Re: [gentoo-user] problems debugging a systemd problem

Canek Peláez Valdés <caneko@×××××.com> wrote:

> On Thu, May 28, 2015 at 1:36 PM, Rich Freeman <rich0@g.o> wrote:
> >
> > On Thu, May 28, 2015 at 2:11 PM, Canek Peláez Valdés <caneko@×××××.com>
> wrote:
> > >
> > > Actually, it does work (see attached screenshot). I set my root= kernel
> > > command line parameter wrong on purpose, and systemd (inside dracut)
> dropped
> > > me inside a rescue shell.
> >
> > Interesting.  Perhaps it just enables shell access.
> >
> > There is a separate option that configures whether dracut drops to a
> > shell at all, or if it just hangs on failure.  The latter might be
> > desirable for security purposes in some cases.
> >
> > Are you sure that you don't get a shell if you don't pass emergency on
> > the command line, but still have an invalid root=?
>
> I wasn't sure, I did a couple of tests more. I comment them below.
>
> > >>  Usually when somebody
> > >> wants a rescue shell, they want it in their root filesystem, and not
> > >> in their initramfs before it has pivoted.  That is why dracut has
> > >> options like rd.break.
> > >
> > > But that doesn't help you at all when the problem is exactly that you
> cannot
> > > mount your root filesystem. With the rescue shell of systemd (inside
> > > dracut), you can analyze the problem, or perhaps even mount your root
> > > filesystem and continue the boot process; the initramfs should have all
> the
> > > necessary tools to do that.
> >
> > rd.break DOES give you a shell before root is mounted, if you tell it to.
> >
> > rd.shell tells dracut to give you a shell if something fails
> >
> > rd.break forces a shell at the specified point, whether something fails
> or not.
> >
> > The official docs do not list emergency as a valid dracut option.
> > Obviously systemd uses it, but again the fact that you had to mangle
> > your root= option sugests that systemd within dracut ignores it if it
> > can mount your root.
>
> No, if you set emergency or rescue, systemd will go to emergency.target and
> rescue.target, respectively.
>
> > >> If the problem were with systemd/services/etc in the actual root
> > >> filesystem (once the actual distro has started booting), then putting
> > >> emergency on the command line should get you a rescue shell.
> > >
> > > Again, what if the problem is before *that*?
> >
> > Then you tell dracut to drop to a shell.  I wasn't aware that the
> > emergency option actually made a difference, though I'm still not 100%
> > sure that was what did it.
>
> I'm now pretty sure it  DOESN'T make a difference when the problem is
> before you can mount root.
>
> > >> The same generally applies to openrc - if the initramfs isn't mounting
> > >> your root filesystem, then passing instructions to openrc won't do
> > >> anything since in that case openrc isn't even running.
> > >
> > > But in this case, systemd *is* inside the initramfs:
> > >
> > > # ls usr/lib/systemd/
> > > network  systemd-cgroups-agent     systemd-journald
>  systemd-shutdown
> > > systemd-vconsole-setup
> > > system   systemd-fsck              systemd-modules-load
>  systemd-sysctl
> > > system-generators
> > > systemd  systemd-hibernate-resume  systemd-reply-password  systemd-udevd
> > >
> > > That's my initramfs. With dracut, systemd *is* the initramfs init
> system.
> >
> > Sure, and that is how mine works as well.  But, obviously systemd in
> > dracut is configured to ignore that parameter when root= is valid,
>
> No, it doesn't ignore it, even if root= is valid.
>
> > otherwise you'd get a shell every time.  I'd have to check the docs,
> > but I suspect that the behavior is configurable, and systemd within
> > the initramfs is configured differently.  If nothing else they could
> > just make the rescue target launch the default target/etc.
>
> As I said, I did the following tests:
>
> 1. Adding "emergency" to the kernel command line, with a valid root=.
> 2. Adding "rescue" to the kernel command line, with a valid root=.
> 2. Leaving root= invalid without adding neither "emergency" nor "rescue".
>
> If root= is valid, with emergency systemd drops you to a shell with your
> root filesystem mounted read-only. With rescue, systemd drops you to a
> shell with all your filesystems mounted read-write.
>
> If root= is invalid, it doesn't matter if you use emergency, rescue, or
> neither, *dracut* drops you to a shell, still inside the initramfs
> obviously. It takes a while; I didn't took the time, but I think it was 3
> minutes. Inside this shell, you can use systemd normally, and if you manage
> to mount the root filesystem, I'm sure you could continue the normal boot
> process. You'll have to pivot root manually, though.
>
> Hope that makes it clear.

How do you pivot route manually?


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