Re: [PATCH] system/rtc: Fix a possible year-2038 integer overflow problem

On Mon, May 18, 2026 at 05:10:36PM +0100, Daniel P. Berrangé wrote:
> On Mon, May 18, 2026 at 06:02:55PM +0200, Thomas Huth wrote:
> > From: Thomas Huth <[email protected]>
> > 
> > rtc_realtime_clock_offset is initialized with:
> > 
> >   rtc_realtime_clock_offset = qemu_clock_get_ms(QEMU_CLOCK_REALTIME) / 1000;
> > 
> > And QEMU_CLOCK_REALTIME might be based on gettimeofday() in certain
> > cases (see get_clock_realtime() in include/qemu/timer.h). So this
> > counter will exceed 32 bits in the year 2038, thus we should not
> > store this value in a normal integer variable. Change it to an int64_t
> > to fix the problem.
> > And while we're at it, also adjust the nearby rtc_host_datetime_offset
> > variable to be on the safe side in the related code.
> > 
> > Signed-off-by: Thomas Huth <[email protected]>
> > ---
> >  system/rtc.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> Reviewed-by: Daniel P. Berrangé <[email protected]>

Actually on second reading I think this patch is not quite right.
The code which uses the value is:

    time_t value = qemu_clock_get_ms(clock) / 1000;
    switch (clock) {
    case QEMU_CLOCK_REALTIME:
        value -= rtc_realtime_clock_offset;


On 64-bit platforms 'int' was 64-bit already and so was time_t so we
have no bug to fix.

On 32-bit platforms the patch fixes 'int' to 'int64' which is fine
but that int64 value is then subtracted from time_t which is usually
going to be 32-bit again, unless the OS was built with 64-bit time_t
on 32-bit which almost no one does.

IOW we only platform this fixes is 32-bit OS with 64-bit time_t.

We can't do any better than that as the caller code paths need a
time_t to pass into gmtime/localtime.

So IMHO we should just declare it as time_t. 

With regards,
Daniel
-- 
|: https://berrange.com       ~~        https://hachyderm.io/@berrange :|
|: https://libvirt.org          ~~          https://entangle-photo.org :|
|: https://pixelfed.art/berrange   ~~    https://fstop138.berrange.com :|