Re: [PATCH 1/7] x86/vdso: Respect COMPAT_32BIT_TIME

Arnd Bergmann
2026-04-13T16:00:21Z

On Fri, Apr 10, 2026, at 09:24, Thomas Weißschuh wrote:
> On Tue, Mar 03, 2026 at 09:50:33PM +0100, Arnd Bergmann wrote:
>> On Tue, Mar 3, 2026, at 19:11, H. Peter Anvin wrote:
>> >
>> > The time zone in the kernel has never worked anyway, as it would require the
>> > kernel to contain at least the forward portion of the zoneinfo/tzdata table in
>> > order to actually work correctly. The only plausible use of it would be for
>> > local time-based filesystems like FAT, but I don't think we bother.
>> >
>> > A bigger question is whether or not we should omit these from the vDSO
>> > completely (potentially causing link failures) or replace them with stubs
>> > returning -ENOSYS.
>>
>> I see no harm in keeping gettimeofday() in the vdso when
>> COMPAT_32BIT_TIME is turned on, as existing code will call it
>> no matter whether it's in the vdso or the syscall.
>
> We would still always keep them for 64-bit ABIs, right?

Yes, I think we can't easily change that now. It was probably
a mistake to keep them in the generic syscall table after
we dropped them for 32-bit non-time32 targets, so riscv64
and loongarch should have not had these in the first place.

>> Equally, I see no point in having either version of
>> gettimeofday() or settimeofday() when COMPAT_32BIT_TIME is
>> disabled, as clearly anything calling it would pass incorrect
>> data for times past 2038.
>
> Should we also drop the syscalls in these cases?
> We will need to keep settimeofday() in some form to support the
> timewarping call done by init.
>
> Recap/Proposal:
>
> * Keep the gettimeofday()/time() syscalls when they are y2038 safe or
> CONFIG_COMPAT_32BIT_TIME is set.
> * The vDSO functions always mirror the systemcall availability.

These sound good.

> * Always provide settimeofday(). If CONFIG_COMPAT_32BIT_TIME is *not*
> set, reject passing any 'tv' argument where it may not be y2038 safe.

This sounds wrong to me now: the case I'm worried about is a 32-bit
system calling settimeofday() based on the value of an RTC or NTP.
The idea of CONFIG_COMPAT_32BIT_TIME=n is to catch this by causing
an intentional ENOSYS error even for valid times, so it doesn't
suddenly start breaking in 2038.

Arnd