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1 | .. SPDX-License-Identifier: GPL-2.0 |
2 | ||
3 | =========================== | |
08d1f215 AG |
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 | |
66d495d0 | 11 | information you need to know on them from:: |
08d1f215 | 12 | |
66d495d0 | 13 | ./include/net/ieee80211_radiotap.h |
08d1f215 AG |
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 | ||
66d495d0 MCC |
27 | :: |
28 | ||
08d1f215 AG |
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 | ||
66d495d0 MCC |
84 | :: |
85 | ||
08d1f215 AG |
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 | ||
179f831b AG |
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 | |
66d495d0 | 99 | its prototypes available in include/net/cfg80211.h. You use it like this:: |
179f831b | 100 | |
66d495d0 | 101 | #include <net/cfg80211.h> |
179f831b | 102 | |
66d495d0 | 103 | /* buf points to the start of the radiotap header part */ |
179f831b | 104 | |
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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); | |
179f831b | 110 | |
66d495d0 | 111 | while (!ret) { |
179f831b | 112 | |
66d495d0 | 113 | ret = ieee80211_radiotap_iterator_next(&iterator); |
179f831b | 114 | |
66d495d0 MCC |
115 | if (ret) |
116 | continue; | |
179f831b | 117 | |
66d495d0 | 118 | /* see if this argument is something we can use */ |
179f831b | 119 | |
66d495d0 MCC |
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; | |
179f831b | 133 | |
66d495d0 MCC |
134 | case IEEE80211_RADIOTAP_ANTENNA: |
135 | /* radiotap uses 0 for 1st ant */ | |
136 | antenna = *iterator.this_arg); | |
137 | break; | |
179f831b | 138 | |
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139 | case IEEE80211_RADIOTAP_DBM_TX_POWER: |
140 | pwr = *iterator.this_arg; | |
141 | break; | |
179f831b | 142 | |
66d495d0 MCC |
143 | default: |
144 | break; | |
145 | } | |
146 | } /* while more rt headers */ | |
179f831b | 147 | |
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148 | if (ret != -ENOENT) |
149 | return TXRX_DROP; | |
179f831b | 150 | |
66d495d0 MCC |
151 | /* discard the radiotap header part */ |
152 | buf += iterator.max_length; | |
153 | buflen -= iterator.max_length; | |
179f831b | 154 | |
66d495d0 | 155 | ... |
179f831b | 156 | |
66d495d0 | 157 | } |
179f831b | 158 | |
08d1f215 | 159 | Andy Green <andy@warmcat.com> |