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

["Arnd Bergmann" <arnd@arndb.de>]

On Tue, Nov 8, 2022, at 13:51, Adhemerval Zanella Netto wrote:
> On 08/11/22 09:28, Arnd Bergmann wrote:
>
> 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).

It's possible that I have misread what glibc does at the moment,
as I see there are checks for __ASSUME_TIME64_SYSCALLS in the
same code path.

To clarify: I think an important configuration is one where
an embedded system assumes both kernel and userspace are
always modern and only support time64. This means
CONFIG_COMPAT_32BIT_TIME is disabled in the kernel, and glibc
is built to only support a modern enough kernel to assume
time64 support is always available. I have not tested whether
this works at the moment, but it probably does not require
a large rework if there is something still missing. Sorry
for having hijacked this thread if this is already supported.

If glibc is configured to support older linux-3.2 and includes
the time32 fallbacks for that, it's not unreasonable to require
CONFIG_COMPAT_32BIT_TIME=y for those. It would be
good to document this dependency, or even have an early
runtime check similar to the "kernel too old" error that
glibc produces when there is a version mismatch.

      Arnd