Re: Bug#855203: hwclock-set: Synchronize from hwclock despite systemd presence
- Date: Fri, 17 Mar 2017 19:55:31 +0100
- From: Lukas Wunner <lukas@xxxxxxxxx>
- Subject: Re: Bug#855203: hwclock-set: Synchronize from hwclock despite systemd presence
[cc += pkg-sysvinit-devel@xxxxxxxxxxxxxxxxxxxxxxx,
(as requested by Andreas Henriksson)]
[cc += #785445, #785419, Rick Thomas (bug reporter)]
sorry for the delay. Attached is a new patch to address your review
comments. (Thanks a lot for them!)
The patch no longer merely fixes the issue relating to systemd presence,
but seeks to tackle the problem more comprehensively by making hwclock-set
behave identically to hwclock.sh. That way, users can switch between
sysvinit and systemd-udevd and get the same behavior without having to
change their hwclock configuration or experiencing unpleasant surprises.
On Tue, Feb 28, 2017 at 10:20:24PM +0100, Andreas Henriksson wrote:
> On Mon, Feb 27, 2017 at 02:34:34PM +0100, Lukas Wunner wrote:
> > Okay, let's define a policy first. How about:
> > (1) Users can set HWCLOCKACCESS=no in /etc/default/hwclock to prohibit
> > setting the system time or time zone from any RTC. (The parameter
> > is already defined in the config file, but it's not honored by
> > hwclock-set.)
> > (2) Users can set HCTOSYS_DEVICE in /etc/default/hwclock to constrain
> > setting the system time or time zone to a specific RTC. (Same as
> > above, parameter is already defined but not honored by hwclock-set.
> > This will also fix #785445.) If this parameter is not set,
> > hwclock-set will pick the first RTC that becomes available.
I've changed (2) to pick rtc0 (instead of the first RTC that becomes
available), otherwise I'd introduce a change of behavior (as you've
correctly pointed out).
> > (3) If the kernel has already set the system time from *any* hwclock,
> > we don't set it once more in hwclock-set. (We only adjust the
> > timezone.)
> (3. would unfix #785445 from 2. again though, right? Maybe also support
> HWCLOCKACCESS=force which skips 3. would really offer a solution
> option for #785445 ? Reminder: This might also need 'force' to be
> converted to simply 'yes' (default) in /etc/init.d/hwclock.sh)
Good catch. I've changed (3) to not set the system time if the kernel
has already done so AND the kernel used the same clock that was chosen
by the user in HCTOSYS_DEVICE (or rtc0 if not set by the user).
Thus, in a situation as described in #785445, the user would configure
HCTOSYS_DEVICE=rtc1, thereby instructing hwclock-set to override the
unreliable rtc0 used by the kernel. This is actually just like hwclock.sh
behaves (if udev is not used).
@Rick Thomas: Could you verify if the attached patch solves this issue
> Targeting this at Debian (doesn't suffer from the problem you're trying
> to fix in any officially supported system AFAIK) we'll need to be
> careful to not introduce regressions for any currently existing
> configuration. (There are unfortunately many (non-default) combinations
> to consider, like sysvinit, etc.)
The issue relating to systemd presence does affect Debian if the RTC's
driver is compiled as a module, which is true for these arches:
$ git clone https://anonscm.debian.org/git/kernel/linux.git
$ egrep -rc 'CONFIG_RTC_DRV.*=m$' linux/debian/config | egrep -v ':0$'
Agreed on being careful to avoid regressions though.
> > diff --git a/debian/hwclock.rules b/debian/hwclock.rules
> > index 972e4aa..8584deb 100644
> > --- a/debian/hwclock.rules
> > +++ b/debian/hwclock.rules
> > @@ -1,4 +1,4 @@
> > # Set the System Time from the Hardware Clock and set the kernel's timezone
> > # value to the local timezone when the kernel clock module is loaded.
> > -KERNEL=="rtc0", RUN+="/lib/udev/hwclock-set $root/$name"
> > +KERNEL=="rtc*", RUN+="/lib/udev/hwclock-set $root/$name $kernel"
> In general taking the initial policy and inlining it as comments in the
> sources would be useful for future generations trying to figure out what
> the idea was when this was changes. Also rephrasing it as eg.
> # Don't touch timezone when running systemd, because .....
