Generic code to walk through the fields in a radiotap header, accounting
for nasties like extended "field present" bitfields and alignment rules
Try #11
- parser returns enum retcode - Johannes Berg
Try #10 comments from Michael Wu
- kill some spaces between * and var/arg
- weed out cut-n-pasted #includes that aren't actually used
- kill vertical justification
- kill unneccessary parenthesis in some expressions
- sizeof() on dereferenced vars not types
- ulong pointer arithmetic! Good catch!
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 <[email protected]>
===================================================
---
Documentation/networking/radiotap-headers.txt | 60 ++++++
include/net/cfg80211.h | 45 ++++
net/wireless/Makefile | 2
net/wireless/radiotap.c | 245 ++++++++++++++++++++++++++
4 files changed, 351 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 +74,51 @@ 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;
+};
+
+typedef enum {
+ RADIOTAP_PARSER_OK = 0,
+ RADIOTAP_PARSER_DONE,
+ RADIOTAP_PARSER_INVALID
+} ieee80211_radiotap_parser_retcode_t;
+
+extern ieee80211_radiotap_parser_retcode_t ieee80211_radiotap_iterator_init(
+ struct ieee80211_radiotap_iterator *iterator,
+ struct ieee80211_radiotap_header *radiotap_header,
+ int max_length
+);
+
+extern ieee80211_radiotap_parser_retcode_t 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 +1,245 @@
+/*
+ * Radiotap parser
+ *
+ * Copyright 2007 Andy Green <[email protected]>
+ */
+
+#include <linux/err.h>
+#include <net/cfg80211.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: RADIOTAP_PARSER_OK or RADIOTAP_PARSER_INVALID if there is
+ * a problem.
+ *
+ * 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)
+ * checking for a good RADIOTAP_PARSER_OK return code. Then loop calling
+ * __ieee80211_radiotap_iterator_next()... it returns either RADIOTAP_PARSER_OK,
+ * RADIOTAP_PARSER_DONE if there are no more args to parse, or
+ * RADIOTAP_PARSER_INVALID if there is a problem.
+ * 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
+ */
+
+ieee80211_radiotap_parser_retcode_t 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 RADIOTAP_PARSER_INVALID;
+
+ /* sanity check for allowed length and radiotap length field */
+ if (max_length < le16_to_cpu(radiotap_header->it_len))
+ return RADIOTAP_PARSER_INVALID;
+
+ 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(*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 (((ulong)iterator->arg -
+ (ulong)iterator->rtheader) > iterator->max_length)
+ return RADIOTAP_PARSER_INVALID;
+ }
+
+ 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 RADIOTAP_PARSER_OK;
+}
+
+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: RADIOTAP_PARSER_OK if there is an argument to handle, or
+ * RADIOTAP_PARSER_DONE if there are no more args or RADIOTAP_PARSER_INVALID
+ * if there is something else wrong.
+ *
+ * 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.
+ */
+
+ieee80211_radiotap_parser_retcode_t 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 (((ulong)iterator->arg - (ulong)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 (((ulong)iterator->arg - (ulong)iterator->rtheader) >
+ iterator->max_length)
+ return RADIOTAP_PARSER_INVALID;
+
+ 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 RADIOTAP_PARSER_OK;
+ }
+
+ /* we don't know how to handle any more args, we're done */
+ return RADIOTAP_PARSER_DONE;
+}
+
+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 +76,64 @@ 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;
+ ieee80211_radiotap_parser_retcode_t ret =
+ ieee80211_radiotap_iterator_init(&iterator, buf, buflen);
+
+ while (ret == RADIOTAP_PARSER_OK) {
+
+ ret = ieee80211_radiotap_iterator_next(&iterator);
+
+ if (ret != RADIOTAP_PARSER_OK)
+ continue;
+
+ /* 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 */
+
+ if (ret != RADIOTAP_PARSER_DONE)
+ return TXRX_DROP;
+
+ /* discard the radiotap header part */
+ buf += iterator.max_length;
+ buflen -= iterator.max_length;
+
+ ...
+
+}
+
Andy Green <[email protected]>
--
On Tuesday 12 June 2007 06:04, [email protected] wrote:
> +typedef enum {
> + RADIOTAP_PARSER_OK = 0,
> + RADIOTAP_PARSER_DONE,
> + RADIOTAP_PARSER_INVALID
> +} ieee80211_radiotap_parser_retcode_t;
Yuck. I much prefer the standard error codes used in the previous version..
