linux-kernel - Re: [PATCH v4 seccomp 3/5] x86: Enable seccomp architecture tracking

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Message-ID: <CALCETrUcsQhYM3+y+geFNmVzscv30Rg=8P50zNtEpLBgEwf9Pg@mail.gmail.com>
Date:   Fri, 9 Oct 2020 13:59:31 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     YiFei Zhu <zhuyifei1999@...il.com>
Cc:     Linux Containers <containers@...ts.linux-foundation.org>,
        YiFei Zhu <yifeifz2@...inois.edu>, bpf <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Laight <David.Laight@...lab.com>,
        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 v4 seccomp 3/5] x86: Enable seccomp architecture tracking

On Fri, Oct 9, 2020 at 11:32 AM YiFei Zhu <zhuyifei1999@...il.com> wrote:
>
> On Fri, Oct 9, 2020 at 12:25 PM Andy Lutomirski <luto@...capital.net> wrote:
> > Is the idea that any syscall that's out of range for this (e.g. all of
> > the x32 syscalls) is unoptimized?  I'm okay with this, but I think it
> > could use a comment.
>
> Yes, any syscall number that is out of range is unoptimized. Where do
> you think I should put a comment? seccomp_cache_check_allow_bitmap
> above `if (unlikely(syscall_nr < 0 || syscall_nr >= bitmap_size))`,
> with something like "any syscall number out of range is unoptimized"?
>

I was imagining a comment near the new macros explaining that this is
the range of syscalls that seccomp will optimize, that behavior is
still correct (albeit slower) for out of range syscalls, and that x32
is intentionally not optimized.

This avoids people like future me reading this code, not remembering
the context, and thinking it looks buggy.