Generic code to walk through the fields in a radiotap header, accounting
for nasties like extended "field present" bitfields and alignment rules
Try #9 (of the radiotap patchset) thanks to Johannes Berg's feedback
- Add docs to nano kernel docs
- Add docs to Documentation/networking/radiotap-headers.txt
- Fix typos
Try #2
- added sanity check to extended present bitmap u32 walking code - disallow
possibility to walk past the end of the radiotap header length
Try #1
- Based on Johannes Berg's comments, broke out the radiotap parsing into
its own file as part of cfg80211
- From same comments, added mask constant for b31 of arg presence bitfield
to ieee80211_radiotap.h
- Fixed subtle but nasty bug with alignment: args in the radiotap area are
aligned *relative to the start of the header* now. The header is not
guaranteed to align to anything (it is randomly in an skb data area).
Signed-off-by: Andy Green <andy@xxxxxxxxxxx>
===================================================
---
Documentation/networking/radiotap-headers.txt | 56 +++++
include/net/cfg80211.h | 39 +++
net/wireless/Makefile | 2
net/wireless/radiotap.c | 256 ++++++++++++++++++++++++++
4 files changed, 352 insertions(+), 1 deletion(-)
Index: wireless-dev/include/net/cfg80211.h
===================================================================
--- wireless-dev.orig/include/net/cfg80211.h
+++ wireless-dev/include/net/cfg80211.h
@@ -74,6 +74,45 @@ struct key_params {
u32 cipher;
};
+
+/* Radiotap header iteration
+ * implemented in net/wireless/radiotap.c
+ * docs in Documentation/networking/radiotap-headers.txt
+ */
+/**
+ * struct ieee80211_radiotap_iterator - tracks walk thru present radiotap args
+ * @rtheader: pointer to the radiotap header we are walking through
+ * @max_length: length of radiotap header in cpu byte ordering
+ * @this_arg_index: IEEE80211_RADIOTAP_... index of current arg
+ * @this_arg: pointer to current radiotap arg
+ * @arg_index: internal next argument index
+ * @arg: internal next argument pointer
+ * @next_bitmap: internal pointer to next present u32
+ * @bitmap_shifter: internal shifter for curr u32 bitmap, b0 set == arg present
+ */
+
+struct ieee80211_radiotap_iterator {
+ struct ieee80211_radiotap_header *rtheader;
+ int max_length;
+ int this_arg_index;
+ u8 * this_arg;
+
+ int arg_index;
+ u8 * arg;
+ __le32 *next_bitmap;
+ u32 bitmap_shifter;
+};
+
+extern int ieee80211_radiotap_iterator_init(
+ struct ieee80211_radiotap_iterator * iterator,
+ struct ieee80211_radiotap_header * radiotap_header,
+ int max_length
+);
+
+extern int ieee80211_radiotap_iterator_next(
+ struct ieee80211_radiotap_iterator * iterator);
+
+
/* from net/wireless.h */
struct wiphy;
Index: wireless-dev/net/wireless/Makefile
===================================================================
--- wireless-dev.orig/net/wireless/Makefile
+++ wireless-dev/net/wireless/Makefile
@@ -1,5 +1,5 @@
obj-$(CONFIG_WIRELESS_EXT) += wext.o
obj-$(CONFIG_CFG80211) += cfg80211.o
-cfg80211-y += core.o sysfs.o
+cfg80211-y += core.o sysfs.o radiotap.o
cfg80211-$(CONFIG_NL80211) += nl80211.o
Index: wireless-dev/net/wireless/radiotap.c
===================================================================
--- /dev/null
+++ wireless-dev/net/wireless/radiotap.c
@@ -0,0 +1,256 @@
+/*
+ * Radiotap parser
+ *
+ * Copyright 2007 Andy Green <andy@xxxxxxxxxxx>
+ */
+
+#include <linux/if.h>
+#include <linux/err.h>
+#include <linux/mutex.h>
+#include <linux/device.h>
+#include <net/genetlink.h>
+#include <net/cfg80211.h>
+#include <net/wireless.h>
+#include "nl80211.h"
+#include "core.h"
+#include <net/ieee80211_radiotap.h>
+
+/* function prototypes and related defs are in include/net/cfg80211.h */
+
+/**
+ * ieee80211_radiotap_iterator_init - radiotap parser iterator initialization
+ * @iterator: radiotap_iterator to initialize
+ * @radiotap_header: radiotap header to parse
+ * @max_length: total length we can parse into (eg, whole packet length)
+ *
+ * Returns: 0 on success or negative if sanity check fails
+ *
+ * This function initializes an opaque iterator struct which can then
+ * be passed to ieee80211_radiotap_iterator_next() to visit every radiotap
+ * argument which is present in the header. It knows about extended
+ * present headers and handles them.
