net/wireless/radiotap.c

   1 /*
   2  * Radiotap parser
   3  *
   4  * Copyright 2007               Andy Green <andy@warmcat.com>
   5  */
   6
   7 #include <net/cfg80211.h>
   8 #include <net/ieee80211_radiotap.h>
   9 #include <asm/unaligned.h>
  10
  11 /* function prototypes and related defs are in include/net/cfg80211.h */
  12
  13 /**
  14  * ieee80211_radiotap_iterator_init - radiotap parser iterator initialization
  15  * @iterator: radiotap_iterator to initialize
  16  * @radiotap_header: radiotap header to parse
  17  * @max_length: total length we can parse into (eg, whole packet length)
  18  *
  19  * Returns: 0 or a negative error code if there is a problem.
  20  *
  21  * This function initializes an opaque iterator struct which can then
  22  * be passed to ieee80211_radiotap_iterator_next() to visit every radiotap
  23  * argument which is present in the header.  It knows about extended
  24  * present headers and handles them.
  25  *
  26  * How to use:
  27  * call __ieee80211_radiotap_iterator_init() to init a semi-opaque iterator
  28  * struct ieee80211_radiotap_iterator (no need to init the struct beforehand)
  29  * checking for a good 0 return code.  Then loop calling
  30  * __ieee80211_radiotap_iterator_next()... it returns either 0,
  31  * -ENOENT if there are no more args to parse, or -EINVAL if there is a problem.
  32  * The iterator's @this_arg member points to the start of the argument
  33  * associated with the current argument index that is present, which can be
  34  * found in the iterator's @this_arg_index member.  This arg index corresponds
  35  * to the IEEE80211_RADIOTAP_... defines.
  36  *
  37  * Radiotap header length:
  38  * You can find the CPU-endian total radiotap header length in
  39  * iterator->max_length after executing ieee80211_radiotap_iterator_init()
  40  * successfully.
  41  *
  42  * Alignment Gotcha:
  43  * You must take care when dereferencing iterator.this_arg
  44  * for multibyte types... the pointer is not aligned.  Use
  45  * get_unaligned((type *)iterator.this_arg) to dereference
  46  * iterator.this_arg for type "type" safely on all arches.
  47  *
  48  * Example code:
  49  * See Documentation/networking/radiotap-headers.txt
  50  */
  51
  52 int ieee80211_radiotap_iterator_init(
  53     struct ieee80211_radiotap_iterator *iterator,
  54     struct ieee80211_radiotap_header *radiotap_header,
  55     int max_length)
  56 {
  57         /* Linux only supports version 0 radiotap format */
  58         if (radiotap_header->it_version)
  59                 return -EINVAL;
  60
  61         /* sanity check for allowed length and radiotap length field */
  62         if (max_length < le16_to_cpu(get_unaligned(&radiotap_header->it_len)))
  63                 return -EINVAL;
  64
  65         iterator->rtheader = radiotap_header;
  66         iterator->max_length = le16_to_cpu(get_unaligned(
  67                                                 &radiotap_header->it_len));
  68         iterator->arg_index = 0;
  69         iterator->bitmap_shifter = le32_to_cpu(get_unaligned(
  70                                                 &radiotap_header->it_present));
  71         iterator->arg = (u8 *)radiotap_header + sizeof(*radiotap_header);
  72         iterator->this_arg = NULL;
  73
  74         /* find payload start allowing for extended bitmap(s) */
  75
  76         if (unlikely(iterator->bitmap_shifter & (1<<IEEE80211_RADIOTAP_EXT))) {
  77                 while (le32_to_cpu(get_unaligned((__le32 *)iterator->arg)) &
  78                                    (1<<IEEE80211_RADIOTAP_EXT)) {
  79                         iterator->arg += sizeof(u32);
  80
  81                         /*
  82                          * check for insanity where the present bitmaps
  83                          * keep claiming to extend up to or even beyond the
  84                          * stated radiotap header length
  85                          */
  86
  87                         if (((ulong)iterator->arg -
  88                              (ulong)iterator->rtheader) > iterator->max_length)
  89                                 return -EINVAL;
  90                 }
  91
  92                 iterator->arg += sizeof(u32);
  93
  94                 /*
  95                  * no need to check again for blowing past stated radiotap
  96                  * header length, because ieee80211_radiotap_iterator_next
  97                  * checks it before it is dereferenced
  98                  */
  99         }
 100
 101         /* we are all initialized happily */
 102
 103         return 0;
 104 }
 105 EXPORT_SYMBOL(ieee80211_radiotap_iterator_init);
 106
 107
 108 /**
 109  * ieee80211_radiotap_iterator_next - return next radiotap parser iterator arg
 110  * @iterator: radiotap_iterator to move to next arg (if any)
 111  *
 112  * Returns: 0 if there is an argument to handle,
 113  * -ENOENT if there are no more args or -EINVAL
 114  * if there is something else wrong.
 115  *
 116  * This function provides the next radiotap arg index (IEEE80211_RADIOTAP_*)
 117  * in @this_arg_index and sets @this_arg to point to the
 118  * payload for the field.  It takes care of alignment handling and extended
 119  * present fields.  @this_arg can be changed by the caller (eg,
 120  * incremented to move inside a compound argument like
 121  * IEEE80211_RADIOTAP_CHANNEL).  The args pointed to are in
 122  * little-endian format whatever the endianess of your CPU.
 123  *
 124  * Alignment Gotcha:
 125  * You must take care when dereferencing iterator.this_arg
 126  * for multibyte types... the pointer is not aligned.  Use
 127  * get_unaligned((type *)iterator.this_arg) to dereference
 128  * iterator.this_arg for type "type" safely on all arches.
