Re: [PATCH] sshd-regen-keys: Fix sshd deadlock on boot

[Harald Seiler <hws@denx.de>]

On Thu, 2018-12-13 at 13:46 +0100, Henning Schild wrote:
> Am Thu, 13 Dec 2018 09:56:58 +0100
> schrieb Harald Seiler <hws@denx.de>:
> 
> > Currently, when sshd-regen-keys runs dpkg-reconfigure, this
> > will lead to a call to `systemctl restart ssh`.  This call blocks
> > forever because of course the sshd-regen-keys unit, which is a
> > dependency of sshd, hasn't finished at this point and can't do so
> > because it is waiting as well.
> > 
> > To circumvent this deadlock, this commit changes sshd-regen-keys'
> > behavior so sshd is first disabled and only reenabled after the
> > job is done.
> > 
> > Signed-off-by: Harald Seiler <hws@denx.de>
> > ---
> >  .../sshd-regen-keys/files/sshd-regen-keys.service     |  2 +-
> >  .../sshd-regen-keys/files/sshd-regen-keys.sh          | 19
> > +++++++++++++++++++ .../sshd-regen-keys/sshd-regen-keys_0.1.bb
> > >  7 +++++-- 3 files changed, 25 insertions(+), 3 deletions(-)
> > 
> >  create mode 100644
> > meta/recipes-support/sshd-regen-keys/files/sshd-regen-keys.sh
> > 
> > diff --git
> > a/meta/recipes-support/sshd-regen-keys/files/sshd-regen-keys.service
> > b/meta/recipes-support/sshd-regen-keys/files/sshd-regen-keys.service
> > index 3b8231f..a05e1a9 100644 ---
> > a/meta/recipes-support/sshd-regen-keys/files/sshd-regen-keys.service
> > +++
> > b/meta/recipes-support/sshd-regen-keys/files/sshd-regen-keys.service
> > @@ -10,7 +10,7 @@ ConditionPathIsReadWrite=/etc Type=oneshot
> > RemainAfterExit=yes Environment=DEBIAN_FRONTEND=noninteractive
> > -ExecStart=/bin/sh -c "rm -v /etc/ssh/ssh_host_*_key*;
> > dpkg-reconfigure openssh-server"
> > +ExecStart=/usr/sbin/sshd-regen-keys.sh ExecStartPost=-/bin/systemctl
> > disable sshd-regen-keys.service StandardOutput=syslog
> > StandardError=syslog diff --git
> > a/meta/recipes-support/sshd-regen-keys/files/sshd-regen-keys.sh
> > b/meta/recipes-support/sshd-regen-keys/files/sshd-regen-keys.sh new
> > file mode 100644 index 0000000..294e8fa --- /dev/null
> > +++ b/meta/recipes-support/sshd-regen-keys/files/sshd-regen-keys.sh
> > @@ -0,0 +1,19 @@
> > +#!/usr/bin/env sh
> > +
> > +echo -n "SSH server is "
> > +if systemctl is-enabled ssh; then
> > +    SSHD_ENABLED="true"
> > +    systemctl disable --no-reload ssh
> > +fi
> > +
> > +echo "Removing keys ..."
> > +rm -v /etc/ssh/ssh_host_*_key*
> > +
> > +echo "Regenerating keys ..."
> > +dpkg-reconfigure openssh-server
> > +
> > +if test -n $SSHD_ENABLED; then
> > +    echo "Reenabling ssh server ..."
> > +    systemctl enable --no-reload ssh
> > +    systemctl start --no-block ssh
> 
> Do we need the start? Is that not in fact taking us into the same
> issue? ... i guess that is the "--no-block".

Yes, `systemctl start --no-block` queues the unit for starting
and immediately returns, which prevents us from running into the
same deadlock again.

> Henning
> 
> > +fi
> > diff --git
> > a/meta/recipes-support/sshd-regen-keys/sshd-regen-keys_0.1.bb
> > b/meta/recipes-support/sshd-regen-keys/sshd-regen-keys_0.1.bb index
> > 02e9e25..6f12414 100644 ---
> > a/meta/recipes-support/sshd-regen-keys/sshd-regen-keys_0.1.bb +++
> > b/meta/recipes-support/sshd-regen-keys/sshd-regen-keys_0.1.bb @@ -6,9
> > +6,12 @@ MAINTAINER = "isar-users <isar-users@googlegroups.com>"
> > DEBIAN_DEPENDS = "openssh-server, systemd" SRC_URI = "file://postinst
> > \
> > -           file://sshd-regen-keys.service"
> > +           file://sshd-regen-keys.service \
> > +           file://sshd-regen-keys.sh"
> >  
> > +do_install[cleandirs] = "${D}/lib/systemd/system \
> > +                         ${D}/usr/sbin"
> >  do_install() {
> > -    install -v -d -m 755 "${D}/lib/systemd/system"
> >      install -v -m 644 "${WORKDIR}/sshd-regen-keys.service"
> > "${D}/lib/systemd/system/sshd-regen-keys.service"
> > +    install -v -m 755 "${WORKDIR}/sshd-regen-keys.sh"
> > "${D}/usr/sbin/sshd-regen-keys.sh" }
> 
> 
-- 
Harald

DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
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Phone: +49-8142-66989-62  Fax: +49-8142-66989-80   Email: hws@denx.de