[erlang-bugs] [BUG] OTP R12B-3: HTTP "space after headers" bug in inet_drv.c

Edwin Fine <>
Wed Jul 2 01:06:52 CEST 2008


First of all, many thanks to Claes Wikstrom for isolating and finding the
module in which this bug lives (and for doing it so quickly). I originally
thought this was a Yaws bug, and emailed Claes with the symptoms. It's quite
repeatable (test code at end of email). He traced the bug to inet_drv.c and
emailed me back with a small test program and details, and I offered to
report the bug. Then I got curious as to what it could have been, so I did
some debugging.

The bug is in inet_drv.c. It will cause the end of the HTTP headers to be
missed if the first character following the last CRLF (the one on its "own
line") is a space or tab. This causes Yaws to time out after 30 seconds
waiting for more header data that will never arrive. I am sure this will be
true for httpd, too.

The culprit (well, sort of; if you read on you will see why) is the
following code on line 8324:

*if (SP(ptr2+1)) {*
    ptr1 = ptr2+1;
    len = n - plen;
}
else
    goto done;

Consider the following HTTP POST data:

"POST /invalid/url HTTP/1.1\r\n"
"Connection: close\r\n"
"Host: localhost:8000\r\n"
"User-Agent: perl post\r\n"
"Content-Length: 4\r\n"
"Content-Type: text/xml; charset=utf-8\r\n"
"\r\n"
" postdata..." <-- Note this data starts with a space character

When the inet_drv.c code gets to the last CRLF, it doesn't detect that it's
the last CRLF (end of headers) and checks to see if the next character is a
space or tab using the macro SP(ptr2+1). If there's any data following the
last CRLF and it starts with a space or tab, the code as it stands now will
think it's another header line and try to get more data, then time out when
nothing arrives.

What needed to be done was to check to see if the data (in that particular
state) was an LF or CRLF standing on its own, and only check for a space
following that if not. This check correctly detects the CRLF that terminates
the header data. I added two lines of code to inet_drv.c to fix that error,
and the Erlang test code (kindly supplied by Claes, and at the end of this
email) now runs without error. I can't guarantee that the fix will work
under all circumstances, and it needs to be tested thoroughly, but it's a
starting point and illustrates the problem. (I would say that the HTTP
header parsing in inet_drv.c probably could be beefed up a little).

The patch to inet_drv.c is as follows (tabs may not be correct because I
reformatted the code):

8318a8319,8323
>
> /* Test needed in case buffer is in form "\r\n\srequestdata" where \s is
SP or TAB */
> if (((plen == 1) && NL(ptr)) || ((plen == 2) && CRNL(ptr)))
>     goto done;
>

Claes's test code is here (slightly modified by me to make it easier to test
the normal case and the "bug" case). You can run it as
post:p(Host,Port,data) or post:p(Host,Port,bug).

Regards,
Edwin Fine
======================================
-module(post).
-compile(export_all).

p() ->
    p({127,0,0,1},8000,data).

p(Host, Port, What) when What =:= bug; What =:= data ->
   {ok, S} = gen_tcp:connect(Host, Port, [{active, true}]),
   gen_tcp:send(S, select_data(What)),
   recloop().

recloop() ->
   receive
       X ->
           io:format("GOT ~p~n", [X]),
           recloop()
   after 4000 ->
           timeout
   end.

select_data(What) ->
    case What of
        bug ->
            bug();
        data ->
            data()
    end.

data() ->
   Msg = "dfoo",
   H = "POST /invalid/url HTTP/1.1\r\n"
       "Connection: close\r\n"
       "Host: localhost:8000\r\n"
       "User-Agent: perl post\r\n"
       "Content-Length: 4\r\n"
       "Content-Type: text/xml; charset=utf-8\r\n"
       "\r\n",
   H ++ Msg.


%% space bug
bug() ->
   Msg = " foo",
   H = "POST /invalid/url HTTP/1.1\r\n"
       "Connection: close\r\n"
       "Host: localhost:8000\r\n"
       "User-Agent: perl post\r\n"
       "Content-Length: 4\r\n"
       "Content-Type: text/xml; charset=utf-8\r\n"
       "\r\n",
   H ++ Msg.



-- 
The great enemy of the truth is very often not the lie -- deliberate,
contrived and dishonest, but the myth, persistent, persuasive, and
unrealistic. Belief in myths allows the comfort of opinion without the
discomfort of thought.
John F. Kennedy 35th president of US 1961-1963 (1917 - 1963)
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