Bug 24821

Summary:	errno is lost if activation fork() or exec() fails
Product:	dbus	Reporter:	Carl Worth <cworth>
Component:	core	Assignee:	Simon McVittie <smcv>
Status:	RESOLVED FIXED	QA Contact:	D-Bus Maintainers <dbus>
Severity:	normal
Priority:	medium	CC:	chengwei.yang.cn, msniko14, rosslagerwall
Version:	1.5	Keywords:	patch
Hardware:	All
OS:	All
Whiteboard:	review?
i915 platform:		i915 features:
Attachments:	dbus-spawn: do not forget the exec() errno when the grandchild exits

Description Carl Worth 2009-10-30 13:54:13 UTC

A recent "event" on my machine caused the ownership of a few files to
be flipped fairly randomly. One file that happened to be hit was:

	/usr/lib/dbus-1.0/dbus-daemon-launch-helper

It saw its group change from "messagebus" to a random different
group. This meant that dbus could not launch any helpers which led to
random and hard-to-diagnose misbehavior[*] on the system.

I would have loved it if dbus would have made some obvious noise in my
syslog about the problem, but it was silent as far as I could tell. I
ended up having to do more strace-ing and reverse-engineering than I
really felt like doing.

Thanks,

-Carl

[*] Specifically, network-manager failed to launch the
nm-systems-settings daemon, which led to it being unable to ask for
the current hostname, which in turn caused it to set my hostname to
"localhost.localdomain" every time a device connected. All of this
combined meant that if I plugged in a network cable, then suddenly I
couldn't talk to my X server anymore, (since the hostname had change
the Xauthority tokens were no longer valid).

Comment 1 Simon McVittie 2011-07-21 05:44:44 UTC

(In reply to comment #0)
> I would have loved it if dbus would have made some obvious noise in my
> syslog about the problem

Colin added system-bus activation logging to syslog for Bug #35705, which was released in 1.4.10. Here's what I get for a similar failure mode:

Jul 21 13:31:38 reptile dbus[1570]: [system] Activated service 'org.freedesktop.SystemToolsBackends.SMBConfig' failed: Failed to execute program /usr/lib/dbus-1.0/dbus-daemon-launch-helper: Success

For some reason the errno for execve() failing (which should be EACCESS) is apparently getting lost somewhere, but other than that, it works...

Comment 2 Chengwei Yang 2013-12-07 12:20:53 UTC

(In reply to comment #1)
> (In reply to comment #0)
> > I would have loved it if dbus would have made some obvious noise in my
> > syslog about the problem
> 
> Colin added system-bus activation logging to syslog for Bug #35705, which
> was released in 1.4.10. Here's what I get for a similar failure mode:
> 
> Jul 21 13:31:38 reptile dbus[1570]: [system] Activated service
> 'org.freedesktop.SystemToolsBackends.SMBConfig' failed: Failed to execute
> program /usr/lib/dbus-1.0/dbus-daemon-launch-helper: Success
> 
> For some reason the errno for execve() failing (which should be EACCESS) is
> apparently getting lost somewhere, but other than that, it works...

Still a bug, the errno (EACCES) was recorded, but then overwritten. Related logs like below.

[dbus-spawn.c(1000):do_exec] Child process has PID 15253
15230: [dbus-spawn.c(560):handle_babysitter_socket] Reading data from babysitter
15230: [dbus-spawn.c(496):read_data] recorded grandchild pid 15253
15230: [dbus-spawn.c(576):handle_error_pipe] Reading data from child error
15230: [dbus-spawn.c(471):read_data] recorded exec errnum 13
15230: [dbus-spawn.c(532):close_error_pipe_from_child] Closing child error
15230: [dbus-watch.c(418):_dbus_watch_list_remove_watch] Removing watch on fd 10
15230: [dbus-watch.c(629):dbus_watch_set_data] Setting watch fd -1 data to data = (nil) function = (nil) from data = (nil) function = (nil)
15252: [dbus-spawn.c(1069):check_babysit_events] reaped child pid 15253
15230: [dbus-spawn.c(560):handle_babysitter_socket] Reading data from babysitter
15230: [dbus-spawn.c(459):read_data] recorded child status exited = 1 signaled = 0 exitstatus = 1 termsig = 0
15230: [dbus-spawn.c(560):handle_babysitter_socket] Reading data from babysitter
15230: [dbus-spawn.c(511):close_socket_to_babysitter] Closing babysitter
15230: [dbus-watch.c(418):_dbus_watch_list_remove_watch] Removing watch on fd 12
15230: [dbus-watch.c(629):dbus_watch_set_data] Setting watch fd -1 data to data = (nil) function = (nil) from data = (nil) function = (nil)

Comment 3 Simon McVittie 2014-09-11 09:47:05 UTC

OK, so the bug as reported has been fixed: failure to start dbus-daemon-launch-helper is logged.

The remaining bug is errno being lost, and I can see why now.

Comment 4 Simon McVittie 2014-09-11 10:01:26 UTC

Created attachment 106123 [details] [review]
dbus-spawn: do not forget the exec() errno when the  grandchild exits

As is already noted in a comment in
_dbus_babysitter_set_child_exit_error(), if the grandchild fails
to exec() the desired process, we get both CHILD_EXEC_FAILED (with
an errno) and CHILD_EXITED (with a status), and we want to report
the former, since it is more informative. However, clearing
sitter->errnum meant we lose the errno value.

---

I haven't tested this in a live system yet, but I believe it should address the missing errno.

Comment 5 Simon McVittie 2014-10-16 18:29:34 UTC

Comment on attachment 106123 [details] [review]
dbus-spawn: do not forget the exec() errno when the  grandchild exits

I've tested this with a dbus-daemon-launch-helper chmod'd to be non-executable, and it does work as intended.

Comment 6 Ross Lagerwall 2014-10-21 19:20:16 UTC

*** Bug 85171 has been marked as a duplicate of this bug. ***

Comment 7 Simon McVittie 2014-10-29 16:23:27 UTC

Fixed in git for 1.9.2

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