Request for help debugging screen problem

Shaddy Baddah
Sun Feb 7 15:48:00 GMT 2010


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 

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)

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???


More information about the Cygwin-developers mailing list