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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 10 Oct 2018 16:18:02 +0200
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Subject: Re: [PATCH v9 00/24] ILP32 for ARM64
To:     Eugene Syromiatnikov <esyr@redhat.com>
Cc:     Yury Norov <ynorov@caviumnetworks.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-api@vger.kernel.org, Adam Borowski <kilobyte@angband.pl>,
        Alexander Graf <agraf@suse.de>,
        Alexey Klimov <klimov.linux@gmail.com>,
        Andreas Schwab <schwab@suse.de>,
        Andrew Pinski <pinskia@gmail.com>,
        Bamvor Zhangjian <bamv2005@gmail.com>,
        Chris Metcalf <cmetcalf@mellanox.com>,
        Christoph Muellner <christoph.muellner@theobroma-systems.com>,
        Dave Martin <Dave.Martin@arm.com>,
        "David S . Miller" <davem@davemloft.net>,
        Florian Weimer <fweimer@redhat.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        James Hogan <james.hogan@imgtec.com>,
        James Morse <james.morse@arm.com>,
        Joseph Myers <joseph@codesourcery.com>,
        Lin Yongting <linyongting@huawei.com>,
        Manuel Montezelo <manuel.montezelo@gmail.com>,
        Mark Brown <broonie@kernel.org>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@linaro.org>,
        Nathan_Lynch <Nathan_Lynch@mentor.com>,
        Philipp Tomsich <philipp.tomsich@theobroma-systems.com>,
        Prasun Kapoor <Prasun.Kapoor@caviumnetworks.com>,
        Ramana Radhakrishnan <ramana.gcc@googlemail.com>,
        Steve Ellcey <sellcey@caviumnetworks.com>,
        Szabolcs Nagy <szabolcs.nagy@arm.com>,
        Pavel Machek <pavel@ucw.cz>,
        Palmer Dabbelt <palmer@sifive.com>,
        Wookey <wookey@wookware.org>
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On 10/10/18, Eugene Syromiatnikov <esyr@redhat.com> wrote:
> On Tue, Jul 24, 2018 at 08:39:57PM +0300, Yury Norov wrote:
>> Hi all,
>>
>> + Pavel Machek, Palmer Dabbelt, Wookey.
>>
>> On Wed, May 16, 2018 at 11:18:45AM +0300, Yury Norov 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.
>> >
>> > Based on kernel v4.16. Tested with LTP, glibc testsuite, trinity,
>> > lmbench,
>> > CPUSpec.
>
>> This is the update of the series based on 4.17 kernel
>> https://github.com/norov/linux/tree/ilp32-4.17
>
> Hello.
>
> I have some questions regarding AArch64 ILP32 implementation for which I
> failed to find an answer myself:
>  * How ptrace() tracer is supposed to distinguish between ILP32 and LP64
>    tracees? For MIPS N32 and x32 this is possible based on syscall
>    number, but for AArch64 ILP32 I do not see such a sign. There's also
>    ARM_ip is employed for signalling entering/exiting, I wonder whether
>    it's possible to employ it also for signalling tracee's personality.

Don't know.

>  * What's the reasoning behind capping syscall arguments to 32 bit? x32
>    and MIPS N32 do not have such a restriction (and do not need special
>    wrappers for syscalls that pass 64-bit values as a result, except
>    when they do,  as it is the case for preadv2 on x32); moreover, that
>    would lead to insurmountable difficulties for AArch64 ILP32 tracers
>    that try to trace LP64 tracees, as it would be impossible to pass
>    64-bit addresses to process_vm_{read,write} or ptrace PEEK/POKE.

The idea is to keep the syscall entry points as similar as possible
between arm (aarch32) emulation and aarch64-ilp32 mode when
you have a kernel that supports both.

      Arnd