linux-kernel - Re: [PATCH v2 seccomp 2/6] asm/syscall.h: Add syscall

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Message-ID: <CABqSeAQ2mek4ZJ0SFZDy6obJ_JveoWX9GDi-_Q_e8o8ZPpF-XQ@mail.gmail.com>
Date:   Thu, 24 Sep 2020 11:02:10 -0500
From:   YiFei Zhu <zhuyifei1999@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "containers@...ts.linux-foundation.org" 
        <containers@...ts.linux-foundation.org>,
        YiFei Zhu <yifeifz2@...inois.edu>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dimitrios Skarlatos <dskarlat@...cmu.edu>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Hubertus Franke <frankeh@...ibm.com>,
        Jack Chen <jianyan2@...inois.edu>,
        Jann Horn <jannh@...gle.com>,
        Josep Torrellas <torrella@...inois.edu>,
        Kees Cook <keescook@...omium.org>,
        Tianyin Xu <tyxu@...inois.edu>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Tycho Andersen <tycho@...ho.pizza>,
        Valentin Rothberg <vrothber@...hat.com>,
        Will Drewry <wad@...omium.org>
Subject: Re: [PATCH v2 seccomp 2/6] asm/syscall.h: Add syscall_arches[] array

On Thu, Sep 24, 2020 at 9:37 AM YiFei Zhu <zhuyifei1999@...il.com> wrote:
> > Try with a slghtly older gcc.
> > I think that entire optimisation (discarding const arrays)
> > is very recent.
>
> Will try, will take a while to get an old GCC to run, however :/

Possibly one of the oldest I can easily get to work is GCC 6.5.0, and
unrolling seems is still the case:

0000000000001560 <__seccomp_filter>:
[...]
    15d4:       41 8b 74 24 04          mov    0x4(%r12),%esi
    15d9:       bf 08 01 00 00          mov    $0x108,%edi
    15de:       81 fe 3e 00 00 c0       cmp    $0xc000003e,%esi
    15e4:       75 30                   jne    1616 <__seccomp_filter+0xb6>
[...]
    1616:       81 fe 03 00 00 40       cmp    $0x40000003,%esi
    161c:       bf 40 01 00 00          mov    $0x140,%edi
    1621:       74 c3                   je     15e6 <__seccomp_filter+0x86>
    1623:       0f 0b                   ud2

Am I overlooking something or should I go further back in the compiler version?

YiFei Zhu