[tor-bugs] #32034 [Core Tor/Tor]: tor reads PT protocol messages from stderr

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#32034: tor reads PT protocol messages from stderr
------------------------------+--------------------------------
     Reporter:  dcf           |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:
    Component:  Core Tor/Tor  |    Version:  Tor: 0.4.2.2-alpha
     Severity:  Normal        |   Keywords:
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
------------------------------+--------------------------------
 Create a fake server transport plugin that writes PT protocol strings to
 stderr, not stdout:

 {{{#!sh
 #!/bin/sh
 (
 echo "VERSION 1"
 echo "SMETHOD testpt 127.0.0.1:9999"
 echo "SMETHODS DONE"
 ) 2>&1
 }}}

 Verify that it writes nothing to stdout:

 {{{
 $ ./testpt.stderr.sh >/dev/null | wc
       0       0       0
 }}}

 Run tor with this torrc:

 {{{
 PublishServerDescriptor 0
 AssumeReachable
 SOCKSPort 0

 ORPort auto
 ServerTransportPlugin testpt exec ./testpt.stderr.sh
 Bridge testpt 127.0.0.1:9999
 }}}

 A log line shows that tor has interpreted PT protocol messages from
 stderr, when it should only be looking for them on stdout.

 {{{
 Oct 10 16:21:11.000 [notice] Registered server transport 'testpt' at
 '127.0.0.1:9999'
 }}}

 ----

 I think this is a bug. [https://gitweb.torproject.org/torspec.git/tree/pt-
 spec.txt?id=d890052d5525a09829c798ab0ad6dcdcede1a8ef#n368 pt-spec says]
 "All Pluggable Transport Proxies communicate to the parent process via
 writing NL-terminated lines to stdout." It does not say that tor should
 inspect stderr and interpret any lines that happen to be a well-formed PT
 protocol message that it finds there, as if they had been sent on stdout.

 The only mention of stderr in the spec is
 [https://gitweb.torproject.org/torspec.git/tree/pt-
 spec.txt?id=d890052d5525a09829c798ab0ad6dcdcede1a8ef#n599 in the context
 of LOG and STATUS], and that is
 [https://gitweb.torproject.org/torspec.git/commit/pt-
 spec.txt?id=d890052d5525a09829c798ab0ad6dcdcede1a8ef a fairly recent
 addition] with #28179/#28181. I always thought that the intention was to
 copy anything seen on stderr to tor's log, as if it had been part of a LOG
 message on stdout. The mention of stderr in relation to STATUS is
 something I missed during review of that part of the spec, because I don't
 think it makes sense there.

 What I expected to see was this, treating verbatim stderr lines as things
 to be logged:
 {{{
 [notice] Unknown line received by managed proxy (VERSION 1)
 [notice] Unknown line received by managed proxy (SMETHOD testpt
 127.0.0.1:9999)
 [notice] Unknown line received by managed proxy (SMETHODS DONE)
 }}}

 Otherwise, tor is essentially holding transport plugins to a contract that
 is unstated in the spec: transport plugins cannot safely write anything to
 stderr at all, because there is always a danger that a future version of
 tor will recognize it as a PT message and interpret it, instead of just
 logging it.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/32034>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online

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