linux-kernel - Re: [PATCH v9 00/24] ILP32 for ARM64

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Date:   Sun, 14 Oct 2018 21:49:01 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Yury Norov <ynorov@...iumnetworks.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Adam Borowski <kilobyte@...band.pl>,
        Alexander Graf <agraf@...e.de>,
        Alexey Klimov <klimov.linux@...il.com>,
        Andreas Schwab <schwab@...e.de>,
        Andrew Pinski <pinskia@...il.com>,
        Bamvor Zhangjian <bamv2005@...il.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Christoph Muellner <christoph.muellner@...obroma-systems.com>,
        Dave Martin <Dave.Martin@....com>,
        David Miller <davem@...emloft.net>,
        Florian Weimer <fweimer@...hat.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        James Hogan <james.hogan@...tec.com>,
        James Morse <james.morse@....com>,
        Joseph Myers <joseph@...esourcery.com>,
        Lin Yongting <linyongting@...wei.com>,
        "Manuel A. Fernandez Montecelo" <manuel.montezelo@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
        Nathan Lynch <Nathan_Lynch@...tor.com>,
        Philipp Tomsich <philipp.tomsich@...obroma-systems.com>,
        Prasun Kapoor <Prasun.Kapoor@...iumnetworks.com>,
        Ramana Radhakrishnan <ramana.gcc@...glemail.com>,
        Steve Ellcey <sellcey@...iumnetworks.com>,
        Szabolcs Nagy <szabolcs.nagy@....com>
Subject: Re: [PATCH v9 00/24] ILP32 for ARM64

On Sat, Oct 13, 2018 at 9:36 PM Andy Lutomirski <luto@...capital.net> wrote:
>
> On Wed, May 16, 2018 at 1:19 AM Yury Norov <ynorov@...iumnetworks.com> wrote:
> >
> > This series enables AARCH64 with ILP32 mode.
> >
> > As supporting work, it introduces ARCH_32BIT_OFF_T configuration
> > option that is enabled for existing 32-bit architectures but disabled
> > for new arches (so 64-bit off_t userspace type is used by new userspace).
> > Also it deprecates getrlimit and setrlimit syscalls prior to prlimit64.
>
> Second, ILP32 user code is highly unlikely
> to end up with the same struct layout as ILP64 code.  The latter seems
> like it should be solved entirely in userspace by adding a way to
> annotate a structure as being a kernel ABI structure and getting the
> toolchain to lay it out as if it were ILP64 even though the target is
> ILP32.

The syscall ABI could be almost completely abstracted in glibc, the
main issue is ioctl and a couple of related interfaces that pass data
structures (read() on /dev/input/*, mmap on /dev/snd/*
or raw sockets, fcntl).

The question whether a data type is laid out like a 64-bit architecture
would cannot be a property of the type in most of those cases,
because the same types are used elsewhere. Many ioctls just
take a pointer to a 'long' or similar, and then you have structures
like 'timespec' that are used both in syscall/ioctl ABI and in normal
user space code, but are required to be laid out differently there.
(timespec is a bad example because y2038 of course, but it
illustrates the point).

> 2. I think you should make a conscious decision as to whether the
> ILP32-ness of a syscall is a property of the task or of the syscall.
> On x86, x32-ness is a property of the syscall, but historically it
> also got rather entangled with the state of the task, and the result
> was a mess.  It looks like you're making it be a property of the task,
> which is fine, but you're making it impossible for very clever ILP32
> libraries to include little ILP64 stubs that do fancy things with full
> 64-bit syscalls.
>
> 3. Make very certain that you aren't exploitable by malicious
> processes that set the high bits in ILP32 syscall args.  x86 compat
> has issues like that in the past.

This point was actually the most important one for keeping the
aarch64 ilp32 interface as restricted as it is: it doesn't allow
anything that the normal aarch32/armv7 emulation doesn't
already provide.

        Arnd