NAK: [F][PATCH 0/1] rtc: class: support hctosys from modular RTC drivers

[Dimitri John Ledkov]

Hi,


On Thu, Nov 25, 2021 at 10:09 AM Jesse Sung <jesse.sung at canonical.com> wrote:
>
> On Thu, Nov 25, 2021 at 12:00 AM Dimitri John Ledkov
> <dimitri.ledkov at canonical.com> wrote:
> >
> > But doesn't this break assumptions that systemd makes? I.e. it would
> > sync it's time from the timesync state file, and then loading rtc
> > module would trump this.
> >
> > This change in v5.7 kernel has caused breakage. Does introducing this
> > in v5.4 kernel going to break UC20 machines which sync a better/secure
> > time in the initrd; and then will get broken time from the rtc module
> > because it happens to not be battery backed. Leading to inability to
> > snap refresh?
>
> IMHO a proper solution for this is to have a way to add a list of
> modules when we use ubuntu-core-initramfs to generate the initrd.
> Something like /etc/initramfs-tools/modules in the classic image. For
> example:
> ubuntu-core-initram --extra-kernel-modules <mod1>,<mod2>,<mod3>
> So on one hand we don't need to make every RTC driver built-in, and on
> the other hand hctosys happens before systemd tries to access system
> time hence it acts like a built-in driver to systemd.
>

That has not been true for a while now in recent updates of UC20.

ubuntu-core-initrd starts systemd inside the initrd, it looks up the
timestamp from /usr/lib/clock-epoch which is updated at every initrd
creation time and sets that as a system clock, if this timestamp is
older than the current system time.

Then, snapd-bootstrap runs and reads & validates signed assertions. If
it finds any valid assertions with a newer timestamp than the current
time, it again updates the system clock.

Thus all UC20 builds of recent enough kernels (Since around March
2021) come up correctly with correct time of MAX ( initrd built time,
any assertions used by the image). Since implementing this feature we
have had no bug reports about UC20 machines failing to boot due to
wrong time, including from people using stock kernel, yet brand new
models/account-keys/etc.


> >
> > Can you please explain what is prompting to backport this change into
> > v5.4 kernels?
>
> For systems which don't have Internet access but do have valid RTC
> time, if the RTC driver is a module, the RTC time will never be used
> and instead UC20 would start from a fixed time which causes problems
> in some projects.
>

This sounds not true since Mar 2021. Given the above.

Strong NAK on applying this patch, given the core-initrd solutions
available since Mar 2021, which this patch will undo. Cause it will
reset time to random one, upon loading RTC module which may go back in
time to the past prior to initrd time-epoch & snapd assertions used to
boot.

The kernel RTC / hctosys must be fixed to not reset the clock into the
past; if it has already been adjusted by the userspace. At the moment
hctosys & userspace are racing each other, with userspace having
access to more time information than a battery-less RTC module.


> Thanks,
> Jesse
>
> >
> >
> > --
> > Regards,
> >
> > Dimitri.
> >
> > On Wed, Nov 24, 2021 at 11:01 AM Wen-chien Jesse Sung
> > <jesse.sung at canonical.com> wrote:
> > >
> > > BugLink: https://launchpad.net/bugs/1952077
> > >
> > > == Impact ==
> > > Although we have CONFIG_RTC_HCTOSYS enabled and set CONFIG_RTC_HCTOSYS_DEVICE
> > > to "rtc0", system time would only be set if the RTC driver is built-in. This
> > > may not be an issue on most amd64 systems because CONFIG_RTC_DRV_CMOS is y
> > > for amd64, however it is an issue for other architectures since there may not
> > > be a proper RTC driver enabled as built-in. Also making every RTC driver that
> > > may be used as built-in is not a good option.
> > >
> > > == Fix ==
> > > The commit is in mainline since v5.7:
> > >
> > > == Risk of Regression ==
> > > It's a clean cherry-pick for v5.4 and looks straight forward so risk should be low.
> > >
> > >
> > > Steve Muckle (1):
> > >   rtc: class: support hctosys from modular RTC drivers
> > >
> > >  drivers/rtc/Kconfig   |  3 --
> > >  drivers/rtc/Makefile  |  1 -
> > >  drivers/rtc/class.c   | 61 ++++++++++++++++++++++++++++++++++++++
> > >  drivers/rtc/hctosys.c | 69 -------------------------------------------
> > >  4 files changed, 61 insertions(+), 73 deletions(-)
> > >  delete mode 100644 drivers/rtc/hctosys.c
> > >
> > > --
> > > 2.25.1
> > >
> > >
> > > --
> > > kernel-team mailing list
> > > kernel-team at lists.ubuntu.com
> > > https://lists.ubuntu.com/mailman/listinfo/kernel-team