[Buildroot] [PATCH] package/haveged: Allow service to run early

Jérémy ROSEN jeremy.rosen at smile.fr
Sun Jun 7 19:47:08 UTC 2020


Le dim. 7 juin 2020 à 21:42, Norbert Lange <nolange79 at gmail.com> a écrit :

> Am So., 7. Juni 2020 um 21:27 Uhr schrieb Jérémy ROSEN <
> jeremy.rosen at smile.fr>:
> >
> >
> >
> > Le dim. 7 juin 2020 à 21:18, Norbert Lange <nolange79 at gmail.com> a
> écrit :
> >>
> >> Am So., 7. Juni 2020 um 21:05 Uhr schrieb Jérémy ROSEN <
> jeremy.rosen at smile.fr>:
> >> >
> >> >
> >> >
> >> > Le dim. 7 juin 2020 à 20:35, Norbert Lange <nolange79 at gmail.com> a
> écrit :
> >> >>
> >> >> Am So., 7. Juni 2020 um 12:35 Uhr schrieb Jérémy ROSEN <
> jeremy.rosen at smile.fr>:
> >> >> >
> >> >> >
> >> >> >
> >> >> > Le sam. 6 juin 2020 à 00:55, Norbert Lange <nolange79 at gmail.com>
> a écrit :
> >> >> >>
> >> >> >> Drop default dependencies, haveged needs nothing but
> >> >> >> local sockets and /dev/random.
> >> >> >> This allows the entropy to be available earlier.
> >> >> >>
> >> >> >> Signed-off-by: Norbert Lange <nolange79 at gmail.com>
> >> >> >> ---
> >> >> >>  package/haveged/haveged.service | 5 ++++-
> >> >> >>  1 file changed, 4 insertions(+), 1 deletion(-)
> >> >> >>
> >> >> >> diff --git a/package/haveged/haveged.service
> b/package/haveged/haveged.service
> >> >> >> index 91035c6711..ed9f043a91 100644
> >> >> >> --- a/package/haveged/haveged.service
> >> >> >> +++ b/package/haveged/haveged.service
> >> >> >> @@ -1,10 +1,13 @@
> >> >> >>  [Unit]
> >> >> >>  Description=Entropy Harvesting Daemon
> >> >> >>  Documentation=man:haveged(8)
> >> >> >> +DefaultDependencies=no
> >> >> >> +Conflicts=shutdown.target
> >> >> >> +Before=sysinit.target shutdown.target systemd-update-done.service
> >> >> >>
> >> >> > Since v243 (quoting  systemd's NEWS file) :
> >> >> >
> >> >> > * systemd-random-seed.service is now a synchronization point for
> full
> >> >> >           initialization of the kernel's entropy pool. Services
> that require
> >> >> >           /dev/urandom to be correctly initialized should be
> ordered after this
> >> >> >           service.
> >> >> >
> >> >> > so... in that case it might make sense to add
> Before=systemd-random-seed.service
> >> >> > (assuming that works. This a very early point, so it needs some
> testing)
> >> >>
> >> >>
> >> >> No, doesnt make sense, a "before"  only means the service is
> *started before*,
> >> >> not the process did do anything worthwile. (That's the reason for my
> >> >> openssh patch BTW)
> >> >> systemd-random-seed.service is a one-shot, so a "before" *that*
> >> >> service means the dependants wait till it is finished.
> >> >>
> >> > (note that I don't really understand what haveged does, so I might be
> completely wrong in my
> >> > understanding)
> >> >
> >> > systemd-random-seed is the sync point, that means that any service
> that needs
> >> > random numbers should  be ordered after random-seed.
> >> >
> >> > So, to me, it makes sense that we want to have haveged started before
> that point. Thus
> >> > having haveged ordered before random-seed.
> >> >
> >> > and I don't really understand your sentence above...
> >> > if you add Before=systemd-random-seed, it's systemd-random-seed that
> will wait for
> >> > haveged to be started, not the other way round. (which seems to be
> what you say ??)
> >>
> >>
> >>  Yes, systemd-random-seed will wait for haveged to be *startet* (and
> >> maybe its immediatly preempted),
> >> but it takes several seconds after that till /dev/urandom is "ready"
> >> (what that means changed in the last few years multiple times)
> >>
> > ... yeah ...
> >>
> >> >
> >> > now... if haveged needs some time to "do something usefull" then it
> means that haveged
> >> > is badly synchronised on startup. The Ready signal in systemd is
> defined as "the point
> >> > at which the service is able to render the service expected for it'
> >>
> >>
> >> Yes, but haveged does not send such a signal (and systemd only wait
> >> for that if the Type=notify).
> >
> >
> > There is a workaround for that. Add an ExecStartPost that detects
> readiness.
> > (mentionning that for completeness sake, not sure if it's worth doing
> here.)
> >
> >>
> >> Further the service only feeds data to the kernel, and the only valid
> >> meaning of /dev/urandom is "ready"
> >> comes from the kernel.
> >>
> > and random-seed will wait for that, so syncronisation will work correctly
>
> Nope, it does not. It's a (poor) substitute to wait after some entropy
> was fed to the kernel.
>
> comment at the bottom of random-seed.service:

# This service waits until the kernel's entropy pool is initialized, and
may be
# used as ordering barrier for service that require an initialized entropy
# pool. Since initialization can take a while on entropy-starved systems,
let's
# increase the time-out substantially here.

and from the man page

 This new seed is  retrieved synchronously from the kernel, which means the
service will not complete start-up until the random pool is fully
initialized.

 I might misread stuff, as you stated randomness handing in linux is
complicated
but I really think that the initial idea behind that service was
synchronisation...

>
> >>
> >>  The point of waiting for "systemd-random-seed" is a substitute if you
> >> are in a really tough spot,
> >> and independent of what haveged does.
> >>
> > No, it's more than a substitute. it's a sync point.
> > It doesn't just load entropy into the kernel, it also blocks until the
> kernel reports
> > that /dev/urandom is ready
>
> Show me where?
>
> >
> > IOW the only way to make sure that haveged is started "at the right
> point" in a generic
> > way is to add that Before=
> >
> > That would allow any service that needs reliable randomness to be
> correctly synced
> > with haveged, even if they are not aware that haveged is here...
>
> Well, consider that systemd-random-seed would block for that (which it
> does not),
> then it does not matter *when* haveged is started, only that it is not
> blocked (and it is not).
> In other words, it would make no difference.
>
> Norbert.
>


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