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

[Adhemerval Zanella Netto via Libc-alpha <libc-alpha@sourceware.org>]

On 08/11/22 09:28, Arnd Bergmann wrote:
> On Tue, Nov 8, 2022, at 12:39, Florian Weimer wrote:
>>> On Tue, Nov 8, 2022, at 12:17, Florian Weimer wrote:
>>>> * Arnd Bergmann:
>>>>> On Tue, Nov 8, 2022, at 05:49, YunQiang Su wrote:
>>>>
>>>> Isn't that an invalid kernel configuration?
>>>
>>> No, this is what we will use anyway after 2038 (possibly
>>> earlier), so the option is useful to ensure that all userspace
>>> has been converted away from the time32 interfaces.
>>
>> What happened to not breaking userspace?
> 
> It's a configuration option that makes sense for some users
> but not others, just like a lot of other conditional system
> calls that we have (UID16, IPC, SGETMASK_SYSCALL, FUTEX, EVENTPOLL,
> INOTIFY, IO_URING, AIO, TIMERFD, QUOTA, MODULES).
> 
> If you turn these compile-time symbols off, you naturally break
> compatibility with any applications and libraries that call the
> corresponding syscalls, but nevertheless there are embedded
> systems that never need to call them and prefer to leave the
> symbols disabled.
> 
> If you know you run a modern glibc, you probably don't
> care about uid16 syscalls or sgetmask/ssetmask and can
> leave them disabled. Similarly, system calls that have
> not been used for a long time eventually get removed
> completely, with extreme caution. Examples include prof(),
> lock(), afs_syscall(), pkey_set(), or ftime(). uselib() may
> be next on the list.
> 
> For embedded systems, being able to turn off the broken
> time32 interfaces right now is fairly important, otherwise
> it is excessively hard to validate that a system is not
> accidentally calling the wrong interfaces or relying on
> fallbacks that no longer work after 2038.
> 
> For a desktop distro, that obviously does not apply, as
> users need to be able to run arbitrary binaries that
> were built for time32 or uid16.

Sigh, I have added the use of 64 bit syscall if required for some
sysmbols because I assumed that 32 bit time_t syscall would be always
available.  This is small optimization to avoid issuing a lot of
64 bit syscall, specially for syscall like futex.

But if kernel does indeed assume that COMPAT_32BIT_TIME is a supported
configuration I think we will need to roll out the optimization and
always issue the 64 bit time_t syscalls anyway.  From a glibc point of
view it should not matter much besides some small overhead for newer
glibc running on older kernels (we might add a global to disable the
64-bit call if kernel does not support, as we do for some specific
syscalls).

> 
>> Those with 32-bit applications will want to run their legacy stuff in a
>> time-shifted environment, so these system calls must remain supported
>> outside specialized applications.
> 
> We have had several discussions about adding time-shifting
> for CLOCK_REALTIME to the kernel, and always concluded that
> the added complexity is not worth the possible benefit
> (unlike shifting CLOCK_MONOTONIC, which has use cases for
> checkpoint/restart).
> 
> Clearly one can do the same thing using some LD_PRELOAD
> library or a virtual machine, but then again if you do this,
> you can also have that LD_PRELOAD library do the conversion
> to the time64 interfaces, or have the VM run a kernel with
> the time32 syscalls still present.
> 
>      Arnd