Re: Request for help debugging screen problem

[Shaddy Baddah <helium at shaddybaddah dot name>]

Hi,

On 7/02/2010 3:21 PM, Corinna Vinschen wrote:
> On Feb  7 14:01, Shaddy Baddah wrote:
> > I'm sorry, I have abandoned inspect what the issue with cygserver is
> > because I've realised what the real situation is. int
> > fhandler_tty_slave::open (int, mode_t) needs to call OpenProcess
> > with PROCESS_DUP_HANDLE on the tty master process. When logged in
> > via ssh, this is the dedicated sshd process still owned by
> > cyg_server.
>
> Huh?  That's not how you explained the situation originally.  IIUYC, the
> situation is that a desktop user created a screen session and then the
> same user trying to connect to the screen session from a ssh session
> gets a permission denied.  In that case, the screen application is the
> pty master and when trying to connect from the ssh session, it has to
> open the screen process.

Yes, sorry about this. I intended to ammend the scenario in my last 
email and forgot to. I'm now at a very very confusing point in my analysis.

Before I begin, the situation I was faced with was that regardless of 
who created the screen session (inside ssh session or desktop), only the 
desktop user could reattach. I apologise if I didn't communicate that 
clearly.

However, I'm now all at sea. The very initial problem I faced is this. 
Anytime I try to perform a screen -r in a ssh session, I get no output 
and just get returned to the prompt:

shaddy@*** ~
$ (cd && exec screen)
[detached]

shaddy@*** ~
$ screen -r

shaddy@*** ~
$

So, I tried an strace:

shaddy@*** ~
$ strace -o /tmp/screen-r+cygserver-down.strace screen -r
  27629 [main] screen 4940 C:\software\cygwin\bin\screen.exe: *** fatal 
error - couldn't initialize fd 0 for /dev/tty2

At this point, I assumed that by running strace, I was revealing the 
real reason I was being returned to a prompt without attaching to the 
screen session.

However, I now have a new problem. Say I have no screen sessions running 
at all:

shaddy@*** ~
$ screen -r
There is no screen to be resumed.

But I try an strace:

shaddy@*** ~
$ strace -o /tmp/screen-r+cygserver-down.strace screen -r
  23025 [main] screen 5340 C:\software\cygwin\bin\screen.exe: *** fatal 
error - couldn't initialize fd 0 for /dev/tty2

I've completely lost faith in my strace scenarios now. This last 
scenario is how I came to determine that screen was trying to call 
OpenProcess() on the cyg_server owned sshd process. I was intending to 
mention it as it as it was unusual.

Why doesn't this last strace just output the same "There is no screen to 
be resumed." message? I am now hesistant to go on using strace, as I'm 
not sure it is helping me analyse my initial problem. It seems to be 
creating problems of its own???

Regards,
Shaddy