> The 'because ...' part makes it easy for future generations to evaluate
> if the the check is still valid.
I've added some code comments to hwclock-set to hopefully improve clarity,
however I'm skeptical about keeping old code as comments, the git history
should be sufficient for archeologists. :-)
All your other comments make total sense to me and I've amended hwclock-set
accordingly. Let me know if there is something else you want changed or if
I've missed anything.
>From aac97ffc67512182041715264a9b6559adec22f2 Mon Sep 17 00:00:00 2001
From: Lukas Wunner <lukas@xxxxxxxxx>
Date: Thu, 16 Mar 2017 19:50:39 +0100
Subject: [PATCH] hwclock-set: Behave like hwclock.sh
If udev is installed, "/etc/init.d/hwclock.sh start" becomes a no-op and
the system clock is instead supposed to be set up by a udev rule which
invokes the script /lib/udev/hwclock-set.
Unfortunately this script contains bugs which lead to a behavior different
from that of hwclock.sh and which may have the fatal consequence that
the system clock stays set to the epoch despite presence of an RTC.
Worse, this incorrect system time will then be synced back to the RTC
by the kernel, overwriting the correct time that was stored there.
* The user may configure HWCLOCKACCESS=no in /etc/default/hwclock to
forbid usage of any RTC, e.g. if none is reliable. While hwclock.sh
honors this, hwclock-set does not. Same for HWCLOCKPARS.
* The user may configure a different hwclock than rtc0 (the default),
e.g. if rtc0 is unreliable but another known-good hwclock is present.
Again, hwclock.sh honors this but hwclock-set does not. (Because the
udev rule invoking the script has rtc0 hard-coded.) See #785445.
* If systemd is running, the system clock is not set from the hwclock at
all. Thus, unless the kernel has set the system clock, it will stay
set to the epoch. This is the case if the hwclock's driver was compiled
as a module or if the kernel was compiled with CONFIG_RTC_HCTOSYS
This bug was introduced in 2011 while trying to fix #629811: According
to the bug report, "systemd > 25 sets the hwclock itself, based on the
contents of /etc/adjtime, so /lib/udev/hwclock-set should not do
anything if running under systemd." Unfortunately that assumption is
only half-correct: systemd determines if the RTC runs at UTC or local
time, and warps the system clock accordingly. It does *not* however
set the system clock from the RTC.
The bug particularly affects the Raspberry Pi because Raspbian ships
with a kernel which disables CONFIG_RTC_HCTOSYS and also compiles all
RTC drivers as a module. This makes sense because the Raspberry Pi
does not possess an RTC. Instead, people use add-on boards with
various types of clocks and it wouldn't be reasonable to link all of
their drivers into the kernel. Googling for "systemd" "hwclock-set"
gets 1230 results, the vast majority are recipes for the Raspberry Pi
to manually remove the systemd condition from hwclock-set, cargo-culted
over and over again for the last 6 years.
Fix these issues to make hwclock-set behave like hwclock.sh, thereby
giving users the freedom to switch between sysvinit and systemd-udevd
without having to adjust their configuration settings or hack scripts.
Note that hwclock deliberately continues to differ from hwclock.sh in
the following ways, which may be worth porting over to hwclock.sh in a
* If the kernel has already initialized the system clock from the same
RTC that was configured by the user, hwclock-set does not pointlessly
do so again. (The RTC may have drifted in the meantime.)
* If the kernel has already initialized the system clock from an RTC
and the user has not chosen a specific RTC in /etc/default/hwclock,
hwclock-set will not set the system clock again. This is useful if
the user has compiled a custom kernel and set CONFIG_RTC_HCTOSYS_DEVICE
to something else than rtc0 (the default). By contrast, hwclock.sh
will just blindly overwrite the system clock with rtc0.
This patch was developed for and tested successfully on a Revolution Pi
by Kunbus GmbH.
