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Re: Request for help debugging screen problem


Hi,

On 8/02/2010 11:52 AM, Corinna Vinschen wrote:
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).

Indeed.


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.

Thanks for that. I'll give it a try, but I'm sure I'll see the behaviour you described.


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.

No, I didn't notice. I can get a little to eager at times, and miss important steps. Plus I can only devote sporadic time to the issues. I'll read through it and give it a try.


Thanks,
Shaddy


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