Fail to open a shell

[xavierhardy]

Bug report

Using libssh2 examples, I tried opening shell inside a channel in ssh2-python and it failed. I need to have a channel and a shell kept open for re-use. I cannot use exec because using exec to run a command closes the channel (which is expected).

Here is an example in the tests of libssh2:
https://github.com/libssh2/libssh2/blob/master/tests/ssh2.c#L152
libssh2_channel_shell is a macro:
https://github.com/libssh2/libssh2/blob/master/include/libssh2.h#L775

I get a LIBSSH2_ERROR_SOCKET_DISCONNECT error code (-13) when trying to open the shell.
https://github.com/libssh2/libssh2/blob/master/include/libssh2.h#L451

Steps to reproduce:

Run the SSH daemon locally and add an SSH public key to the authorized_hosts file of any user (here root)

Run the following Python script.

#!/usr/bin/env python
import socket

from ssh2.session import Session
from ssh2.utils import version

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(("localhost", 22))
session = Session()
session.handshake(sock)
session.userauth_publickey_fromfile(
    "root", "/root/.ssh/id_rsa.pub", "/root/.ssh/id_rsa", ""
)
channel = session.open_session()
print("pty")
err_code = channel.pty()
print(err_code)
print("shell")
err_code = channel.process_startup("shell", "")
print(err_code)

channel.close()

pty
0
shell
-13

Expected behaviour: 0 error code on shell request

Actual behaviour: error on shell request

Additional info:
libssh2-1:amd64 1.5.0-2ubuntu0.1
libssh2-1-dev:amd64 1.5.0-2ubuntu0.1
python-libssh2 1.0.0-1.2

[pkittenis]

Hi there,

Thanks for the interest and report.

Yes, this is an issue with the process_startup function. For shell requests, no message should be sent but the function does not allow that. An empty string as the message (as opposed to no message), is not accepted by the server which terminates the connection.

The next version will have a channel.shell() function which was missing - the libssh2 macro for shell process startup - as well as a revised process_startup which allows for no message.

From next version onwards:

channel = session.open_session()
channel.shell()
channel.write('my command\n')
size, data = channel.read()
<..>

Note that a pty is not explicitly required for interactive shells though it may be used.

[xavierhardy]

Yes indeed, the macro in libssh2 uses a null pointer as message. My knowledge of Cython is limited, but I'll try to submit a pull request if I find the time. Thanks a lot!

[xavierhardy]

That was fast, it's small fix but still. Thanks a lot, though I will open a new ticket. I would like to test your branch and the only way to find how to build it was to look at Travis. Keep up the good work!