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Generic code to walk through the fields in a radiotap header, accounting for nasties like extended "field present" bitfields and alignment rules Signed-off-by: Andy Green <andy@warmcat.com> Signed-off-by: Jiri Benc <jbenc@suse.cz> Signed-off-by: John W. Linville <linville@tuxdriver.com>
152 lines
5 KiB
Text
152 lines
5 KiB
Text
How to use radiotap headers
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===========================
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Pointer to the radiotap include file
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------------------------------------
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Radiotap headers are variable-length and extensible, you can get most of the
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information you need to know on them from:
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./include/net/ieee80211_radiotap.h
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This document gives an overview and warns on some corner cases.
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Structure of the header
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-----------------------
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There is a fixed portion at the start which contains a u32 bitmap that defines
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if the possible argument associated with that bit is present or not. So if b0
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of the it_present member of ieee80211_radiotap_header is set, it means that
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the header for argument index 0 (IEEE80211_RADIOTAP_TSFT) is present in the
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argument area.
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< 8-byte ieee80211_radiotap_header >
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[ <possible argument bitmap extensions ... > ]
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[ <argument> ... ]
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At the moment there are only 13 possible argument indexes defined, but in case
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we run out of space in the u32 it_present member, it is defined that b31 set
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indicates that there is another u32 bitmap following (shown as "possible
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argument bitmap extensions..." above), and the start of the arguments is moved
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forward 4 bytes each time.
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Note also that the it_len member __le16 is set to the total number of bytes
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covered by the ieee80211_radiotap_header and any arguments following.
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Requirements for arguments
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--------------------------
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After the fixed part of the header, the arguments follow for each argument
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index whose matching bit is set in the it_present member of
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ieee80211_radiotap_header.
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- the arguments are all stored little-endian!
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- the argument payload for a given argument index has a fixed size. So
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IEEE80211_RADIOTAP_TSFT being present always indicates an 8-byte argument is
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present. See the comments in ./include/net/ieee80211_radiotap.h for a nice
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breakdown of all the argument sizes
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- the arguments must be aligned to a boundary of the argument size using
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padding. So a u16 argument must start on the next u16 boundary if it isn't
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already on one, a u32 must start on the next u32 boundary and so on.
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- "alignment" is relative to the start of the ieee80211_radiotap_header, ie,
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the first byte of the radiotap header. The absolute alignment of that first
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byte isn't defined. So even if the whole radiotap header is starting at, eg,
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address 0x00000003, still the first byte of the radiotap header is treated as
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0 for alignment purposes.
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- the above point that there may be no absolute alignment for multibyte
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entities in the fixed radiotap header or the argument region means that you
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have to take special evasive action when trying to access these multibyte
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entities. Some arches like Blackfin cannot deal with an attempt to
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dereference, eg, a u16 pointer that is pointing to an odd address. Instead
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you have to use a kernel API get_unaligned() to dereference the pointer,
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which will do it bytewise on the arches that require that.
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- The arguments for a given argument index can be a compound of multiple types
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together. For example IEEE80211_RADIOTAP_CHANNEL has an argument payload
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consisting of two u16s of total length 4. When this happens, the padding
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rule is applied dealing with a u16, NOT dealing with a 4-byte single entity.
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Example valid radiotap header
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-----------------------------
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0x00, 0x00, // <-- radiotap version + pad byte
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0x0b, 0x00, // <- radiotap header length
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0x04, 0x0c, 0x00, 0x00, // <-- bitmap
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0x6c, // <-- rate (in 500kHz units)
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0x0c, //<-- tx power
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0x01 //<-- antenna
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Using the Radiotap Parser
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-------------------------
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If you are having to parse a radiotap struct, you can radically simplify the
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job by using the radiotap parser that lives in net/wireless/radiotap.c and has
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its prototypes available in include/net/cfg80211.h. You use it like this:
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#include <net/cfg80211.h>
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/* buf points to the start of the radiotap header part */
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int MyFunction(u8 * buf, int buflen)
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{
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int pkt_rate_100kHz = 0, antenna = 0, pwr = 0;
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struct ieee80211_radiotap_iterator iterator;
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int ret = ieee80211_radiotap_iterator_init(&iterator, buf, buflen);
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while (!ret) {
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ret = ieee80211_radiotap_iterator_next(&iterator);
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if (ret)
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continue;
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/* see if this argument is something we can use */
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switch (iterator.this_arg_index) {
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/*
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* You must take care when dereferencing iterator.this_arg
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* for multibyte types... the pointer is not aligned. Use
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* get_unaligned((type *)iterator.this_arg) to dereference
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* iterator.this_arg for type "type" safely on all arches.
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*/
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case IEEE80211_RADIOTAP_RATE:
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/* radiotap "rate" u8 is in
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* 500kbps units, eg, 0x02=1Mbps
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*/
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pkt_rate_100kHz = (*iterator.this_arg) * 5;
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break;
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case IEEE80211_RADIOTAP_ANTENNA:
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/* radiotap uses 0 for 1st ant */
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antenna = *iterator.this_arg);
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break;
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case IEEE80211_RADIOTAP_DBM_TX_POWER:
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pwr = *iterator.this_arg;
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break;
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default:
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break;
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}
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} /* while more rt headers */
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if (ret != -ENOENT)
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return TXRX_DROP;
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/* discard the radiotap header part */
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buf += iterator.max_length;
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buflen -= iterator.max_length;
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...
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}
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Andy Green <andy@warmcat.com>
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