Re: [PATCH v3] Define in_int32_t_range to check if the 64 bit time_t syscall should be used

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 08/11/22 11:07, Arnd Bergmann wrote:
> On Tue, Nov 8, 2022, at 14:49, Adhemerval Zanella Netto wrote:
>> On 08/11/22 10:27, Arnd Bergmann wrote:
>>
>> Yes, the 32 bit fallback assumes that you either use the default minimum
>> kernel or configure with --enable-kernel with a value lower than 5.1.
>> And the optimization such as ecf2661281c was added on the basis that for
>> such configuration the 32 time_t is always present.  
>>
>> For __ASSUME_TIME64_SYSCALLS (default fro 64 bit time_t ABI and for 32
>> bit time_t with --enable-kernel=5.1) the 32 bit syscall should not be
>> issued.
> 
> Ok, good. What is the amount of testing that this combination
> (--enable-kernel=5.1, CONFIG_COMPAT_32_BIT_TIME=n) has seen? Is
> this something that is already part of some regression test setup,
> or rather something that is supposed to work but isn't in
> widespread use?

I would say not much I am aware off, I usually check only against stock 
kernels from usual distributions (ubuntu, fedora, etc) and our patchwork 
instance also uses default kernels.

> 
>> There are still the issue for a default configured glibc when running
>> on kernels with CONFIG_COMPAT_32BIT_TIME=y, this would require to remove
>> the fallback optimizations for !__ASSUME_TIME64_SYSCALLS.
> 
> You mean CONFIG_COMPAT_32BIT_TIME=n with !__ASSUME_TIME64_SYSCALLS,
> right? As I said, I'm not too worried about this corner case, as
> long as there is some kind of use feedback that tells users to
> change either glibc or kernel configuration.
> 
> My impression is that the optimizing for running on old kernels
> is not overly helpful, and that changing the fallback logic
> would be better, but this does not feel like a correctness
> issue when general-purpose distros always enable
> CONFIG_COMPAT_32BIT_TIME, and embedded users that want it
> disabled always build a matching glibc as well.

I added the optimization mainly because to disable the use of 32 bit
time_t support requires a configure switch.  It is worries me a bit
the combination of CONFIG_COMPAT_32BIT_TIME=n with 
!__ASSUME_TIME64_SYSCALLS as potential source of obscures bugs, but
I think that it would be unlikely in future when newer kernels will
be more present.