Re: Can we drop upstream Linux x32 support?

[Rich Felker <dalias@libc.org>]

On Wed, Dec 12, 2018 at 08:39:53AM -0800, Andy Lutomirski wrote:
> > On Dec 11, 2018, at 6:33 PM, Thorsten Glaser <tg@mirbsd.de> wrote:
> >
> > Andy Lutomirski dixit:
> >
> 
> >
> >> IMO the real right solution would be to push the whole problem to
> >> userspace: get an ILP32 system working with almost or entirely LP64
> >
> > Is this a reflex of Linux kernel developers? ;-)
> >
> > I doubt that userspace is the right place for this, remember
> > the recent glibc vs. syscalls debate. It would also need to
> > multiply across various libcs.
> >
> >> How hard would it be to have __attribute__((ilp64)), with an optional
> >> warning if any embedded structs are not ilp64?  This plus a wrapper to
> >
> > You mean LP64. Impossible, because LP64 vs. ILP32 is not the only
> > difference between amd64 and x32.
> 
> I mean LP64.  And I'm not suggesting that ILP32 is the only difference
> between x32 and x86_64, nor am I suggesting that a technique like this
> would implement x32 -- I'm suggesting it would implement something
> better than x32.
> 
> The kernel, as a practical matter, supports two ABIs on 64-bit builds:
> LP64 and ILP32.  ILP32 is what the kernel calls "compat".  ("compat"
> comes with other baggage -- it generally has a 32-bit signal frame,
> syscall arguments are mostly limited to 32 bits, etc.)  Allowing a
> user program that runs in 64-bit mode to issue compat syscalls is not
> really a big deal.  x86_64 has allowed this forever using int $0x80 --
> it's just slow.  Adding a faster mechanism would be straightforward.
> As I understand it, the arm64 ilp32 proposal involves using a genuine
> ILP32 model for user code, so the syscalls will all (except for signal
> handling) go through the compat path.
> 
> x32 is not this at all.  The kernel ABI part of x32 isn't ILP32.  It's
> IP32, 32-bit size_t, and *64-bit* long.  The core kernel doesn't
> really support this.  The only good things I can think of about it are
> that (a) it proves that somewhat odd ABIs are possible, at least in
> principle, and (b) three users have come out of the woodwork to say
> that they use it.
> 
> I'm proposing another alternative.  Given that x32 already proves that
> the user bitness model doesn't have to match the kernel model (in x32,
> user "long" is 32-bit but the kernel ABI "long" is 64-bit), I'm
> proposing extending this to just make the kernel ABI be LP64.  So
> __kernel_size_t would be 64-bit and pointers in kernel data structures
> would be 64-bit.  In other words, most or all of the kernel ABI would
> just match x86_64.
> 
> As far as I can tell, the only thing that really needs unusual
> toolchain features here is that C doesn't have an extra-wide pointer
> type.  The kernel headers would need a way to say "this pointer is
> still logically a pointer, and user code may assume that it's 32 bits,
> but it has 8-byte alignment."

None of this works on the userspace/C side, nor should any attempt be
made to make it work. Types fundamentally cannot have alignments
larger than their size. If you want to make the alignment of some
pointers 8, you have to make their size 8, and then you just have LP64
again if you did it for all pointers.

If on the other hand you tried to make just some pointers "wide
pointers", you'd also be completely breaking the specified API
contracts of standard interfaces. For example in struct iovec's
iov_base, &foo->iov_base is no longer a valid pointer to an object of
type void* that you can pass to interfaces expecting void**. Sloppy
misunderstandings like what you're making now are exactly why x32 is
already broken and buggy (&foo->tv_nsec already has wrong type for
struct timespec foo).

If you wanted to do this and have it work, libc would have to
completely wrap and convert all the types from userspace structures
with the right types matching the specified API contracts to
kernel-interface structures that can be passed to the LP64 syscalls.
This could be done, but it's a big task, and I don't think anyone
wants to do it, especially if it would be yet another new thing that
doesn't exactly match the already-existing x32. The cost-benefit ratio
is just too high.

> There's an added benefit to my proposal over x32.  With my proposal,
> an ILP32 program could plausibly call into a true 64-bit C library
> using a straightforward FFI translation.

Unless it's a thin, "pure" library that doesn't need anything from
libc, or needs sufficiently little that it could be satisfied by some
shims, this would necessarily require having two libcs in the same
process, which is not going to work.

Rich