1.. SPDX-License-Identifier: GPL-2.0 2 3=========================== 4How to use radiotap headers 5=========================== 6 7Pointer to the radiotap include file 8------------------------------------ 9 10Radiotap headers are variable-length and extensible, you can get most of the 11information you need to know on them from:: 12 13 ./include/net/ieee80211_radiotap.h 14 15This document gives an overview and warns on some corner cases. 16 17 18Structure of the header 19----------------------- 20 21There is a fixed portion at the start which contains a u32 bitmap that defines 22if the possible argument associated with that bit is present or not. So if b0 23of the it_present member of ieee80211_radiotap_header is set, it means that 24the header for argument index 0 (IEEE80211_RADIOTAP_TSFT) is present in the 25argument area. 26 27:: 28 29 < 8-byte ieee80211_radiotap_header > 30 [ <possible argument bitmap extensions ... > ] 31 [ <argument> ... ] 32 33At the moment there are only 13 possible argument indexes defined, but in case 34we run out of space in the u32 it_present member, it is defined that b31 set 35indicates that there is another u32 bitmap following (shown as "possible 36argument bitmap extensions..." above), and the start of the arguments is moved 37forward 4 bytes each time. 38 39Note also that the it_len member __le16 is set to the total number of bytes 40covered by the ieee80211_radiotap_header and any arguments following. 41 42 43Requirements for arguments 44-------------------------- 45 46After the fixed part of the header, the arguments follow for each argument 47index whose matching bit is set in the it_present member of 48ieee80211_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 81Example 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 94Using the Radiotap Parser 95------------------------- 96 97If you are having to parse a radiotap struct, you can radically simplify the 98job by using the radiotap parser that lives in net/wireless/radiotap.c and has 99its 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 159Andy Green <andy@warmcat.com> 160