[Bug 50971] Re: kdesu will not start apps

Nicolai Hähnle prefect_ at gmx.net
Wed Feb 21 14:20:34 UTC 2007

Okay, things appear to be slightly more complicated. Here's an excerpt
of the processes running when the bug appears.

 5555 ?        S      0:00  \_ kdesu -u root -c adept_manager
 5558 pts/1    Ss+    0:00  |   \_ /usr/bin/kdesu_stub -
 5561 pts/2    Ts+    0:00  |   \_ /usr/bin/sudo -u root /usr/bin/kdesu_stub -

Since sudo performs an exec() and thus gets out of the way once it has
succeeded, this means that kdesu has run sudo twice. The first time
appears to be due to the call of checkNeedPassword() at
kdebase/kdesu/kdesu/kdesu.cpp:338. This is the instance on pseudo
terminal pts/1, which probably caused kdesu to think that it needed no
password (see my explanation in the previous post). This explains why
kdesu does not show the password prompt.

The second time is the real sudo attempt. However, because - for some
reason - the kdesu_stub from the first attempt is still hanging around,
the second sudo is started on the second pseudo terminal. Therefore,
this second sudo attempt does require a password and therefore hangs
during sudo.

Now the code in ConverseSU() in kdelibs/kdesu/su.cpp looks like it
should take care of that problem, but apparently it doesn't. The effect
is that according to backtraces, the kdesu process is already in
ConverseStub(), while the sudo child process is still waiting for a

So it looks like there are two problems in the code, which together
cause this bug:

1) Kdesu uses checkNeedPassword(), but this function is fundamently
unable to predict whether the next sudo attempt will prompt for a
password or not. The code as it currently stands is inherently racy and
makes assumptions that are fundamentally ***wrong***.

2) Some problem in ConverseSU() or the surrounding code causes us to
miss the fact that sudo is still waiting for a password.

kdesu will not start apps

More information about the kubuntu-bugs mailing list