dovecot - Mar 2019 - getrandom() before forking daemon is blocking init system

Hello

When booting from a slow machine, I can observe dovecot blocking the
whole boot process. I traced it down to the getrandom() system call in
lib/randgen.c, which blocks until the random number generator is
initialized (dmesg "random: crng init done"). This can take up to
three
minutes (!) on my machine, as there is not much entropy available (no
hardware RNG, network VPN is also waiting for random).

Unfortunately dovecot calls getrandom() before forking a daemon, which
as a consequence blocks the whole init process (OpenRC on Gentoo Linux).

I believe this behavior has changed in kernel 4.14:

https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=v4.14.40&id=6e513bc20ca63f594632eca4e1968791240b8f18

Quoting getrandom(2):
"If the urandom source has not yet been initialized, then getrandom()
will block, unless GRND_NONBLOCK is specified in flags."


Dovecot: 2.3.4.1 (f79e8e7e4)

Linux: 4.19.26-gentoo #2 SMP Thu Feb 28 20:30:23 CET 2019 x86_64 AMD
G-T40E Processor AuthenticAMD GNU/Linux


Regards,

Axel

On Tue, Mar 05, 2019 at 05:39:28PM +0100, Axel Burri via dovecot
wrote:
> Hello
> 
> When booting from a slow machine, I can observe dovecot blocking the
> whole boot process. I traced it down to the getrandom() system call in
> lib/randgen.c, which blocks until the random number generator is
> initialized (dmesg "random: crng init done"). This can take up to
three
> minutes (!) on my machine, as there is not much entropy available (no
> hardware RNG, network VPN is also waiting for random).
> 
> Unfortunately dovecot calls getrandom() before forking a daemon, which
> as a consequence blocks the whole init process (OpenRC on Gentoo Linux).
> 
> I believe this behavior has changed in kernel 4.14:
> 
>
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=v4.14.40&id=6e513bc20ca63f594632eca4e1968791240b8f18
> 
> Quoting getrandom(2):
> "If the urandom source has not yet been initialized, then getrandom()
> will block, unless GRND_NONBLOCK is specified in flags."
> 
> 
> Dovecot: 2.3.4.1 (f79e8e7e4)
> 
> Linux: 4.19.26-gentoo #2 SMP Thu Feb 28 20:30:23 CET 2019 x86_64 AMD
> G-T40E Processor AuthenticAMD GNU/Linux
> 
> 
> Regards,
> 
> Axel
> 
It should either block or fail to start. I personally like the idea of 
blocking so it starts up successfully.

Have you tried installing an entropy daemon or something to provide more
entropy? I've seen people suggest haveged before.

On a side note.. I thought you want to call getrandom() after forking 
otherwise all children have the same rng sequence.

> On 05 March 2019 at 18:51 William Taylor via dovecot <dovecot at
dovecot.org> wrote:
> 
> 
> On Tue, Mar 05, 2019 at 05:39:28PM +0100, Axel Burri via dovecot wrote:
> > Hello
> > 
> > When booting from a slow machine, I can observe dovecot blocking the
> > whole boot process. I traced it down to the getrandom() system call in
> > lib/randgen.c, which blocks until the random number generator is
> > initialized (dmesg "random: crng init done"). This can take
up to three
> > minutes (!) on my machine, as there is not much entropy available (no
> > hardware RNG, network VPN is also waiting for random).
> > 
> > Unfortunately dovecot calls getrandom() before forking a daemon, which
> > as a consequence blocks the whole init process (OpenRC on Gentoo
Linux).
> > 
> > I believe this behavior has changed in kernel 4.14:
> > 
> >
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=v4.14.40&id=6e513bc20ca63f594632eca4e1968791240b8f18
> > 
> > Quoting getrandom(2):
> > "If the urandom source has not yet been initialized, then
getrandom()
> > will block, unless GRND_NONBLOCK is specified in flags."
> > 
> > 
> > Dovecot: 2.3.4.1 (f79e8e7e4)
> > 
> > Linux: 4.19.26-gentoo #2 SMP Thu Feb 28 20:30:23 CET 2019 x86_64 AMD
> > G-T40E Processor AuthenticAMD GNU/Linux
> > 
> > 
> > Regards,
> > 
> > Axel
> > 
> 
> It should either block or fail to start. I personally like the idea of 
> blocking so it starts up successfully.
> 
> Have you tried installing an entropy daemon or something to provide more
> entropy? I've seen people suggest haveged before.
> 
> On a side note.. I thought you want to call getrandom() after forking 
> otherwise all children have the same rng sequence.
>
Entropy daemon is very recommended for your server in any case, otherwise
you'll have lots of trouble with SSL.

Aki