[PATCH] cfg80211: Radiotap parser
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>
diff --git a/net/wireless/radiotap.c b/net/wireless/radiotap.c
new file mode 100644
index 0000000..68c11d0
--- /dev/null
+++ b/net/wireless/radiotap.c
@@ -0,0 +1,257 @@
+/*
+ * 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 < le16_to_cpu(get_unaligned(&radiotap_header->it_len)))
+ return -EINVAL;
+
+ iterator->rtheader = radiotap_header;
+ iterator->max_length = le16_to_cpu(get_unaligned(
+ &radiotap_header->it_len));
+ iterator->arg_index = 0;
+ iterator->bitmap_shifter = le32_to_cpu(get_unaligned(
+ &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 (le32_to_cpu(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
+ /*
+ * 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 = le32_to_cpu(
+ get_unaligned(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);