linux-kernel - Re: Can we drop upstream Linux x32 support?

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Date:   Fri, 14 Dec 2018 11:13:19 -0500
From:   Rich Felker <dalias@...c.org>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Andy Lutomirski <luto@...nel.org>, tg@...bsd.de,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Mike Frysinger <vapier@...too.org>,
        "H. J. Lu" <hjl.tools@...il.com>, x32@...ldd.debian.org,
        Arnd Bergmann <arnd@...db.de>,
        Will Deacon <will.deacon@....com>
Subject: Re: Can we drop upstream Linux x32 support?

On Fri, Dec 14, 2018 at 12:42:34PM +0100, Florian Weimer wrote:
> * Rich Felker:
> 
> > On Thu, Dec 13, 2018 at 05:04:59PM +0100, Florian Weimer wrote:
> >> * Rich Felker:
> >> 
> >> >> If the compiler can handle the zeroing, that would be great, though not
> >> >> sure how (some __attribute__((zero)) which generates a type constructor
> >> >> for such structure; it kind of departs from what the C language offers).
> >> >
> >> > The compiler fundamentally can't. At the very least it would require
> >> > effective type tracking, which requires shadow memory and is even more
> >> > controversial than -fstrict-aliasing (because in a sense it's a
> >> > stronger version thereof).
> >> 
> >> It's possible to do it with the right types.  See _Bool on 32-bit Darwin
> >> PowerPC for an example, which is four bytes instead of the usual one.
> >> 
> >> Similarly, we could have integer types with trap representations.
> >> Whether it is a good idea is a different matter, but the amount of
> >> compiler magic required is actually limited.
> >
> > If you do this you just have LP64 with value range restricted to
> > 32-bit.
> 
> You have to a type different from long int for the relevant struct
> fields.  This type would have zero padding.

Just upthread (Message-ID: <20181212165237.GT23599@...ghtrain.aerifal.cx>)
I explained why this does not work:

>>> 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).

Rich