Re: [Xen-devel] [systemd-devel] [PATCH v5 12/14] autoconf: xen: enable explicit preference option for xenstored preference

["Luis R. Rodriguez" <mcgrof@xxxxxxxx>]

On Thu, Jun 05, 2014 at 01:22:13PM +0200, Lennart Poettering wrote:
> On Thu, 05.06.14 02:31, Luis R. Rodriguez (mcgrof@xxxxxxxx) wrote:
> 
> > On Sun, Jun 01, 2014 at 08:15:47AM +0200, Lennart Poettering wrote:
> > > On Fri, 30.05.14 01:29, Luis R. Rodriguez (mcgrof@xxxxxxxx) wrote:
> > > 
> > > > I'm cc'ing a few security folks as I'd appreciate review on the ideas 
> > > > here,
> > > > in particular that of a launcher idea on system to replace alternatives 
> > > > on the
> > > > ExecStart= line of a systemd service unit file, alternative ideas are of
> > > > course welcomed. I'm also Cc'ing systemd-devel as this subject was 
> > > > reviewed
> > > > a little while ago with nothing concrete being recommended but instead 
> > > > a few
> > > > options being now archived as possibilities. I'm looking for a bit wider
> > > > review of the approaches and recomendations.
> > > > 
> > > > Some general background for non xen folks: old xen requires the launch 
> > > > of
> > > > a daemon which implements supports of the xenstore, which is the 
> > > > database
> > > > that xen uses for information about guests / dom0. There are two 
> > > > supported
> > > > daemons, xenstored (C version) and oxenstored (Ocaml version) but they 
> > > > do the
> > > > same thing. Right now old init lets you override which one you pick 
> > > > through
> > > > an environment variable on /etc/{sysconfig,default}/xencommons, the 
> > > > script
> > > > will use the appropriate on there. Systemd doesn't let you use 
> > > > variables on
> > > > the ExecStart line of a service unit file so alternatives are required.
> > > > 
> > > > The reason I'm being very careful here this could set a precedent and at
> > > > least for the launcher idea it'd require the usage of getenv() and 
> > > > execve(),
> > > > and secure alternatives for these (secure_getenv(), execve_nosecurity())
> > > > have either been merged or suggested before for Linux. The systemd 
> > > > discussion
> > > > is only specific to Linux but if we have a launcher we could consider 
> > > > it for
> > > > other supported OSes. All that said I'd like proper review of the 
> > > > security
> > > > implications of *all* strategies but obviously in particular the 
> > > > launcher
> > > > idea. I want to tread carefuly before setting precedents.
> > > 
> > > You can also just invoke a shell script from ExecStart=. I mean, we try
> > > to deemphesize them in the boot process, but there's nothing wrong with
> > > using shell, if you need to parse shell configuraiton fragments and just
> > > want to execute on ot another program...
> > 
> > I tried this and it didn't work given that systemd expects sd_notify()
> > to be called from the parent process, in this case the shell script.
> 
> Hmm? You should "exec" the real daemon binary at the end, not just fork
> it off. That wait the shell script process is replaced by the daemon
> binary, which is what you want.

I tried both just running it and also running exec foo; both presented
the same issue given that shell exec does not really execve.

  Luis

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