Cc: Mathias Duckeck <m.duckeck@xxxxxxxxx>
Cc: Phil Elwell <pelwell@xxxxxxxxxxxxxxx>
Cc: Andreas Henriksson <andreas@xxxxxxxx>
Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>
debian/hwclock-set | 40 ++++++++++++++++++++++++++++-----------
debian/hwclock.5 | 7 ++++---
debian/hwclock.rules | 2 +-
debian/util-linux.hwclock.default | 6 ++++--
4 files changed, 38 insertions(+), 17 deletions(-)
diff --git a/debian/hwclock-set b/debian/hwclock-set
index eacf948df..c36137140 100755
@@ -1,12 +1,13 @@
# Reset the System Clock to UTC if the hardware clock from which it
-# was copied by the kernel was in localtime.
+# was copied by the kernel was in localtime and systemd hasn't already
+# done so.
+# Set the System Clock from the hardware clock if the kernel hasn't already
+# done so or if the user has chosen a different hardware clock than the one
+# used by the kernel.
-if [ -e /run/systemd/system ] ; then
- exit 0
if [ -e /run/udev/hwclock-set ]; then
@@ -20,17 +21,34 @@ fi
+HCTOSYS_DEVICE= # defaults to rtc0 unless set by the user, see below
if [ -f /etc/default/hwclock ] ; then
+# Bail out if the user has generally forbidden hwclock access or
+# has chosen a different rtc
+if [ "$HWCLOCKACCESS" != yes ] ||
+ ( [ -n "$HCTOSYS_DEVICE" ] && [ "$rtc" != "$HCTOSYS_DEVICE" ] ) ||
+ ( [ -z "$HCTOSYS_DEVICE" ] && [ "$rtc" != rtc0 ] ) ; then
+ exit 0
if [ yes = "$BADYEAR" ] ; then
- /sbin/hwclock --rtc=$dev --systz --badyear
- /sbin/hwclock --rtc=$dev --hctosys --badyear
- /sbin/hwclock --rtc=$dev --systz
- /sbin/hwclock --rtc=$dev --hctosys
+# Determine the hwclock used by the kernel, if any
+kernel_hctosys=$(grep -l '^1$' /sys/class/rtc/*/hctosys 2>/dev/null)
+if [ ! -e /run/systemd/system ] ; then
+ /sbin/hwclock --rtc=$dev --systz $HWCLOCKPARS $badyear
+if [ -z "$kernel_hctosys" ] ||
+ ( [ -n "$HCTOSYS_DEVICE" ] && [ "$kernel_hctosys" != "$HCTOSYS_DEVICE" ] ) ; then
+ /sbin/hwclock --rtc=$dev --hctosys $HWCLOCKPARS $badyear
# Note 'touch' may not be available in initramfs
diff --git a/debian/hwclock.5 b/debian/hwclock.5
index fb6355f27..51be5cc89 100644
@@ -28,9 +28,10 @@ Additional options for hwclock. Unset by default. For example, this
may be use to specify the machine hardware clock type for Alphas.
-The hardware clock device you want to use. Defaults to \fBrtc0\fP.
-It should probably match the CONFIG_RTC_HCTOSYS_DEVICE kernel config
+The hardware clock device you want to use. The kernel tries to
+initialize the system clock from \fBrtc0\fP by default (subject to the
+CONFIG_RTC_HCTOSYS_DEVICE kernel config option). If rtc0 is unreliable,
+you may override it with a different hardware clock here.
Roger Leigh <rleigh@xxxxxxxxxx>
diff --git a/debian/hwclock.rules b/debian/hwclock.rules
index 972e4aab3..8584deb48 100644
@@ -1,4 +1,4 @@
# Set the System Time from the Hardware Clock and set the kernel's timezone
# value to the local timezone when the kernel clock module is loaded.
-KERNEL=="rtc0", RUN+="/lib/udev/hwclock-set $root/$name"
+KERNEL=="rtc*", RUN+="/lib/udev/hwclock-set $root/$name $kernel"
diff --git a/debian/util-linux.hwclock.default b/debian/util-linux.hwclock.default
index dcf5451fe..0ccf3d53f 100644
@@ -13,7 +13,9 @@
# as machine hardware clock type for Alphas.
-# Set this to the hardware clock device you want to use, it should
-# probably match the CONFIG_RTC_HCTOSYS_DEVICE kernel config option.
+# Set this to the hardware clock device you want to use. The kernel
+# tries to initialize the system clock from rtc0 by default (subject
+# to the CONFIG_RTC_HCTOSYS_DEVICE kernel config option). If rtc0 is
+# unreliable, you may override it with a different hardware clock here.