linux-kernel - Re: [PATCH v5 1/2] selinux: add brief info to policydb

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Date:   Wed, 17 May 2017 09:22:06 -0700
From:   William Roberts <bill.c.roberts@...il.com>
To:     Sebastien Buisson <sbuisson.ddn@...il.com>
Cc:     Stephen Smalley <sds@...ho.nsa.gov>,
        "selinux@...ho.nsa.gov" <selinux@...ho.nsa.gov>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-security-module@...r.kernel.org,
        Sebastien Buisson <sbuisson@....com>,
        James Morris <james.l.morris@...cle.com>
Subject: Re: [PATCH v5 1/2] selinux: add brief info to policydb

On Wed, May 17, 2017 at 9:04 AM, William Roberts
<bill.c.roberts@...il.com> wrote:
> On Wed, May 17, 2017 at 8:43 AM, Sebastien Buisson
> <sbuisson.ddn@...il.com> wrote:
>> 2017-05-17 17:34 GMT+02:00 William Roberts <bill.c.roberts@...il.com>:
>>>>>>> Is there a particular reason to not just return policybrief_len here as
>>>>>>> well, for consistency in the interface?  How do you intend to use this
>>>>>>> value in the caller?
>>>>>>
>>>>>> As called in the other patch to expose policy brief via selinuxfs
>>>>>> (sel_read_policybrief), the intent is to provide the caller with the
>>>>>> length of the string returned.
>>>>>> Or should I set *len to policy brief_len here, and just make the
>>>>>> caller aware that the returned length is in fact the length of the
>>>>>> buffer (i.e. including terminating NUL byte)?
>>>>>
>>>>> What is the caller supposed to do with length? This interface seemed kind of
>>>>> odd. If it's guaranteed NUL byte terminated, do they even need length?
>>>>
>>>> The length is useful as an input parameter in case the caller provides
>>>> its own buffer (instead of letting the function allocate one), and as
>>>
>>> This is what I don't get, why doesn't the function just always allocate?
>>
>> For performance reasons mainly. The caller would have a statically
>> allocated buffer, reused every time it needs to get the policy brief
>> info.
>
> I'm assuming in the Lustre code you're going to call security_policy_brief(),
> how would the caller know how big that buffer is going to be?
>
> I'm looking at both v5 patches, I don't see where it's being called with alloc
> set to false.
>
> I don't see how this works with LSM stacking, I would imagine the security
> hook needs to call this routine for each LSM and join them together in
> some module name spaced way, which was mentioned before, but I don't
> see that either, am I missing it?

Disregard this last statement, I thought more of stacking was in.