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

On 19/5/26 11:26, Thomas Huth wrote:
> On 19/05/2026 11.18, Thomas Huth wrote:
>> On 19/05/2026 10.24, Philippe Mathieu-Daudé wrote:
>>> On 18/5/26 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.
>>>>
>>>> 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.
>>>
>>> "32-bit *host* OS".
>>>
>>> Per ./configure script, we still allow:
>>> - x86_64 x32 ABI      (CPU_CFLAGS="-mx32")
>>
>> We should maybe deprecate that x32 ABI ...
>>
>>> - sparc32             (CPU_CFLAGS="-m32 -mv8plus -mcpu=ultrasparc")
>>
>> ... and that one as well?
>>
>>> - s390                (CPU_CFLAGS="-m31")
>>
>> Uh, we really still allow 31-bit s390 hosts here? I think this could 
>> be removed nowadays, also upstream kernel recently ditched the 31-bit 
>> userspace:
>>
>>   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/ 
>> commit/?id=8e0b986c59c67e08ada646249f834655a9e6da16
>>
>> Question is whether we can remove it immediately, or whether it needs 
>> to be deprecated first?
> 
> Ah, wait, we completely disallow disallow 32-bit hosts nowadays:
> 
>   https://gitlab.com/qemu-project/qemu/-/ 
> commit/372ec46b9f1215f48a4717f2b7ed969f65bfadc6
> 
> ... so these checks in the configure script are just dead code nowadays 
> that could be removed now? Or is there still some hidden magic here that 
> depends on these lines?

Ah think, I remember reviewing something like that but was looking in
./configure history and forgot to look at meson.build. So this is indeed
dead code.