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Bug#611487: xterm: immediately exits upon running: exec login USER

On Tue, 8 Feb 2011, Thomas Dickey wrote:


On Thu, 3 Feb 2011, Thomas Dickey wrote:


On Thu, 3 Feb 2011, Jeffrey Sheinberg wrote:


On Thu, Feb 03, 2011 at 05:47:46AM -0500, Thomas Dickey wrote:

From: Thomas Dickey <dickey@his.com>
To: Thomas Dickey <dickey@his.com>, 611487@bugs.debian.org
Date: Thu, 3 Feb 2011 05:47:46 -0500 (EST)
Subject: Re: Bug#611487: xterm: immediately exits upon running: exec login 
 USER
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On Wed, 2 Feb 2011, Thomas Dickey wrote:


On Sat, 29 Jan 2011, Jeffrey Sheinberg wrote:


This is a bug in xterm Version: 261-1 as I originally reported, please
see my above comments.


I can reproduce this difference without the utempter library
(though it's possible that the problem is related to ifdef's for
it).  I'll look for a solution and close out #268 "soon" (have
been busy setting up new development machines).


hmm - my quick check "without" was also without setgid, so there was no
utmp entry.

Reading the source for login, I see that the error message comes
from a check (a) not running as root and (b) no valid utmp entry
found.

Given that, the likely explanation is that login is dropping the
setuid itself.  I don't see that in the unpatched source, but see
comments in the Debian patch which indicate that's intended
behavior.

Looks like a possible workaround would be to use sudo or other
wrapper that holds the setuid behavior.



Hi Thomas,

1. As I originally wrote, I can "exec login" from xterm-235-2
successfully, but not from xterm-261-1, with no other changes to
my system.


right - essentially that's because #235 is setgid to utmp,
and opens the utmp file directly.  With utempter, there's a
separate process that opens it.


2. Cyril noted in his reply that xfce4-terminal behaves similar to
xterm-261-1, so I checked this out, I also tried it for lxterminal - but
they have a different behavior than xterm-261-1,

   A. Xterm-261-1 seems to exit without ever invoking the login
   authentication process.

   B. Both xfce4-terminal and lxterminal invoke login and the
   authentication process is successful (it's in the syslog), and then
   they each exit.


thanks - I'll look/see why they're different from xterm.  ldd tells me
they're not using utempter, for instance.


So I still maintain that there is a bug in xterm-261-1 compared to
xterm-235-2.

Thanks,


no problem (will keep studying it)


looking at lxterminal now, and compiling the login program to add some
traces, I'm seeing _that_ fail at the same point, with the same cause.
(I'll compare with xfce4-terminal this evening, and investigate whether
one of the Debian packages might change this behavior).


I did look further, found that in my configuration at least, that I was
unable to get a login prompt from the unmodified setuid-login program
for either lxterminal or xfce4-terminal (or even gnome-terminal).

I simply copied /bin/login to /usr/local/bin/root-login, making it setuid
and setgid to root:staff

Since that prompt is what I understood "authentication process" to
refer to above, I'm puzzled - need more info to reproduce the case where
I would be seeing the VTE-based terminals working as you describe.

--
Thomas E. Dickey
http://invisible-island.net
ftp://invisible-island.net
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