Re: [PATCHv3 00/24] ILP32 support in ARM64

[Rich Felker <dalias at libc dot org>]

On Thu, Feb 12, 2015 at 08:30:10AM -0800, H.J. Lu wrote:
> On Thu, Feb 12, 2015 at 7:50 AM, Catalin Marinas
> <catalin.marinas@arm.com> wrote:
> > On Wed, Feb 11, 2015 at 12:15:56PM -0800, Andy Lutomirski wrote:
> >> On 02/11/2015 11:57 AM, H.J. Lu wrote:
> >> >>>>trivially satisfied if you consider x32 and x86_64 separate
> >> >>>>compilation environments, but it's not related to the core issue: that
> >> >>>>the definition of timespec violates core (not obscure) requirements of
> >> >>>>both POSIX and C11. At the time you were probably unaware of the C11
> >> >>>>requirement. Note that it's a LOT harder to effect change in the C
> >> >>>>standard, so even if the Austin Group would be amenable to changing
> >> >>>>the requirement for timespec to allow something like nseconds_t,
> >> >>>>getting WG14 to make this change to work around a Linux/glibc mistake
> >> >>>>does not sound practical.
> >> >>>
> >> >>>That is very unfortunate.  I consider it is too late for x32 to change.
> >> >>
> >> >>Why? It's hardly an incompatible ABI change, as long as the
> >> >>kernel/libc fills the upper bits (for old programs that read them
> >> >>based on the old headers) when structs are read from the kernel to the
> >> >>application, and ignores the upper bits (potentially set or left
> >> >>uninitialized by the application) when strings are passed from
> >> >>userspace to the kernel. Newly built apps using the struct definition
> >> >>with 32-bit tv_nsec would need new libc to ensure that the high bits
> >> >>aren't interpreted, but this could be handled by symbol versioning.
> >> >>
> >> >
> >> >We have considered this option.  But since kernel wouldn't change
> >> >tv_nsec/tv_usec handling just for x32, it wasn't selected.
> >>
> >> Did anyone *ask* the kernel people (e.g. hpa)?
> >
> > It seems so:
> >
> > https://lkml.org/lkml/2011/8/31/244
> >
> > Couple of more replies from hpa:
> >
> > https://lkml.org/lkml/2011/8/31/261
> > https://lkml.org/lkml/2012/2/8/408
> >
> > It looks like hpa was going to talk the POSIX committee but I don't know
> > what the conclusion was and didn't follow the thread (at the time I
> > wasn't interested in ARM ILP32).
> 
> Just for the record,  tv_nsec/tv_usec can be changed to long
> as long as kernel always read them as 32 bits and write them
> as 64 bits for both LP64 and ILP32 in 64-bit  imespec amd timeval.

No; currently userspace relies on the kernel to produce EINVAL when
tv_nsec is not in the range [0,999999999]. If the kernel just reads it
as 32-bit unconditionally, tv_nsec=0x100000000 would fail to produce
EINVAL in LP64 models where tv_nsec is a 64-bit object in userspace.

> In glibc, they can be changed to long without breaking existing binaries.

This is true only if glibc or the kernel ignores the upper bits.
Otherwise, programs could end up passing junk that glibc and/or the
kernel interprets.

> For x86-32, 64-bit __time_t must be 64-bit aligned.  Otherwise, there will
> be no padding in 64-bit timespec nor timeval.

Just adding an explicit padding member when long is 32-bit would be
cleaner. This makes it possible to manually set/clear/inspect the bits
without memset. I don't see any reason to require actual alignment of
the struct on x86-32 unless you're going with a whole new ABI where
64-bit types are aligned. Of course if we're thinking about making
64-bit time_t on 32-bit archs, that's an incompatible ABI already and
would be a great time to make lots of other ABI fixes... But I wonder
if anyone is going to care about actual x86-32 hardware as Y2038
approaches.

Rich