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

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Date:   Tue, 11 Dec 2018 10:02:45 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "H.J. Lu" <hjl.tools@...il.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <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>,
        Florian Weimer <fweimer@...hat.com>, vapier@...too.org,
        Rich Felker <dalias@...c.org>, x32@...ldd.debian.org,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Can we drop upstream Linux x32 support?

On Tue, Dec 11, 2018 at 6:35 AM Andy Lutomirski <luto@...nel.org> wrote:
> On Mon, Dec 10, 2018 at 7:15 PM H.J. Lu <hjl.tools@...il.com> wrote:
> > On Mon, Dec 10, 2018 at 5:23 PM Andy Lutomirski <luto@...nel.org> wrote:
> Right.  My question wasn't whether x32 had developers -- it was
> whether it had users.  If the only users are a small handful of people
> who keep the toolchain and working and some people who benchmark it,
> then I think the case for keeping it in upstream Linux is a bit weak.

+1

> > > Conversely, if you call a syscall in the 512 range with bit 31
> > > *clear*, then the compat entry is set with in_compat_syscall()
> > > *clear*.  This is also nutty.
> >
> > This is to share syscalls between LP64 and ILP32 (x32) in x86-64 kernel.
> >
>
> I tried to understand what's going on.  As far as I can tell, most of
> the magic is the fact that __kernel_long_t and __kernel_ulong_t are
> 64-bit as seen by x32 user code.  This means that a decent number of
> uapi structures are the same on x32 and x86_64.  Syscalls that only
> use structures like this should route to the x86_64 entry points.  But
> the implementation is still highly dubious -- in_compat_syscall() will
> be *true* in such system calls,

I think the fundamental issue was that the intention had always been
to use only the 64-bit entry points for system calls, but the most
complex one we have -- ioctl() -- has to use the compat entry point
because device drivers define their own data structures using 'long'
and pointer members and they need translation, as well as
matching in_compat_syscall() checks. This in turn breaks down
again whenever a driver defines an ioctl command that takes
a __kernel_long_t or a derived type like timespec as its argument.

> which means that, if someone changes:
>
...
> where one argument has x32 and x86_64 matching but the other has x32
> and x86_32 matching.
>
> This whole thing seems extremely fragile.

It definitely is. We have lots of workarounds specifically for
x32 in device drivers, but in the time_t conversion for y2038
I still found ones that had not been caught earlier, and for each
y2038 conversion that someone did to a driver or syscall, we have
to make sure that it doesn't break x32 in the process.

         Arnd