Fix FCS on frames without lengths

Pull out the checksum from ambiguous frames before parsing them again.
msantos · Sep 3, 2011 · 29e6d77 · 29e6d77
1 parent e157342
commit 29e6d77
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Showing 2 changed files with 9 additions and 5 deletions.
diff --git a/README.md b/README.md
@@ -195,7 +195,8 @@ CAP\_NET\_ADMIN privileges:
 
     Attempt to write a frame to the wireless device.
 
-    wierl_monitor:frame(Socket, Frame) -> {Radiotap, FrameControl, FrameBody}
+    wierl_monitor:frame(Socket, Frame) ->
+        {Radiotap, FrameControl, FrameBody, FrameCheckSequence}
     wierl_monitor:frame(Socket, {FrameControl, FrameBody}) -> Frame
 
         Types   Socket = pid()
@@ -211,6 +212,7 @@ CAP\_NET\_ADMIN privileges:
                     | #ieee802_11_cf_bar{},
                     | #ieee802_11_cf_ba{},
                     | #ieee802_11_data{}
+                FrameCheckSequence = integer()
 
     Encode or decode an 802.11 wireless frame between binaries and
     records. Include the wierl_frame header to have access to the

diff --git a/src/wierl_monitor.erl b/src/wierl_monitor.erl
@@ -121,7 +121,7 @@ frame(Ref, Frame) when is_binary(Frame) ->
             {FB, 0};
         <<N:4/unsigned-integer-unit:8>> ->
             % Driver provides FCS
-            % XXX flag is set in state  on EVERY frame
+            % XXX flag is set in state on EVERY frame
             ok = gen_server:call(Ref, {fcs, true}),
             {FB, N};
         false ->
@@ -131,9 +131,11 @@ frame(Ref, Frame) when is_binary(Frame) ->
 
             case Include of
                 true ->
-                    Len = byte_size(FB) - 4,
-                    <<F1:Len/bytes, F2:4/unsigned-integer-unit:8>> = FB,
-                    {F1, F2};
+                    % Remove 4 bytes from the frame body and re-parse the frame
+                    Len = byte_size(Data2) - 4,
+                    <<Body:Len/bytes, RealFCS:4/unsigned-integer-unit:8>> = Data2,
+                    {FrameBody, _} = wierl_frame:type(FC, Body),
+                    {FrameBody, RealFCS};
                 false ->
                     {FB, 0}
             end