 129  */
 130
 131 int ieee80211_radiotap_iterator_next(
 132     struct ieee80211_radiotap_iterator *iterator)
 133 {
 134
 135         /*
 136          * small length lookup table for all radiotap types we heard of
 137          * starting from b0 in the bitmap, so we can walk the payload
 138          * area of the radiotap header
 139          *
 140          * There is a requirement to pad args, so that args
 141          * of a given length must begin at a boundary of that length
 142          * -- but note that compound args are allowed (eg, 2 x u16
 143          * for IEEE80211_RADIOTAP_CHANNEL) so total arg length is not
 144          * a reliable indicator of alignment requirement.
 145          *
 146          * upper nybble: content alignment for arg
 147          * lower nybble: content length for arg
 148          */
 149
 150         static const u8 rt_sizes[] = {
 151                 [IEEE80211_RADIOTAP_TSFT] = 0x88,
 152                 [IEEE80211_RADIOTAP_FLAGS] = 0x11,
 153                 [IEEE80211_RADIOTAP_RATE] = 0x11,
 154                 [IEEE80211_RADIOTAP_CHANNEL] = 0x24,
 155                 [IEEE80211_RADIOTAP_FHSS] = 0x22,
 156                 [IEEE80211_RADIOTAP_DBM_ANTSIGNAL] = 0x11,
 157                 [IEEE80211_RADIOTAP_DBM_ANTNOISE] = 0x11,
 158                 [IEEE80211_RADIOTAP_LOCK_QUALITY] = 0x22,
 159                 [IEEE80211_RADIOTAP_TX_ATTENUATION] = 0x22,
 160                 [IEEE80211_RADIOTAP_DB_TX_ATTENUATION] = 0x22,
 161                 [IEEE80211_RADIOTAP_DBM_TX_POWER] = 0x11,
 162                 [IEEE80211_RADIOTAP_ANTENNA] = 0x11,
 163                 [IEEE80211_RADIOTAP_DB_ANTSIGNAL] = 0x11,
 164                 [IEEE80211_RADIOTAP_DB_ANTNOISE] = 0x11
 165                 /*
 166                  * add more here as they are defined in
 167                  * include/net/ieee80211_radiotap.h
 168                  */
 169         };
 170
 171         /*
 172          * for every radiotap entry we can at
 173          * least skip (by knowing the length)...
 174          */
 175
 176         while (iterator->arg_index < sizeof(rt_sizes)) {
 177                 int hit = 0;
 178                 int pad;
 179
 180                 if (!(iterator->bitmap_shifter & 1))
 181                         goto next_entry; /* arg not present */
 182
 183                 /*
 184                  * arg is present, account for alignment padding
 185                  *  8-bit args can be at any alignment
 186                  * 16-bit args must start on 16-bit boundary
 187                  * 32-bit args must start on 32-bit boundary
 188                  * 64-bit args must start on 64-bit boundary
 189                  *
 190                  * note that total arg size can differ from alignment of
 191                  * elements inside arg, so we use upper nybble of length
 192                  * table to base alignment on
 193                  *
 194                  * also note: these alignments are ** relative to the
 195                  * start of the radiotap header **.  There is no guarantee
 196                  * that the radiotap header itself is aligned on any
 197                  * kind of boundary.
 198                  *
 199                  * the above is why get_unaligned() is used to dereference
 200                  * multibyte elements from the radiotap area
 201                  */
 202
 203                 pad = (((ulong)iterator->arg) -
 204                         ((ulong)iterator->rtheader)) &
 205                         ((rt_sizes[iterator->arg_index] >> 4) - 1);
 206
 207                 if (pad)
 208                         iterator->arg +=
 209                                 (rt_sizes[iterator->arg_index] >> 4) - pad;
 210
 211                 /*
 212                  * this is what we will return to user, but we need to
 213                  * move on first so next call has something fresh to test
 214                  */
 215                 iterator->this_arg_index = iterator->arg_index;
 216                 iterator->this_arg = iterator->arg;
 217                 hit = 1;
 218
 219                 /* internally move on the size of this arg */
 220                 iterator->arg += rt_sizes[iterator->arg_index] & 0x0f;
 221
 222                 /*
 223                  * check for insanity where we are given a bitmap that
 224                  * claims to have more arg content than the length of the
 225                  * radiotap section.  We will normally end up equalling this
 226                  * max_length on the last arg, never exceeding it.
 227                  */
 228
 229                 if (((ulong)iterator->arg - (ulong)iterator->rtheader) >
 230                     iterator->max_length)
 231                         return -EINVAL;
 232
 233         next_entry:
 234                 iterator->arg_index++;
 235                 if (unlikely((iterator->arg_index & 31) == 0)) {
 236                         /* completed current u32 bitmap */
 237                         if (iterator->bitmap_shifter & 1) {
 238                                 /* b31 was set, there is more */
 239                                 /* move to next u32 bitmap */
 240                                 iterator->bitmap_shifter = le32_to_cpu(
 241                                         get_unaligned(iterator->next_bitmap));
 242                                 iterator->next_bitmap++;
 243                         } else
 244                                 /* no more bitmaps: end */
 245                                 iterator->arg_index = sizeof(rt_sizes);
 246                 } else /* just try the next bit */
 247                         iterator->bitmap_shifter >>= 1;
 248
 249                 /* if we found a valid arg earlier, return it now */
 250                 if (hit)
 251                         return 0;
 252         }
 253
 254         /* we don't know how to handle any more args, we're done */
 255         return -ENOENT;
 256 }
 257 EXPORT_SYMBOL(ieee80211_radiotap_iterator_next);