]) {
* // It is safe to directly access the attribute payload without
* // any further checks since nlmsg_parse() enforced the policy.
* uint32_t foo = nla_get_u32(attrs[ATTR_FOO]);
* }
*
* if (attrs[ATTR_BAR]) {
* struct nlattr *nested[NESTED_MAX+1];
*
* // Attributes nested in a container can be parsed the same way
* // as top level attributes.
* if ((err = nla_parse_nested(nested, NESTED_MAX, attrs[ATTR_BAR],
* nested_policy)) < 0)
* goto errout;
*
* // Process nested attributes here.
* }
*
* err = 0;
* errout:
* return err;
* }
* @endcode
*
* @{
*/
/**
* @name Attribute Size Calculation
* @{
*/
/** @} */
/**
* @name Parsing Attributes
* @{
*/
/**
* Check if the attribute header and payload can be accessed safely.
* @arg nla Attribute of any kind.
* @arg remaining Number of bytes remaining in attribute stream.
*
* Verifies that the header and payload do not exceed the number of
* bytes left in the attribute stream. This function must be called
* before access the attribute header or payload when iterating over
* the attribute stream using nla_next().
*
* @return True if the attribute can be accessed safely, false otherwise.
*/
int nla_ok(const struct nlattr *nla, int remaining)
{
return remaining >= sizeof(*nla) &&
nla->nla_len >= sizeof(*nla) &&
nla->nla_len <= remaining;
}
/**
* Return next attribute in a stream of attributes.
* @arg nla Attribute of any kind.
* @arg remaining Variable to count remaining bytes in stream.
*
* Calculates the offset to the next attribute based on the attribute
* given. The attribute provided is assumed to be accessible, the
* caller is responsible to use nla_ok() beforehand. The offset (length
* of specified attribute including padding) is then subtracted from
* the remaining bytes variable and a pointer to the next attribute is
* returned.
*
* nla_next() can be called as long as remainig is >0.
*
* @return Pointer to next attribute.
*/
struct nlattr *nla_next(const struct nlattr *nla, int *remaining)
{
int totlen = NLA_ALIGN(nla->nla_len);
*remaining -= totlen;
return (struct nlattr *) ((char *) nla + totlen);
}
static uint16_t nla_attr_minlen[NLA_TYPE_MAX+1] = {
[NLA_U8] = sizeof(uint8_t),
[NLA_U16] = sizeof(uint16_t),
[NLA_U32] = sizeof(uint32_t),
[NLA_U64] = sizeof(uint64_t),
[NLA_STRING] = 1,
};
static int validate_nla(struct nlattr *nla, int maxtype,
struct nla_policy *policy)
{
struct nla_policy *pt;
int minlen = 0, type = nla_type(nla);
if (type <= 0 || type > maxtype)
return 0;
pt = &policy[type];
if (pt->type > NLA_TYPE_MAX)
BUG();
if (pt->minlen)
minlen = pt->minlen;
else if (pt->type != NLA_UNSPEC)
minlen = nla_attr_minlen[pt->type];
if (pt->type == NLA_FLAG && nla_len(nla) > 0)
return -NLE_RANGE;
if (nla_len(nla) < minlen)
return -NLE_RANGE;
if (pt->maxlen && nla_len(nla) > pt->maxlen)
return -NLE_RANGE;
if (pt->type == NLA_STRING) {
char *data = nla_data(nla);
if (data[nla_len(nla) - 1] != '\0')
return -NLE_INVAL;
}
return 0;
}
/**
* Create attribute index based on a stream of attributes.
* @arg tb Index array to be filled (maxtype+1 elements).
* @arg maxtype Maximum attribute type expected and accepted.
* @arg head Head of attribute stream.
* @arg len Length of attribute stream.
* @arg policy Attribute validation policy.
*
* Iterates over the stream of attributes and stores a pointer to each
* attribute in the index array using the attribute type as index to
* the array. Attribute with a type greater than the maximum type
* specified will be silently ignored in order to maintain backwards
* compatibility. If \a policy is not NULL, the attribute will be
* validated using the specified policy.
*
* @see nla_validate
* @return 0 on success or a negative error code.