+ *
+ * How to use:
+ * call __ieee80211_radiotap_iterator_init() to init a semi-opaque iterator
+ * struct ieee80211_radiotap_iterator (no need to init the struct beforehand)
+ * then loop calling __ieee80211_radiotap_iterator_next()... it returns either a
+ * negative error code if there are no more args in the header, or the
+ * next argument type index that is present. The iterator's this_arg member
+ * points to the start of the argument associated with the current argument
+ * index that is present, which can be found in the iterator's this_arg_index
+ * member. This arg index corresponds to the IEEE80211_RADIOTAP_... defines.
+ *
+ * Radiotap header length:
+ * You can find the CPU-endian total radiotap header length in
+ * iterator->max_length after executing ieee80211_radiotap_iterator_init()
+ * successfully.
+ *
+ * Example code:
+ * See Documentation/networking/radiotap-headers.txt
+ */
+
+int ieee80211_radiotap_iterator_init(
+ struct ieee80211_radiotap_iterator * iterator,
+ struct ieee80211_radiotap_header * radiotap_header,
+ int max_length)
+{
+ /* Linux only supports version 0 radiotap format */
+
+ if (radiotap_header->it_version)
+ return -EINVAL;
+
+ /* sanity check for allowed length and radiotap length field */
+
+ if (max_length < (le16_to_cpu(radiotap_header->it_len)))
+ return -EINVAL;
+
+ iterator->rtheader = radiotap_header;
+ iterator->max_length = le16_to_cpu(radiotap_header->it_len);
+ iterator->arg_index = 0;
+ iterator->bitmap_shifter = le32_to_cpu(radiotap_header->it_present);
+ iterator->arg = ((u8 *)radiotap_header) +
+ sizeof(struct ieee80211_radiotap_header);
+ iterator->this_arg = 0;
+
+ /* find payload start allowing for extended bitmap(s) */
+
+ if (unlikely(iterator->bitmap_shifter &
+ IEEE80211_RADIOTAP_PRESENT_EXTEND_MASK)) {
+ while (le32_to_cpu(*((u32 *)iterator->arg)) &
+ IEEE80211_RADIOTAP_PRESENT_EXTEND_MASK) {
+ iterator->arg += sizeof(u32);
+
+ /*
+ * check for insanity where the present bitmaps
+ * keep claiming to extend up to or even beyond the
+ * stated radiotap header length
+ */
+
+ if ((((int)iterator->arg) - ((int)iterator->rtheader)) >
+ iterator->max_length)
+ return -EINVAL;
+ }
+
+ iterator->arg += sizeof(u32);
+
+ /*
+ * no need to check again for blowing past stated radiotap
+ * header length, because ieee80211_radiotap_iterator_next
+ * checks it before it is dereferenced
+ */
+ }
+
+ /* we are all initialized happily */
+
+ return 0;
+}
+
+EXPORT_SYMBOL(ieee80211_radiotap_iterator_init);
+
+
+/**
+ * ieee80211_radiotap_iterator_next - return next radiotap parser iterator arg
+ * @iterator: radiotap_iterator to move to next arg (if any)
+ *
+ * Returns: next present arg index on success or negative if no more or error
+ *
+ * This function returns the next radiotap arg index (IEEE80211_RADIOTAP_*)
+ * and sets iterator->this_arg to point to the payload for the arg. It takes
+ * care of alignment handling and extended present fields. iterator->this_arg
+ * can be changed by the caller (eg, incremented to move inside a compound
+ * argument like IEEE80211_RADIOTAP_CHANNEL). The args pointed to are in
+ * little-endian format whatever the endianess of your CPU.
+ */
+
+int ieee80211_radiotap_iterator_next(
+ struct ieee80211_radiotap_iterator * iterator)
+{
+
+ /*
+ * small length lookup table for all radiotap types we heard of
+ * starting from b0 in the bitmap, so we can walk the payload
+ * area of the radiotap header
+ *
+ * There is a requirement to pad args, so that args
+ * of a given length must begin at a boundary of that length
+ * -- but note that compound args are allowed (eg, 2 x u16
+ * for IEEE80211_RADIOTAP_CHANNEL) so total arg length is not
+ * a reliable indicator of alignment requirement.
