On Tue, Nov 13, 2018 at 10:04:33AM +0000, Daniel P. Berrangé wrote:
> On Fri, Nov 09, 2018 at 12:19:30PM +0000, Richard W.M. Jones wrote:
> > Peter Dimitrov and myself were debugging a very peculiar bug when
> > libguestfs is run as a plugin from collectd:
> > 
> >   https://www.redhat.com/archives/libguestfs/2018-November/thread.html#00023
> > 
> > The long story short is that collectd leaks SIGCHLD == SIG_IGN setting
> > into plugins:
> > 
> >   https://www.redhat.com/archives/libguestfs/2018-November/msg00095.html
> > 
> > This means that any plugin that does the usual pattern of:
> > 
> >   pid = fork ();
> >   ...
> >   if (waitpid (pid, NULL, 0) == -1) {
> >     perror ("waitpid");
> >     exit (EXIT_FAILURE);
> >   }
> > 
> > will fail, because the forked subprocess is automatically reaped
> > before waitpid is called, resulting in the wait failing with errno ==
> > ECHILD.
> > 
> > It is possible to work around this by adding:
> > 
> >   signal (SIGCHLD, SIG_DFL);
> > 
> > to the plugin.  However I believe this is a bug in collectd, and it
> > should sanitize signals (and maybe other things) before running
> > plugins.
> 
> Yes, I'm inclined to agree. If an app or library is spawning external
> processes it should take care to restore signal setup to a "normal" state.
> This means not only setting SIG_DFL for all signals, but also just as
> importantly, the signal mask. It should be set to all ones before calling
> fork, and set back to all zeroes immedaitely before execve, as illustrated
> in libvirt to avoid races:
> 
> https://libvirt.org/git/?p=libvirt.git;a=blob;f=src/util/vircommand.c;h=de937f6f9aa91abb518eac98bfac9dcf37e1f5df;hb=HEAD#l304
> 
> https://libvirt.org/git/?p=libvirt.git;a=blob;f=src/util/vircommand.c;h=de937f6f9aa91abb518eac98bfac9dcf37e1f5df;hb=HEAD#l360

This is absolutely right.  What's interesting and new here is that
collectd is actually using dlopen to call external plugins (so not
forking itself).  See collectd.git/src/daemon/plugin.c

However it's still not sanitizing the environment sufficiently around
these calls, so it's the same sort of problem, but in a different
context that I'd not seen/understood before.

nbdkit has the same problem :-(  We already dealt with umask a few
years ago, now I've got to check our signal handlers too ...

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org

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