[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>

2 files changed, 258 insertions, 1 deletions

diff --git a/net/wireless/Makefile b/net/wireless/Makefile
index 3a96ae60271c..092116e390b6 100644
--- a/net/wireless/Makefile
+++ b/net/wireless/Makefile
@@ -1,4 +1,4 @@
 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
diff --git a/net/wireless/radiotap.c b/net/wireless/radiotap.c
new file mode 100644
index 000000000000..68c11d099917
--- /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);