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

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Date:   Tue, 11 Dec 2018 12:37:42 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Arnd Bergmann <arnd@...db.de>, Andy Lutomirski <luto@...nel.org>,
        "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>, vapier@...too.org,
        Rich Felker <dalias@...c.org>, x32@...ldd.debian.org,
        Will Deacon <will.deacon@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Can we drop upstream Linux x32 support?

* Catalin Marinas:

> On Tue, Dec 11, 2018 at 10:02:45AM +0100, Arnd Bergmann wrote:
>> On Tue, Dec 11, 2018 at 6:35 AM Andy Lutomirski <luto@...nel.org> wrote:
>> > 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.
>
> With arm64 ILP32 we tried to avoid the ioctl() problem by having
> __kernel_long_t 32-bit, IOW mimicking the arm32 ABI (compat). The
> biggest pain point is signals where the state is completely different
> from arm32 (more, wider registers) and can't be dealt with by the compat
> layer.

I would expect to approach this from the opposite direction: use 64-bit
types in places where the 64-bit kernel interface uses 64-bit types.
After all, not everyone who is interested in ILP32 has a companion
32-bit architecture which could serve as a model for the application
ABI.

(If there are conflicts with POSIX, then POSIX needs to be fixed to
support this.)

Thanks,
Florian