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Date:   Mon, 21 Sep 2020 10:39:16 -0600
From:   Tycho Andersen <tycho@tycho.pizza>
To:     YiFei Zhu <zhuyifei1999@gmail.com>
Cc:     Linux Containers <containers@lists.linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Giuseppe Scrivano <gscrivan@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        YiFei Zhu <yifeifz2@illinois.edu>,
        Tobin Feldman-Fitzthum <tobin@ibm.com>,
        Dimitrios Skarlatos <dskarlat@cs.cmu.edu>,
        Valentin Rothberg <vrothber@redhat.com>,
        Hubertus Franke <frankeh@us.ibm.com>,
        Jack Chen <jianyan2@illinois.edu>,
        Josep Torrellas <torrella@illinois.edu>, bpf@vger.kernel.org,
        Tianyin Xu <tyxu@illinois.edu>,
        Andy Lutomirski <luto@amacapital.net>,
        Will Drewry <wad@chromium.org>, Jann Horn <jannh@google.com>,
        Aleksa Sarai <cyphar@cyphar.com>, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH seccomp 0/2] seccomp: Add bitmap cache of
 arg-independent filter results that allow syscalls
Message-ID: <20200921163916.GE3794348@cisco>
References: <cover.1600661418.git.yifeifz2@illinois.edu>
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 <CABqSeASEw=Qr2CroKEpTyWMRXQkamKVUzXiEe2UsoQTCcv_99A@mail.gmail.com>
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On Mon, Sep 21, 2020 at 10:27:56AM -0500, YiFei Zhu wrote:
> On Mon, Sep 21, 2020 at 8:51 AM Tycho Andersen <tycho@tycho.pizza> wrote:
> > One problem with a kernel config setting is that it's for all tasks.
> > While docker and systemd may make decsisions based on syscall number,
> > other applications may have more nuanced filters, and this cache would
> > yield incorrect results.
> >
> > You could work around this by making this a filter flag instead;
> > filter authors would generally know whether their filter results can
> > be cached and probably be motivated to opt in if their users are
> > complaining about slow syscall execution.
> >
> > Tycho
> 
> Yielding incorrect results should not be possible. The purpose of the
> "emulator" (for the lack of a better term) is to determine whether the
> filter reads any syscall arguments. A read from a syscall argument
> must go through the BPF_LD | BPF_ABS instruction, where the 32 bit
> multiuse field "k" is an offset to struct seccomp_data.

I see, I missed this somehow. So is there a reason to hide this behind
a config option? Isn't it just always better?

Tycho