Re: [OE-core] [PATCH] rng-tools: add systemd-udev-settle wants to service

[Kyle Russell <bkylerussell@gmail.com>]

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Thanks, Claudius.

On Wed, Feb 2, 2022 at 8:08 AM Claudius Heine <ch@denx.de> wrote:

> Hi Kyle,
>
> On 2022-02-02 13:38, Kyle Russell wrote:
> > Is this the correct approach?  Even the systemd-udev-settle.service man
> > pages recommends not using its service.  Were the kernel modules really
> > not loaded when rngd started?  Or is the original problem just a matter
> > of waiting for sufficient entropy?
>
> IIRC, the rngd could not find any random source device node (/dev/hwrng
> in that case), so the service failed to start.
>

If /dev/hwrng didn't exist, this feels like the original problem was a
misconfigured
kernel or module that wasn't being loaded properly.


> The patch you are commenting on only adds `Wants` weak dependency to
> make sure `systemd-udev-settle.service` is pulled in to the job queue,
> the `After` ordering rule was already there.
>

Correct.  Just because an `After` exists does not mean the service gets
pulled into
the job queue, so prior to this change no other service was causing the
deprecated
systemd-udev-settle.service to be run during boot.  But now, every device
including
openssh (which has a default PACKAGECONFIG option for rng-tools) now depends
on this deprecated service, which may cause unexpected boot delays.


> So changing this service file to be triggered by a udev event or maybe
> wrap it in a script, which makes sure the right modules are loaded and
> device nodes are available, could be an improvement, but it would be out
> of scope of this patch IMO.
>

I'm more curious whether this change should be reverted from upstream.  It
seems
like a drop-in file could have been applied to your distro instead of
adding a dependency
on a deprecated service for all openssh users.

Kyle


> >
> > On Fri, Sep 17, 2021 at 4:08 AM Claudius Heine <ch@denx.de
> > <mailto:ch@denx.de>> wrote:
> >
> >     rngd needs to start after `systemd-udev-settle` in order for the
> kernel
> >     modules of the random source hardware to be loaded before it is
> started.
> >
> >     However, since the `rngd.service` does not require or want
> >     `systemd-udev-settle.service` it might not be scheduled for start and
> >     the `After=systemd-udev-settle.service` there has no effect.
> >
> >     Adding `Wants=systemd-udev-settle.service` provides a weak
> requirement
> >     to it, so that the `rngd` is started after it, if possible.
> >
> >     Signed-off-by: Claudius Heine <ch@denx.de <mailto:ch@denx.de>>
> >     ---
> >
> >     Hi,
> >
> >     this is a fix, which should probably be backported to the maintained
> >     releases.
> >
> >     regards,
> >     Claudius
> >
> >       meta/recipes-support/rng-tools/rng-tools/rngd.service | 1 +
> >       1 file changed, 1 insertion(+)
> >
> >     diff --git a/meta/recipes-support/rng-tools/rng-tools/rngd.service
> >     b/meta/recipes-support/rng-tools/rng-tools/rngd.service
> >     index 0559b97991..568686e80e 100644
> >     --- a/meta/recipes-support/rng-tools/rng-tools/rngd.service
> >     +++ b/meta/recipes-support/rng-tools/rng-tools/rngd.service
> >     @@ -3,6 +3,7 @@ Description=Hardware RNG Entropy Gatherer Daemon
> >       DefaultDependencies=no
> >       After=systemd-udev-settle.service
> >       Before=sysinit.target shutdown.target
> >     +Wants=systemd-udev-settle.service
> >       Conflicts=shutdown.target
> >
> >       [Service]
> >     --
> >     2.33.0
> >
> >
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