*/
int nla_parse(struct nlattr *tb[], int maxtype, struct nlattr *head, int len,
struct nla_policy *policy)
{
struct nlattr *nla;
int rem, err;
memset(tb, 0, sizeof(struct nlattr *) * (maxtype + 1));
nla_for_each_attr(nla, head, len, rem) {
int type = nla_type(nla);
if (type == 0) {
fprintf(stderr, "Illegal nla->nla_type == 0\n");
continue;
}
if (type <= maxtype) {
if (policy) {
err = validate_nla(nla, maxtype, policy);
if (err < 0)
goto errout;
}
tb[type] = nla;
}
}
if (rem > 0)
fprintf(stderr, "netlink: %d bytes leftover after parsing "
"attributes.\n", rem);
err = 0;
errout:
return err;
}
/**
* Validate a stream of attributes.
* @arg head Head of attributes stream.
* @arg len Length of attributes stream.
* @arg maxtype Maximum attribute type expected and accepted.
* @arg policy Validation policy.
*
* Iterates over the stream of attributes and validates each attribute
* one by one using the specified policy. Attributes with a type greater
* than the maximum type specified will be silently ignored in order to
* maintain backwards compatibility.
*
* See \ref attr_datatypes for more details on what kind of validation
* checks are performed on each attribute data type.
*
* @return 0 on success or a negative error code.
*/
int nla_validate(struct nlattr *head, int len, int maxtype,
struct nla_policy *policy)
{
struct nlattr *nla;
int rem, err;
nla_for_each_attr(nla, head, len, rem) {
err = validate_nla(nla, maxtype, policy);
if (err < 0)
goto errout;
}
err = 0;
errout:
return err;
}
/**
* Find a single attribute in a stream of attributes.
* @arg head Head of attributes stream.
* @arg len Length of attributes stream.
* @arg attrtype Attribute type to look for.
*
* Iterates over the stream of attributes and compares each type with
* the type specified. Returns the first attribute which matches the
* type.
*
* @return Pointer to attribute found or NULL.
*/
struct nlattr *nla_find(struct nlattr *head, int len, int attrtype)
{
struct nlattr *nla;
int rem;
nla_for_each_attr(nla, head, len, rem)
if (nla_type(nla) == attrtype)
return nla;
return NULL;
}
/** @} */
/**
* @name Unspecific Attribute
* @{
*/
/**
* Reserve space for a attribute.
* @arg msg Netlink Message.
* @arg attrtype Attribute Type.
* @arg attrlen Length of payload.
*
* Reserves room for a attribute in the specified netlink message and
* fills in the attribute header (type, length). Returns NULL if there
* is unsuficient space for the attribute.
*
* Any padding between payload and the start of the next attribute is
* zeroed out.
*
* @return Pointer to start of attribute or NULL on failure.
*/
struct nlattr *nla_reserve(struct nl_msg *msg, int attrtype, int attrlen)
{
struct nlattr *nla;
int tlen;
tlen = NLMSG_ALIGN(msg->nm_nlh->nlmsg_len) + nla_total_size(attrlen);
if ((tlen + msg->nm_nlh->nlmsg_len) > msg->nm_size)
return NULL;
nla = (struct nlattr *) nlmsg_tail(msg->nm_nlh);
nla->nla_type = attrtype;
nla->nla_len = nla_attr_size(attrlen);
memset((unsigned char *) nla + nla->nla_len, 0, nla_padlen(attrlen));
msg->nm_nlh->nlmsg_len = tlen;
NL_DBG(2, "msg %p: Reserved %d bytes at offset +%td for attr %d "
"nlmsg_len=%d\n", msg, attrlen,
(void *) nla - nlmsg_data(msg->nm_nlh),
attrtype, msg->nm_nlh->nlmsg_len);
return nla;
}
/**
* Add a unspecific attribute to netlink message.
* @arg msg Netlink message.
* @arg attrtype Attribute type.
* @arg datalen Length of data to be used as payload.
* @arg data Pointer to data to be used as attribute payload.
*
* Reserves room for a unspecific attribute and copies the provided data
* into the message as payload of the attribute. Returns an error if there
* is insufficient space for the attribute.
*
* @see nla_reserve
* @return 0 on success or a negative error code.
*/
int nla_put(struct nl_msg *msg, int attrtype, int datalen, const void *data)
{
struct nlattr *nla;
nla = nla_reserve(msg, attrtype, datalen);
if (!nla)
return -NLE_NOMEM;
memcpy(nla_data(nla), data, datalen);
NL_DBG(2, "msg %p: Wrote %d bytes at offset +%td for attr %d\n",
msg, datalen, (void *) nla - nlmsg_data(msg->nm_nlh), attrtype);
return 0;
}
/** @} */