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

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Date:   Wed, 10 Oct 2018 15:39:07 +0100
From:   Szabolcs Nagy <szabolcs.nagy@....com>
To:     Eugene Syromiatnikov <esyr@...hat.com>,
        Yury Norov <ynorov@...iumnetworks.com>
Cc:     nd@....com, Catalin Marinas <catalin.marinas@....com>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-arch@...r.kernel.org,
        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 S . 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 Montezelo <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>,
        Pavel Machek <pavel@....cz>,
        Palmer Dabbelt <palmer@...ive.com>,
        Wookey <wookey@...kware.org>
Subject: Re: [PATCH v9 00/24] ILP32 for ARM64

On 10/10/18 15:10, Eugene Syromiatnikov wrote:
>  * 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.

but that's necessarily the case for all ilp32 abis:
the userspace syscall function receives 32bit
arguments so even if the kernel abi takes 64bit
args you cannot use that from c code. (the libc
does not even know which args should be sign or
zero extended.)

process_vm_readv/writev is limited by the ilp32
iovec struct, not by the syscall arguments.

ptrace is specified to take void* addr argument,
and void* is 32bit on all ilp32 targets.
so again on the c language level there is no
way around the 32bit limitation.