wireless: add utility function to get P2P attribute

Parsing the P2P attributes can be tricky as their
contents can be split across multiple (vendor) IEs.
Thus, it's not possible to parse them like IEs (by
returning a pointer to the data.) Instead, provide
a function that copies the attribute data into a
caller-provided buffer and returns the size needed
(useful in case the buffer was too small.)
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/cfg80211.h

@@ -3617,6 +3617,25 @@ u32 cfg80211_calculate_bitrate(struct rate_info *rate);
  */
 void cfg80211_unregister_wdev(struct wireless_dev *wdev);
 
+/**
+ * cfg80211_get_p2p_attr - find and copy a P2P attribute from IE buffer
+ * @ies: the input IE buffer
+ * @len: the input length
+ * @attr: the attribute ID to find
+ * @buf: output buffer, can be %NULL if the data isn't needed, e.g.
+ *	if the function is only called to get the needed buffer size
+ * @bufsize: size of the output buffer
+ *
+ * The function finds a given P2P attribute in the (vendor) IEs and
+ * copies its contents to the given buffer.
+ *
+ * The return value is a negative error code (-%EILSEQ or -%ENOENT) if
+ * the data is malformed or the attribute can't be found (respectively),
+ * or the length of the found attribute (which can be zero).
+ */
+unsigned int cfg80211_get_p2p_attr(const u8 *ies, unsigned int len,
+				   u8 attr, u8 *buf, unsigned int bufsize);
+
 /* Logging, debugging and troubleshooting/diagnostic helpers. */
 
 /* wiphy_printk helpers, similar to dev_printk */

net/wireless/util.c

@@ -980,6 +980,105 @@ u32 cfg80211_calculate_bitrate(struct rate_info *rate)
 }
 EXPORT_SYMBOL(cfg80211_calculate_bitrate);
 
+unsigned int cfg80211_get_p2p_attr(const u8 *ies, unsigned int len,
+				   u8 attr, u8 *buf, unsigned int bufsize)
+{
+	u8 *out = buf;
+	u16 attr_remaining = 0;
+	bool desired_attr = false;
+	u16 desired_len = 0;
+
+	while (len > 0) {
+		unsigned int iedatalen;
+		unsigned int copy;
+		const u8 *iedata;
+
+		if (len < 2)
+			return -EILSEQ;
+		iedatalen = ies[1];
+		if (iedatalen + 2 > len)
+			return -EILSEQ;
+
+		if (ies[0] != WLAN_EID_VENDOR_SPECIFIC)
+			goto cont;
+
+		if (iedatalen < 4)
+			goto cont;
+
+		iedata = ies + 2;
+
+		/* check WFA OUI, P2P subtype */
+		if (iedata[0] != 0x50 || iedata[1] != 0x6f ||
+		    iedata[2] != 0x9a || iedata[3] != 0x09)
+			goto cont;
+
+		iedatalen -= 4;
+		iedata += 4;
+
+		/* check attribute continuation into this IE */
+		copy = min_t(unsigned int, attr_remaining, iedatalen);
+		if (copy && desired_attr) {
+			desired_len += copy;
+			if (out) {
+				memcpy(out, iedata, min(bufsize, copy));
+				out += min(bufsize, copy);
+				bufsize -= min(bufsize, copy);
+			}
+
+
+			if (copy == attr_remaining)
+				return desired_len;
+		}
+
+		attr_remaining -= copy;
+		if (attr_remaining)
+			goto cont;
+
+		iedatalen -= copy;
+		iedata += copy;
+
+		while (iedatalen > 0) {
+			u16 attr_len;
+
+			/* P2P attribute ID & size must fit */
+			if (iedatalen < 3)
+				return -EILSEQ;
+			desired_attr = iedata[0] == attr;
+			attr_len = get_unaligned_le16(iedata + 1);
+			iedatalen -= 3;
+			iedata += 3;
+
+			copy = min_t(unsigned int, attr_len, iedatalen);
+
+			if (desired_attr) {
+				desired_len += copy;
+				if (out) {
+					memcpy(out, iedata, min(bufsize, copy));
+					out += min(bufsize, copy);
+					bufsize -= min(bufsize, copy);
+				}
+
+				if (copy == attr_len)
+					return desired_len;
+			}
+
+			iedata += copy;
+			iedatalen -= copy;
+			attr_remaining = attr_len - copy;
+		}
+
+ cont:
+		len -= ies[1] + 2;
+		ies += ies[1] + 2;
+	}
+
+	if (attr_remaining && desired_attr)
+		return -EILSEQ;
+
+	return -ENOENT;
+}
+EXPORT_SYMBOL(cfg80211_get_p2p_attr);
+
 int cfg80211_validate_beacon_int(struct cfg80211_registered_device *rdev,
 				 u32 beacon_int)
 {