dbclient half-close?

Catalin Patulea cat at vv.carleton.ca
Thu Jul 25 09:07:06 WST 2013


Hm, that broke channel-close-by-child-exit. One more try, where we
check for the child exiting and close writefd as a result. If writefd
is the last remaining open pipe to the child, then we also close the
channel as a whole.

On Wed, Jul 17, 2013 at 3:25 PM, Catalin Patulea <cat at vv.carleton.ca> wrote:
> Attached patch should fix both, and use hard tabs so should apply easily.
>
> Rather than replacing readfd with writefd, *both* are checked for
> FD_CLOSED before closing the entire channel. Then each direction can
> be initially closed independently.
>
> On Wed, Jul 17, 2013 at 7:57 PM, Catalin Patulea <cat at vv.carleton.ca> wrote:
>> dbclient handling of remote EOF/local not at EOF also appears broken:
>>
>> openssh:
>> $ ssh -v root at 1.2.3.4 'exec cat >/dev/null 2>/dev/null'
>> [...]
>> debug1: Entering interactive session.
>> debug1: Sending command: exec cat >/dev/null 2>/dev/null
>> # remote has sent EOF by virtue of &>/dev/null, but local->remote
>> # direction still active
>> foo  # local sends
>> bar
>> ^D  # local EOF, channel tears down
>> debug1: client_input_channel_req: channel 1 rtype exit-status reply 0
>> debug1: channel 1: free: client-session, nchannels 2
>> <exit>
>>
>> Note 'exec' is required to replace shell and prevent it from holding a
>> ref to the stdout pipe.
>>
>> dropbear:
>> $ ./dbclient -v root at lun.nanobit.org 'exec cat >/dev/null 2>/dev/null'
>> [...]
>> TRACE  (31787) 1374079900.330457: process_packet: packet type = 96,  len 5
>> TRACE  (31787) 1374079900.330499: enter recv_msg_channel_eof
>> TRACE  (31787) 1374079900.330513: CLOSE some fd 1
>> TRACE  (31787) 1374079900.330526: sending close, readfd is closed
>> # remote EOF should *not* cause entire channel teardown;
>> # local may still have something to write ("foo bar" from openssh
>> # example).
>> TRACE  (31787) 1374079900.330537: enter send_msg_channel_close 0x1f78660
>> TRACE  (31787) 1374079900.330549: enter cli_tty_cleanup
>> TRACE  (31787) 1374079900.330560: leave cli_tty_cleanup: not in raw mode
>> TRACE  (31787) 1374079900.330606: CLOSE some fd 0
>> TRACE  (31787) 1374079900.330618: CLOSE some fd 2
>> <exit>
>>
>>
>> On Sat, Jul 13, 2013 at 12:51 PM, Catalin Patulea <cat at vv.carleton.ca> wrote:
>>> Maybe the check on common-channel.c:338 should be against writefd
>>> instead of readfd? This would get set by
>>> close_chan_fd(channel->writefd) once recv_eof happens.
>>>
>>> This patch indeed causes 'foo' to surface after input EOF:
>>>
>>> diff -r 69cb561cc4c4 common-channel.c
>>> --- a/common-channel.c Sat Jul 13 11:53:24 2013 +0300
>>> +++ b/common-channel.c Sat Jul 13 12:50:41 2013 +0300
>>> @@ -335,7 +335,7 @@
>>>   }
>>>
>>>   /* And if we can't receive any more data from them either, close up */
>>> - if (channel->readfd == FD_CLOSED
>>> + if (channel->writefd == FD_CLOSED
>>>   && (ERRFD_IS_WRITE(channel) || channel->errfd == FD_CLOSED)
>>>   && !channel->sent_close
>>>   && close_allowed
>>>
>>> On Sat, Jul 13, 2013 at 12:31 PM, Catalin Patulea <cat at vv.carleton.ca> wrote:
>>>> Hi,
>>>>
>>>> I'm seeing a difference in how dbclient handles EOF on input compared
>>>> to openssh client. openssh client propagates input EOF to the remote
>>>> command, but continues pumping command stdout. dbclient seems to abort
>>>> before flushing the stdout buffer.
>>>>
>>>> In the following examples, 1.2.3.4 is an openwrt router running
>>>> dropbear server. The remote command is designed to wait for EOF, then
>>>> output something to stdout.
>>>>
>>>> openssh client:
>>>> $ ssh root at 1.2.3.4 'cat; echo foo'
>>>> ^D
>>>> foo
>>>>
>>>> dbclient:
>>>> $ ./dbclient root at 1.2.3.4 'cat; echo foo'
>>>> ^D
>>>> <no output>
>>>>
>>>> I build dropbear with DEBUG_TRACE and these are the last few lines:
>>>> TRACE (...): empty queue dequeing
>>>> ^D
>>>> TRACE (...): send normal readfd
>>>> TRACE (...): enter send_msg_channel_data
>>>> TRACE (...): enter send_msg_channel_data isextended 0 fd 0
>>>> TRACE (...): maxlen 16375
>>>> TRACE (...): CLOSE some fd 0
>>>> TRACE (...): leave send_msg_channel_data: len 0 read err 17 or EOF for fd 0
>>>> TRACE (...): enter send_msg_channel_eof
>>>> TRACE (...): leave send_msg_channel_eof
>>>> TRACE (...): sending close, readfd is closed
>>>> TRACE (...): enter send_msg_channel_close 0xcbfdc0
>>>> TRACE (...): enter cli_tty_cleanup
>>>> TRACE (...): leave cli_tty_cleanup: not in raw mode
>>>> TRACE (...): CLOSE some fd -1
>>>> TRACE (...): CLOSE some fd 2
>>>>
>>>> I tried reading through the relevant sections of channel-common.c but
>>>> I could use some guidance/background. Is this behaviour intentional?
>>>>
>>>> Catalin
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