linux-kernel - Re: [PATCH v3 seccomp 2/5] seccomp/cache: Add "emulator" to check if filter is constant allow

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Date:   Thu, 8 Oct 2020 23:47:17 -0500
From:   YiFei Zhu <zhuyifei1999@...il.com>
To:     Linux Containers <containers@...ts.linux-foundation.org>
Cc:     YiFei Zhu <yifeifz2@...inois.edu>, bpf <bpf@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        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 v3 seccomp 2/5] seccomp/cache: Add "emulator" to check if
 filter is constant allow

On Wed, Sep 30, 2020 at 10:20 AM YiFei Zhu <zhuyifei1999@...il.com> wrote:
> @@ -544,7 +577,8 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
>  {
>         struct seccomp_filter *sfilter;
>         int ret;
> -       const bool save_orig = IS_ENABLED(CONFIG_CHECKPOINT_RESTORE);
> +       const bool save_orig = IS_ENABLED(CONFIG_CHECKPOINT_RESTORE) ||
> +                              IS_ENABLED(CONFIG_SECCOMP_CACHE_NR_ONLY);
>
>         if (fprog->len == 0 || fprog->len > BPF_MAXINSNS)
>                 return ERR_PTR(-EINVAL);

I'm trying to use __is_defined(SECCOMP_ARCH_NATIVE) here, and got this message:

kernel/seccomp.c: In function ‘seccomp_prepare_filter’:
././include/linux/kconfig.h:44:44: error: pasting "__ARG_PLACEHOLDER_"
and "(" does not give a valid preprocessing token
   44 | #define ___is_defined(val)  ____is_defined(__ARG_PLACEHOLDER_##val)
      |                                            ^~~~~~~~~~~~~~~~~~
././include/linux/kconfig.h:43:27: note: in expansion of macro ‘___is_defined’
   43 | #define __is_defined(x)   ___is_defined(x)
      |                           ^~~~~~~~~~~~~
kernel/seccomp.c:629:11: note: in expansion of macro ‘__is_defined’
  629 |           __is_defined(SECCOMP_ARCH_NATIVE);
      |           ^~~~~~~~~~~~

Looking at the implementation of __is_defined, it is:

#define __ARG_PLACEHOLDER_1 0,
#define __take_second_arg(__ignored, val, ...) val
#define __is_defined(x) ___is_defined(x)
#define ___is_defined(val) ____is_defined(__ARG_PLACEHOLDER_##val)
#define ____is_defined(arg1_or_junk) __take_second_arg(arg1_or_junk 1, 0)

Hence, when FOO is defined to be 1, then the expansion would be
__is_defined(FOO) -> ___is_defined(1) ->
____is_defined(__ARG_PLACEHOLDER_1) -> __take_second_arg(0, 1, 0) ->
1,
and when FOO is not defined, the expansion would be __is_defined(FOO)
-> ___is_defined(FOO) -> ____is_defined(__ARG_PLACEHOLDER_FOO) ->
__take_second_arg(__ARG_PLACEHOLDER_FOO 1, 0) -> 0

However, here SECCOMP_ARCH_NATIVE is an expression from an OR of some
bits, and __is_defined(SECCOMP_ARCH_NATIVE) would not expand to
__ARG_PLACEHOLDER_1 during any stage in the preprocessing.

Is there any better way to do this? I'm thinking of just doing #if
defined(CONFIG_CHECKPOINT_RESTORE) || defined(SECCOMP_ARCH_NATIVE)
like in Kee's patch.

YiFei Zhu