Request for help debugging screen problem

Corinna Vinschen
Mon Feb 8 11:52:00 GMT 2010

On Feb  8 10:53, Shaddy Baddah wrote:
> On 8/02/2010 9:51 AM, Corinna Vinschen wrote:
> >It's the same OpenProcess problem in just another shape.
> Just to clarify. If you mean the same problem as reattaching to
> screen, I think the new developments reveal that I have no data at
> all on that. It could well be OpenProcess) as well, but I can't
> determine because screen -r isn't even being run by strace. strace
> is failing before it executes the process (evidenced by no ls output
> when I do a strace ls).


> With regards to the calling OpenProcess() on the master, I'd like to
> outline a disparity here. In my cygwin scenario, the session sshd
> process is not owned by the user, but by the same as its parent,
> cyg_server.
> In linux, the session ssh is setuid'ed to the authenticated user.
> I'm wondering why this is not the case with cygwin.

It's the fact that a full privilege separation implementation in sshd
requires the implementation of descriptor passing via UNIX sockets in
Cygwin.  That's just not available so far.  If you change your sshd_config
file on Linux to specify

 UsePrivilegeSeparation no

and restart sshd, you will notice that the process

  sshd: username [priv]

owned by root is not in the process list anymore and the process

  sshd: username@pts/X

process is now owned by root.  That's identical to what happens in
Cygwin all the time.

> My next step is to build screen and try to debug it to a point where
> I can find where in the cygwin dll (if in fact it is the cygwin at
> fault) that there is an issue. Beyond that, without strace at my
> disposal, I'm a bit stuck on how I'm going to debug cygwin. Help
> would be appreciated.

Did you notice the how-to-debug-cygwin.txt file in the winsup/cygwin
dir?  You should build Cygwin with the -DDEBUGGING flag and then you
have a couple of options using GDB rather than strace.


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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