mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-26 02:39:48 +00:00
![Andy Green](/assets/img/avatar_default.png)
Generic code to walk through the fields in a radiotap header, accounting for nasties like extended "field present" bitfields and alignment rules Signed-off-by: Andy Green <andy@warmcat.com> Signed-off-by: Jiri Benc <jbenc@suse.cz> Signed-off-by: John W. Linville <linville@tuxdriver.com>
152 lines
5 KiB
Text
152 lines
5 KiB
Text
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>
|