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diff --git a/Documentation/networking/radiotap-headers.rst b/Documentation/networking/radiotap-headers.rst new file mode 100644 index 000000000..1a1bd1ec0 --- /dev/null +++ b/Documentation/networking/radiotap-headers.rst @@ -0,0 +1,159 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=========================== +How to use radiotap headers +=========================== + +Pointer to the radiotap include file +------------------------------------ + +Radiotap headers are variable-length and extensible, you can get most of the +information you need to know on them from:: + + ./include/net/ieee80211_radiotap.h + +This document gives an overview and warns on some corner cases. + + +Structure of the header +----------------------- + +There is a fixed portion at the start which contains a u32 bitmap that defines +if the possible argument associated with that bit is present or not. So if b0 +of the it_present member of ieee80211_radiotap_header is set, it means that +the header for argument index 0 (IEEE80211_RADIOTAP_TSFT) is present in the +argument area. + +:: + + < 8-byte ieee80211_radiotap_header > + [ <possible argument bitmap extensions ... > ] + [ <argument> ... ] + +At the moment there are only 13 possible argument indexes defined, but in case +we run out of space in the u32 it_present member, it is defined that b31 set +indicates that there is another u32 bitmap following (shown as "possible +argument bitmap extensions..." above), and the start of the arguments is moved +forward 4 bytes each time. + +Note also that the it_len member __le16 is set to the total number of bytes +covered by the ieee80211_radiotap_header and any arguments following. + + +Requirements for arguments +-------------------------- + +After the fixed part of the header, the arguments follow for each argument +index whose matching bit is set in the it_present member of +ieee80211_radiotap_header. + + - the arguments are all stored little-endian! + + - the argument payload for a given argument index has a fixed size. So + IEEE80211_RADIOTAP_TSFT being present always indicates an 8-byte argument is + present. See the comments in ./include/net/ieee80211_radiotap.h for a nice + breakdown of all the argument sizes + + - the arguments must be aligned to a boundary of the argument size using + padding. So a u16 argument must start on the next u16 boundary if it isn't + already on one, a u32 must start on the next u32 boundary and so on. + + - "alignment" is relative to the start of the ieee80211_radiotap_header, ie, + the first byte of the radiotap header. The absolute alignment of that first + byte isn't defined. So even if the whole radiotap header is starting at, eg, + address 0x00000003, still the first byte of the radiotap header is treated as + 0 for alignment purposes. + + - the above point that there may be no absolute alignment for multibyte + entities in the fixed radiotap header or the argument region means that you + have to take special evasive action when trying to access these multibyte + entities. Some arches like Blackfin cannot deal with an attempt to + dereference, eg, a u16 pointer that is pointing to an odd address. Instead + you have to use a kernel API get_unaligned() to dereference the pointer, + which will do it bytewise on the arches that require that. + + - The arguments for a given argument index can be a compound of multiple types + together. For example IEEE80211_RADIOTAP_CHANNEL has an argument payload + consisting of two u16s of total length 4. When this happens, the padding + rule is applied dealing with a u16, NOT dealing with a 4-byte single entity. + + +Example valid radiotap header +----------------------------- + +:: + + 0x00, 0x00, // <-- radiotap version + pad byte + 0x0b, 0x00, // <- radiotap header length + 0x04, 0x0c, 0x00, 0x00, // <-- bitmap + 0x6c, // <-- rate (in 500kHz units) + 0x0c, //<-- tx power + 0x01 //<-- antenna + + +Using the Radiotap Parser +------------------------- + +If you are having to parse a radiotap struct, you can radically simplify the +job by using the radiotap parser that lives in net/wireless/radiotap.c and has +its prototypes available in include/net/cfg80211.h. You use it like this:: + + #include <net/cfg80211.h> + + /* buf points to the start of the radiotap header part */ + + int MyFunction(u8 * buf, int buflen) + { + int pkt_rate_100kHz = 0, antenna = 0, pwr = 0; + struct ieee80211_radiotap_iterator iterator; + int ret = ieee80211_radiotap_iterator_init(&iterator, buf, buflen); + + while (!ret) { + + ret = ieee80211_radiotap_iterator_next(&iterator); + + if (ret) + continue; + + /* see if this argument is something we can use */ + + switch (iterator.this_arg_index) { + /* + * You must take care when dereferencing iterator.this_arg + * for multibyte types... the pointer is not aligned. Use + * get_unaligned((type *)iterator.this_arg) to dereference + * iterator.this_arg for type "type" safely on all arches. + */ + case IEEE80211_RADIOTAP_RATE: + /* radiotap "rate" u8 is in + * 500kbps units, eg, 0x02=1Mbps + */ + pkt_rate_100kHz = (*iterator.this_arg) * 5; + break; + + case IEEE80211_RADIOTAP_ANTENNA: + /* radiotap uses 0 for 1st ant */ + antenna = *iterator.this_arg); + break; + + case IEEE80211_RADIOTAP_DBM_TX_POWER: + pwr = *iterator.this_arg; + break; + + default: + break; + } + } /* while more rt headers */ + + if (ret != -ENOENT) + return TXRX_DROP; + + /* discard the radiotap header part */ + buf += iterator.max_length; + buflen -= iterator.max_length; + + ... + + } + +Andy Green <andy@warmcat.com> |