Re: GNOME System Monitor will use libgnomesu

[Hongli Lai <h lai chello nl>]

Carlos Garnacho wrote:
> which is exactly the technical problem? GST handles both su and ssh
> quite successfully, and my authentication thingy try (system-auth-agent)
> does this too in a g_spawn fashion.

You want the child process's stdin and stdout. However, su and ssh both 
use stdout to provide information, and read their password from the 
terminal. How do you detect whether your password is wrong? You read 
su/ssh's output and check whether it contains 'Incorrect password'. But 
what if the login actually succeeded, and the child process outputs 
'Incorrect password', not su/ssh?

And how do you know the login succeeded? What if you want to immediately 
return after the login succeeded, and you don't care about the child's 
output? You can't do that with plain su/ssh, unless you don't check for 
'Incorrect password' either.

And furthermore, it's very difficult to correctly use a pseudo terminal.
I wrote a program which communicates with 'su' using a pseudo terminal 
(using getpt() and grandpt(); this program is not related to 
libgnomesu). For some reason, on my computer (FC1), I can't read 
anything from pseudo terminal after su prints the 'Password:' prompt, if 
the login succeeded. So if the child process prints something, I can't 
read it. The strange thing is, I *can* read if the login failed. I 
checked my code 10 times and I went through all the manuals and 
tutorials I can find, and I couldn't figure out why it doesn't work.
The su implementations on different distributions tend to behave 
differently. For example, on Slackware systems without Dropline, su 
insists on reading the password from the *real* terminal, not the pseudo 
terminal I created. Installing Dropline (and PAM) mysteriously fixed the 
problem. On some distributions, getpt() just fails for no good reason. 
And there are other distributions which show other problems.

Back to libgnomesu. I've thoroughly investigated GST's, kdesu's and 
gtksu's source code while writing libgnomesu. My first attempt was to 
use pipes, but newer su's don't like it. forkpty() somehow worked, but 
doesn't work on post-RedHat 7 systems. getpt() is apparently not 
available on FreeBSD: GST uses a different function. But that function 
doesn't seem to work well with all su implementations. There doesn't 
seem to be any other portable functions for creating a pseudo terminal.

Eventually I gave up and just modified GNU su.