Re: CONFIG_COMPAT_32BIT_TIME in the kernel, x86 ABI on x86-64

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

On 23/07/23 12:16, Andreas K. Huettel wrote:
> Dear all, 
> 
> one of my Gentoo colleagues (Mike Gilbert, floppym) found this, so I'm forwarding
> his notes:
> 
> ===
> Using utimensat as an example, the kernel interface is as follows:
> 
> When CONFIG_COMPAT_32BIT_TIME is enabled, the kernel provides both the legact 
> 32-bit interface (utimensat) and the 64-bit interface (utimensat_time64).
> When CONFIG_COMPAT_32BIT_TIME is disabled, only the 64-bit interface 
> (utimensat_time64) is provided. utimensat gives ENOSYS.
> 
> Behavior of utimensat64 libc function before b286eca5d4117b3e17c939e3df56e132ae623df1:
> 
> 1. Try utimensat_time64
> 2. If ENOSYS, fall back to utimensat if the arguments are small enough to fit in 32-bits
> 
> Behavior of utimensat64 libc function after b286eca5d4117b3e17c939e3df56e132ae623df1:
> 
> 1. If the arguments are small enough to fit in 32-bits, use utimensat
> 2. If the arguments are too large, use utimensat_time64
> 
> The new behavior fails when CONFIG_COMPAT_32BIT_TIME is disabled, since utimensat is 
> unavailable.
> 
> Several other glibc functions were updated with similar behavior changes.
> ===
> 
> What's to do?

It has been already discussed [1], and not only glibc but musl creator Rich
agreed [2] that this option is a gratuitous userspace API/ABI stability policy 
breakage that Linux (at least Linus and some other maintainers) is adamant to 
not introduce.

At least for glibc we have the option to configure with a minimum 5.1 kernel
(--enable-kernel=5.1) which should disable the 32 bit syscall fallback.  This
should work with CONFIG_COMPAT_32BIT_TIME, although I have not actually
test it.

[1] https://sourceware.org/pipermail/libc-alpha/2022-November/143330.html
[2] https://sourceware.org/pipermail/libc-alpha/2022-November/143349.html