> +extern ieee80211_radiotap_parser_retcode_t ieee80211_radiotap_iterator_init(
> + struct ieee80211_radiotap_iterator *iterator,
> + struct ieee80211_radiotap_header *radiotap_header,
> + int max_length
> +);
Move the end of the parenthesis up a line to max_length.
> +ieee80211_radiotap_parser_retcode_t ieee80211_radiotap_iterator_init(
> + struct ieee80211_radiotap_iterator * iterator,
^
> + struct ieee80211_radiotap_header * radiotap_header,
^
> + /* 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_parser_retcode_t ieee80211_radiotap_iterator_next(
> + struct ieee80211_radiotap_iterator * iterator)
^
> + if (((ulong)iterator->arg - (ulong)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 part would probably look better if you stored:
((ulong)iterator->arg - (ulong)iterator->rtheader) & ((rt_sizes[iterator->arg_index] >> 4) - 1)
into a temporary variable.
-Michael Wu
On Tue, 2007-06-12 at 22:42 -0700, Michael Wu wrote:
> On Tuesday 12 June 2007 06:04, [email protected] wrote:
> > +typedef enum {
> > + RADIOTAP_PARSER_OK = 0,
> > + RADIOTAP_PARSER_DONE,
> > + RADIOTAP_PARSER_INVALID
> > +} ieee80211_radiotap_parser_retcode_t;
> Yuck. I much prefer the standard error codes used in the previous version..
I would as well, but as I outlined before we basically have four things
that can happen
* we found a next item we understood
* we have a parser error
* we reached the end
* we found an item we can't support (and thus we don't support any
further ones either assuming we add support sequentially)
I would like to be able to distinguish all these cases, the previous
version couldn't distinguish between the last two. However, I'm not
convinced that error codes are reasonable for something that isn't an
error (i.e. reaching the end)
johannes
On Wed, 2007-06-13 at 17:07 +0100, Andy Green wrote:
> Is it useful to report that there were items in the radiotap section
> that were not illegal but were not comprehensible either? Any app that
> is written against the parser will be using the contempary list of
> constants from ieee80211_radiotap.h, and anything added in there should
> be added to radiotap.c. Put another way you won't be in any better
> position to understand or handle radiotap args that cfg80211 doesn't
> understand than cfg80211 is itself. Is there a case where you don't
> want to ignore these unknown later entries that could appear?
Good point, I guess you just want to ignore any new items hoping that
they'll be orthogonal to the ones already there and thus you just don't
handle them. In any case, if we need it we can still change it, so just
leave as-is.
johannes
Johannes Berg wrote:
> On Tue, 2007-06-12 at 22:42 -0700, Michael Wu wrote:
>> On Tuesday 12 June 2007 06:04, [email protected] wrote:
>>> +typedef enum {
>>> + RADIOTAP_PARSER_OK = 0,
>>> + RADIOTAP_PARSER_DONE,
>>> + RADIOTAP_PARSER_INVALID
>>> +} ieee80211_radiotap_parser_retcode_t;
>
>> Yuck. I much prefer the standard error codes used in the previous version..
>
> I would as well, but as I outlined before we basically have four things
> that can happen
> * we found a next item we understood
> * we have a parser error
> * we reached the end
> * we found an item we can't support (and thus we don't support any
> further ones either assuming we add support sequentially)
>
> I would like to be able to distinguish all these cases, the previous
> version couldn't distinguish between the last two. However, I'm not
> convinced that error codes are reasonable for something that isn't an
> error (i.e. reaching the end)
Is it useful to report that there were items in the radiotap section
that were not illegal but were not comprehensible either? Any app that
is written against the parser will be using the contempary list of
constants from ieee80211_radiotap.h, and anything added in there should
be added to radiotap.c. Put another way you won't be in any better
position to understand or handle radiotap args that cfg80211 doesn't
understand than cfg80211 is itself. Is there a case where you don't
want to ignore these unknown later entries that could appear?
This parser is used in userspace apps too, packetspammer, penumbrad,
aircrack and mdk, and hopefully others when injection becomes something
that happens out of the box. The E* error constants are available in
userspace too. Although you can make a case for either way probably
radiotap isn't so wonderfully grand that it deserves its own enum for
its results IMO.
-Andy