1 .. SPDX-License-Identifier: GPL-2.0
2
3 ===========================
4 How to use radiotap headers
5 ===========================
6
7 Pointer to the radiotap include file
8 ------------------------------------
9
10 Radiotap headers are variable-length and extensible, you can get most of the
11 information you need to know on them from::
12
13 ./include/net/ieee80211_radiotap.h
14
15 This document gives an overview and warns on some corner cases.
16
17
18 Structure of the header
19 -----------------------
20
21 There is a fixed portion at the start which contains a u32 bitmap that defines
22 if the possible argument associated with that bit is present or not. So if b0
23 of the it_present member of ieee80211_radiotap_header is set, it means that
24 the header for argument index 0 (IEEE80211_RADIOTAP_TSFT) is present in the
25 argument area.
26
27 ::
28
29 < 8-byte ieee80211_radiotap_header >
30 [ <possible argument bitmap extensions ... > ]
31 [ <argument> ... ]
32
33 At the moment there are only 13 possible argument indexes defined, but in case
34 we run out of space in the u32 it_present member, it is defined that b31 set
35 indicates that there is another u32 bitmap following (shown as "possible
36 argument bitmap extensions..." above), and the start of the arguments is moved
37 forward 4 bytes each time.
38
39 Note also that the it_len member __le16 is set to the total number of bytes
40 covered by the ieee80211_radiotap_header and any arguments following.
41
42
43 Requirements for arguments
44 --------------------------
45
46 After the fixed part of the header, the arguments follow for each argument
47 index whose matching bit is set in the it_present member of
48 ieee80211_radiotap_header.
49
50 - the arguments are all stored little-endian!
51
52 - the argument payload for a given argument index has a fixed size. So
53 IEEE80211_RADIOTAP_TSFT being present always indicates an 8-byte argument is
54 present. See the comments in ./include/net/ieee80211_radiotap.h for a nice
55 breakdown of all the argument sizes
56
57 - the arguments must be aligned to a boundary of the argument size using
58 padding. So a u16 argument must start on the next u16 boundary if it isn't
59 already on one, a u32 must start on the next u32 boundary and so on.
60
61 - "alignment" is relative to the start of the ieee80211_radiotap_header, ie,
62 the first byte of the radiotap header. The absolute alignment of that first
63 byte isn't defined. So even if the whole radiotap header is starting at, eg,
64 address 0x00000003, still the first byte of the radiotap header is treated as
65 0 for alignment purposes.
66
67 - the above point that there may be no absolute alignment for multibyte
68 entities in the fixed radiotap header or the argument region means that you
69 have to take special evasive action when trying to access these multibyte
70 entities. Some arches like Blackfin cannot deal with an attempt to
71 dereference, eg, a u16 pointer that is pointing to an odd address. Instead
72 you have to use a kernel API get_unaligned() to dereference the pointer,
73 which will do it bytewise on the arches that require that.
74
75 - The arguments for a given argument index can be a compound of multiple types
76 together. For example IEEE80211_RADIOTAP_CHANNEL has an argument payload
77 consisting of two u16s of total length 4. When this happens, the padding
78 rule is applied dealing with a u16, NOT dealing with a 4-byte single entity.
79
80
81 Example valid radiotap header
82 -----------------------------
83
84 ::
85
86 0x00, 0x00, // <-- radiotap version + pad byte
87 0x0b, 0x00, // <- radiotap header length
88 0x04, 0x0c, 0x00, 0x00, // <-- bitmap
89 0x6c, // <-- rate (in 500kHz units)
90 0x0c, //<-- tx power
91 0x01 //<-- antenna
92
93
94 Using the Radiotap Parser
95 -------------------------
96
97 If you are having to parse a radiotap struct, you can radically simplify the
98 job by using the radiotap parser that lives in net/wireless/radiotap.c and has
99 its prototypes available in include/net/cfg80211.h. You use it like this::
100
101 #include <net/cfg80211.h>
102
103 /* buf points to the start of the radiotap header part */
104
105 int MyFunction(u8 * buf, int buflen)
106 {
107 int pkt_rate_100kHz = 0, antenna = 0, pwr = 0;
108 struct ieee80211_radiotap_iterator iterator;
109 int ret = ieee80211_radiotap_iterator_init(&iterator, buf, buflen);
110
111 while (!ret) {
112
113 ret = ieee80211_radiotap_iterator_next(&iterator);
114
115 if (ret)
116 continue;
117
118 /* see if this argument is something we can use */
119
120 switch (iterator.this_arg_index) {
121 /*
122 * You must take care when dereferencing iterator.this_arg
123 * for multibyte types... the pointer is not aligned. Use
124 * get_unaligned((type *)iterator.this_arg) to dereference
125 * iterator.this_arg for type "type" safely on all arches.
126 */
127 case IEEE80211_RADIOTAP_RATE:
128 /* radiotap "rate" u8 is in
129 * 500kbps units, eg, 0x02=1Mbps
130 */
131 pkt_rate_100kHz = (*iterator.this_arg) * 5;
132 break;
133
134 case IEEE80211_RADIOTAP_ANTENNA:
135 /* radiotap uses 0 for 1st ant */
136 antenna = *iterator.this_arg);
137 break;
138
139 case IEEE80211_RADIOTAP_DBM_TX_POWER:
140 pwr = *iterator.this_arg;
141 break;
142
143 default:
144 break;
145 }
146 } /* while more rt headers */
147
148 if (ret != -ENOENT)
149 return TXRX_DROP;
150
151 /* discard the radiotap header part */
152 buf += iterator.max_length;
153 buflen -= iterator.max_length;
154
155 ...
156
157 }
158
159 Andy Green <andy@warmcat.com>
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