+ *
+ * upper nybble: content alignment for arg
+ * lower nybble: content length for arg
+ */
+
+ static const u8 rt_sizes[] = {
+ [IEEE80211_RADIOTAP_TSFT] = 0x88,
+ [IEEE80211_RADIOTAP_FLAGS] = 0x11,
+ [IEEE80211_RADIOTAP_RATE] = 0x11,
+ [IEEE80211_RADIOTAP_CHANNEL] = 0x24,
+ [IEEE80211_RADIOTAP_FHSS] = 0x22,
+ [IEEE80211_RADIOTAP_DBM_ANTSIGNAL] = 0x11,
+ [IEEE80211_RADIOTAP_DBM_ANTNOISE] = 0x11,
+ [IEEE80211_RADIOTAP_LOCK_QUALITY] = 0x22,
+ [IEEE80211_RADIOTAP_TX_ATTENUATION] = 0x22,
+ [IEEE80211_RADIOTAP_DB_TX_ATTENUATION] = 0x22,
+ [IEEE80211_RADIOTAP_DBM_TX_POWER] = 0x11,
+ [IEEE80211_RADIOTAP_ANTENNA] = 0x11,
+ [IEEE80211_RADIOTAP_DB_ANTSIGNAL] = 0x11,
+ [IEEE80211_RADIOTAP_DB_ANTNOISE] = 0x11
+ /*
+ * add more here as they are defined in
+ * include/net/ieee80211_radiotap.h
+ */
+ };
+
+ /*
+ * for every radiotap entry we can at
+ * least skip (by knowing the length)...
+ */
+
+ while (iterator->arg_index < sizeof(rt_sizes)) {
+ int hit = 0;
+
+ if (!(iterator->bitmap_shifter & 1))
+ goto next_entry; /* arg not present */
+
+ /*
+ * arg is present, account for alignment padding
+ * 8-bit args can be at any alignment
+ * 16-bit args must start on 16-bit boundary
+ * 32-bit args must start on 32-bit boundary
+ * 64-bit args must start on 64-bit boundary
+ *
+ * note that total arg size can differ from alignment of
+ * elements inside arg, so we use upper nybble of length
+ * table to base alignment on
+ *
+ * also note: these alignments are ** relative to the
+ * start of the radiotap header **. There is no guarantee
+ * that the radiotap header itself is aligned on any
+ * kind of boundary.
+ */
+
+ if ((((int)iterator->arg)-((int)iterator->rtheader)) &
+ ((rt_sizes[iterator->arg_index] >> 4) - 1))
+ iterator->arg_index +=
+ (rt_sizes[iterator->arg_index] >> 4) -
+ ((((int)iterator->arg) -
+ ((int)iterator->rtheader)) &
+ ((rt_sizes[iterator->arg_index] >> 4) - 1));
+
+ /*
+ * this is what we will return to user, but we need to
+ * move on first so next call has something fresh to test
+ */
+
+ iterator->this_arg_index = iterator->arg_index;
+ iterator->this_arg = iterator->arg;
+ hit = 1;
+
+ /* internally move on the size of this arg */
+
+ iterator->arg += rt_sizes[iterator->arg_index] & 0x0f;
+
+ /*
+ * check for insanity where we are given a bitmap that
+ * claims to have more arg content than the length of the
+ * radiotap section. We will normally end up equalling this
+ * max_length on the last arg, never exceeding it.
+ */
+
+ if ((((int)iterator->arg) - ((int)iterator->rtheader)) >
+ iterator->max_length)
+ return -EINVAL;
+
+ next_entry:
+
+ iterator->arg_index++;
+ if (unlikely((iterator->arg_index & 31) == 0)) {
+ /* completed current u32 bitmap */
+ if (iterator->bitmap_shifter & 1) {
+ /* b31 was set, there is more */
+ /* move to next u32 bitmap */
+ iterator->bitmap_shifter = le32_to_cpu(
+ *iterator->next_bitmap);
+ iterator->next_bitmap++;
+ } else {
+ /* no more bitmaps: end */
+ iterator->arg_index = sizeof(rt_sizes);
+ }
+ } else { /* just try the next bit */
+ iterator->bitmap_shifter >>= 1;
+ }
+
+ /* if we found a valid arg earlier, return it now */
+
+ if (hit)
+ return iterator->this_arg_index;
+
+ }
+
+ /* we don't know how to handle any more args, we're done */
+
+ return -ENOENT;
+}
+
+EXPORT_SYMBOL(ieee80211_radiotap_iterator_next);
Index: wireless-dev/Documentation/networking/radiotap-headers.txt
===================================================================
--- wireless-dev.orig/Documentation/networking/radiotap-headers.txt
+++ wireless-dev/Documentation/networking/radiotap-headers.txt
@@ -76,4 +76,60 @@ Example valid radiotap header
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;
+
+ if (ieee80211_radiotap_iterator_init(&iterator, buf, buflen) < 0)
+ return -EINVAL;
+
+ while (ieee80211_radiotap_iterator_next(&iterator) >= 0) {
+ int i, target_rate;
+
+ /* see if this argument is something we can use */
+
+ switch (iterator.this_arg_index) {
+
+ 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 */
+
+ /* discard the radiotap header part */
+
+ buf += iterator->max_length;
+ buflen -= iterator->max_length;
+
+ ...
+
+}
+
Andy Green <andy@xxxxxxxxxxx>
--
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