linux-kernel - Re: [RFC PATCH seccomp 1/2] seccomp/cache: Add "emulator" to check if filter is arg-dependent

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Message-ID: <CAG48ez1YWz9cnp08UZgeieYRhHdqh-ch7aNwc4JRBnGyrmgfMg@mail.gmail.com>
Date:   Tue, 22 Sep 2020 02:25:46 +0200
From:   Jann Horn <jannh@...gle.com>
To:     YiFei Zhu <zhuyifei1999@...il.com>,
        Kees Cook <keescook@...omium.org>
Cc:     Linux Containers <containers@...ts.linux-foundation.org>,
        YiFei Zhu <yifeifz2@...inois.edu>, bpf <bpf@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dimitrios Skarlatos <dskarlat@...cmu.edu>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Hubertus Franke <frankeh@...ibm.com>,
        Jack Chen <jianyan2@...inois.edu>,
        Josep Torrellas <torrella@...inois.edu>,
        Tianyin Xu <tyxu@...inois.edu>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Valentin Rothberg <vrothber@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>,
        Aleksa Sarai <cyphar@...har.com>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH seccomp 1/2] seccomp/cache: Add "emulator" to check if
 filter is arg-dependent

On Tue, Sep 22, 2020 at 1:44 AM YiFei Zhu <zhuyifei1999@...il.com> wrote:
> On Mon, Sep 21, 2020 at 12:47 PM Jann Horn <jannh@...gle.com> wrote:
> > > +       depends on SECCOMP
> > > +       depends on SECCOMP_FILTER
> >
> > SECCOMP_FILTER already depends on SECCOMP, so the "depends on SECCOMP"
> > line is unnecessary.
>
> The reason that this is here is because of the looks in menuconfig.
> SECCOMP is the direct previous entry, so if this depends on SECCOMP
> then the config would be indented. Is this looks not worth keeping or
> is there some better way to do this?

Ah, I didn't realize this.

> > > +       help
> > > +         Seccomp filters can potentially incur large overhead for each
> > > +         system call. This can alleviate some of the overhead.
> > > +
> > > +         If in doubt, select 'none'.
> >
> > This should not be in arch/x86. Other architectures, such as arm64,
> > should also be able to use this without extra work.
>
> In the initial RFC patch I only added to x86. I could add it to any
> arch that has seccomp filters. Though, I'm wondering, why is SECCOMP
> in the arch-specific Kconfigs?

Ugh, yeah, the existing code is already bad... as far as I can tell,
SECCOMP shouldn't be there, and instead the arch-specific Kconfig
should define something like HAVE_ARCH_SECCOMP and then arch/Kconfig
would define SECCOMP and let it depend on HAVE_ARCH_SECCOMP. It's
really gross how the SECCOMP config description has been copypasted
into a dozen different Kconfig files; and looking around a bit, you
can actually see that e.g. s390 has an utterly outdated help text
which still claims that seccomp is controlled via the ancient
"/proc/<pid>/seccomp". I guess this very nicely illustrates why
putting such options into arch-specific Kconfig is a bad idea. :P