linux/net/wireless/radiotap.c

/*
 * Radiotap parser
 *
 * Copyright 2007		Andy Green <andy@warmcat.com>
 */

#include <net/cfg80211.h>
#include <net/ieee80211_radiotap.h>
#include <asm/unaligned.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 or a negative error code 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 0 return code.  Then loop calling
 * __ieee80211_radiotap_iterator_next()... it returns either 0,
 * -ENOENT if there are no more args to parse, or -EINVAL 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.
 *
 * Alignment Gotcha:
 * 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.
 *
 * 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 < get_unaligned_le16(&radiotap_header->it_len))
		return -EINVAL;

	iterator->rtheader = radiotap_header;
	iterator->max_length = get_unaligned_le16(&radiotap_header->it_len);
	iterator->arg_index = 0;
	iterator->bitmap_shifter = get_unaligned_le32(&radiotap_header->it_present);
	iterator->arg = (u8 *)radiotap_header + sizeof(*radiotap_header);
	iterator->this_arg = NULL;

	/* find payload start allowing for extended bitmap(s) */

	if (unlikely(iterator->bitmap_shifter & (1<<IEEE80211_RADIOTAP_EXT))) {
		while (get_unaligned_le32(iterator->arg) &
		       (1 << IEEE80211_RADIOTAP_EXT)) {
			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 -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: 0 if there is an argument to handle,
 * -ENOENT if there are no more args or -EINVAL
 * if there is something else wrong.
 *
 * This function provides the next radiotap arg index (IEEE80211_RADIOTAP_*)
 * in @this_arg_index and sets @this_arg to point to the
 * payload for the field.  It takes care of alignment handling and extended
 * present fields.  @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.
 *
 * Alignment Gotcha:
 * 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.
 */

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,
		[IEEE80211_RADIOTAP_RX_FLAGS] = 0x22,
		[IEEE80211_RADIOTAP_TX_FLAGS] = 0x22,
		[IEEE80211_RADIOTAP_RTS_RETRIES] = 0x11,
		[IEEE80211_RADIOTAP_DATA_RETRIES] = 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;
		int pad;

		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.
		 *
		 * the above is why get_unaligned() is used to dereference
		 * multibyte elements from the radiotap area
		 */

		pad = (((ulong)iterator->arg) -
			((ulong)iterator->rtheader)) &
			((rt_sizes[iterator->arg_index] >> 4) - 1);

		if (pad)
			iterator->arg +=
				(rt_sizes[iterator->arg_index] >> 4) - pad;

		/*
		 * 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 -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 =
				    get_unaligned_le32(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 0;
	}

	/* we don't know how to handle any more args, we're done */
	return -ENOENT;
}
EXPORT_SYMBOL(ieee80211_radiotap_iterator_next);