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

On 18/05/2026 18.18, Daniel P. Berrangé wrote:
> 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.

Uh, did you mix this up with "long" ?

> 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.

Fine for me, too. I'll send a v2...

  Thomas