linux-kernel - Re: [PATCH v3 1/1] security: Add mechanism to safely (un)load LSMs after boot time

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Message-ID: <CAGXu5jLn6c=mr5VC=fUP4LUD18EuPAYyDkYTrv_rpxSuxAz--A@mail.gmail.com>
Date:   Sat, 31 Mar 2018 07:38:54 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     Casey Schaufler <casey@...aufler-ca.com>,
        LSM <linux-security-module@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Igor Stoppa <igor.stoppa@...wei.com>,
        James Morris <jmorris@...ei.org>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Paul Moore <paul@...l-moore.com>, plautrba@...hat.com
Subject: Re: [PATCH v3 1/1] security: Add mechanism to safely (un)load LSMs
 after boot time

On Fri, Mar 30, 2018 at 11:16 PM, Sargun Dhillon <sargun@...gun.me> wrote:
> On Fri, Mar 30, 2018 at 2:39 PM, Casey Schaufler <casey@...aufler-ca.com> wrote:
>>>  static struct security_hook_list     null_hooks[SECURITY_HOOK_COUNT];
>>> -#define HAS_FUNC(SHL, FUNC)  (SHL->hook.FUNC)
>>> +DEFINE_STATIC_SRCU(security_hook_srcu);
>>> +
>>> +static inline bool is_null_hook(struct security_hook_list *shl)
>>> +{
>>> +     union {
>>> +             void *cb_ptr;
>>> +             union security_list_options slo;
>>> +     } hook_options;
>>> +
>>> +     hook_options.slo = shl->hook;
>>> +     return !hook_options.cb_ptr;
>>> +}
>>
>> I like the HAS_FUNC() approach better.
>
>  Just curious, why? I personally prefer small static inline functions
> over macros, if possible.

Generally speaking, small static inline functions are better since
they provide type-checking. In this case, though, it looks like you're
just doing a cast, but with a union. Why isn't this just:

return !!((uintptr_t)shl->hook)

?

Though the security_list_options union exists for callback type
checking, so really, having HAS_FUNC() with the explicit function
you're interested in creates a bit of self-documenting code (even if
it always resolves to the above test).

-Kees

-- 
Kees Cook
Pixel Security