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88 lines
3.5 KiB
Plaintext
88 lines
3.5 KiB
Plaintext
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How to use radiotap headers
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===========================
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Pointer to the radiotap include file
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------------------------------------
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Radiotap headers are variable-length and extensible, you can get most of the
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information you need to know on them from:
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./include/net/ieee80211_radiotap.h
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This document gives an overview and warns on some corner cases.
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Structure of the header
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-----------------------
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There is a fixed portion at the start which contains a u32 bitmap that defines
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if the possible argument associated with that bit is present or not. So if b0
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of the it_present member of ieee80211_radiotap_header is set, it means that
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the header for argument index 0 (IEEE80211_RADIOTAP_TSFT) is present in the
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argument area.
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< 8-byte ieee80211_radiotap_header >
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[ <possible argument bitmap extensions ... > ]
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[ <argument> ... ]
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At the moment there are only 13 possible argument indexes defined, but in case
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we run out of space in the u32 it_present member, it is defined that b31 set
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indicates that there is another u32 bitmap following (shown as "possible
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argument bitmap extensions..." above), and the start of the arguments is moved
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forward 4 bytes each time.
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Note also that the it_len member __le16 is set to the total number of bytes
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covered by the ieee80211_radiotap_header and any arguments following.
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Requirements for arguments
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--------------------------
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After the fixed part of the header, the arguments follow for each argument
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index whose matching bit is set in the it_present member of
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ieee80211_radiotap_header.
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- the arguments are all stored little-endian!
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- the argument payload for a given argument index has a fixed size. So
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IEEE80211_RADIOTAP_TSFT being present always indicates an 8-byte argument is
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present. See the comments in ./include/net/ieee80211_radiotap.h for a nice
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breakdown of all the argument sizes
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- the arguments must be aligned to a boundary of the argument size using
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padding. So a u16 argument must start on the next u16 boundary if it isn't
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already on one, a u32 must start on the next u32 boundary and so on.
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- "alignment" is relative to the start of the ieee80211_radiotap_header, ie,
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the first byte of the radiotap header. The absolute alignment of that first
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byte isn't defined. So even if the whole radiotap header is starting at, eg,
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address 0x00000003, still the first byte of the radiotap header is treated as
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0 for alignment purposes.
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- the above point that there may be no absolute alignment for multibyte
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entities in the fixed radiotap header or the argument region means that you
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have to take special evasive action when trying to access these multibyte
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entities. Some arches like Blackfin cannot deal with an attempt to
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dereference, eg, a u16 pointer that is pointing to an odd address. Instead
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you have to use a kernel API get_unaligned() to dereference the pointer,
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which will do it bytewise on the arches that require that.
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- The arguments for a given argument index can be a compound of multiple types
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together. For example IEEE80211_RADIOTAP_CHANNEL has an argument payload
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consisting of two u16s of total length 4. When this happens, the padding
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rule is applied dealing with a u16, NOT dealing with a 4-byte single entity.
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Example valid radiotap header
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-----------------------------
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0x00, 0x00, // <-- radiotap version + pad byte
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0x0b, 0x00, // <- radiotap header length
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0x04, 0x0c, 0x00, 0x00, // <-- bitmap
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0x6c, // <-- rate (in 500kHz units)
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0x0c, //<-- tx power
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0x01 //<-- antenna
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Andy Green <andy@warmcat.com>
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