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